⎇ⓥ⺖㗴ฬ㧦FEL ߦࠃࠆ‛⾰ߩశ⺃⿠᭴ㅧᄌൻ࡮శ⺃⿠ଔᢙᄌൻߩ⎇ⓥߣ
ࡈࠜ࠻ࡔࡕ࡝࡯᧚ᢱ㐿⊒߳ߩᔕ↪
⎇ⓥઍ⴫⠪䋺ᦸ᦬㩷 ┨੺䋨ᣣᧄᄢቇᢥℂቇㇱ‛ℂቇ⑼䋩㩷
౒ห⎇ⓥ⠪䋺ᵻ࿾㩷 ືᄦ䋨ᣣᧄᄢቇᢥℂቇㇱൻቇ⑼䋩㩷

‫⎇ޣ‬ⓥ⋡⊛‫ޤ‬ᦨㄭ㧘ᚒ‫⎇ߩޘ‬ⓥࠣ࡞࡯ࡊߢ㉄ൻ‛ߩ࿕૕㧘⭯⤑㧘࠽ࡁ☸ሶ⤑߇⚡ᄖ࡟࡯ࠩ࡯శᾖ኿ਅߢߩ㔓
࿐᳇੤឵(㉄⚛Æ⌀ⓨ)ߦ઻ߞߡࡈࠜ࠻࡞ࡒࡀ࠶࠮ࡦࠬ⦡߇นㅒ⊛ᄌൻߒ㧘ᄌൻߒߚ⁁ᘒߪ࡟࡯ࠩ
࡯శࠍᾖ኿ߒߥߌࠇ߫ቶౝἮߩਅߢᅤ૗ߥࠆ㔓࿐᳇੤឵ߦࠃߞߡ߽᳗ਭ⊛ߦ଻ሽߐࠇࠆ੐ࠍ⊒⷗ߒߚ㧚ᧄ⎇ⓥߪߎࠇࠄߩ⎇ⓥࠍၮ␆ߦߒߡ㧘ቶ᷷ߢ቟ቯߦ⿠ߎࠆนㅒ(ᶖ෰น⢻)ߥ࡟࡯ࠩ࡯శ
⺃⿠‛ᕈᄌൻ⃻⽎ࠍ␜ߔ‛⾰ߩត⚝෸߮ഃ⵾ߣߎࠇࠄ‛⾰⴫㕙߳ߩ࠽ࡁࡔ࡯࠻࡞ࠨࠗ࠭ߩࠞ࡜
࡯࠼࠶࠻⸥㍳ߦߟ޿ߡએਅߩ⎇ⓥࠍⴕ߁㧚(1) ⚡ᄖ࡟࡯ࠩ࡯శߩᾖ኿ߢ⊒శ⦡߇⿒⦡߆ࠄ⊕⦡㧘㕍⦡߆ࠄ⊕⦡╬ߦ᳗ਭ⊛ߦォ⒖ߒ(ᖱႎߩ⸥㍳ߦኻᔕ)㧘ߎߩォ⒖߇೎ߩᵄ㐳ߩ࡟࡯ࠩ࡯శ߿⇣ߥࠆ㔓࿐᳇ߢߩ⚡ᄖ࡟࡯ࠩ࡯శᾖ኿ߢరߩ⊒శ⦡ߦᚯߖࠆ(⸥㍳ߐࠇߚᖱႎߩᶖ෰ߦኻᔕ)ᅤߊߩᕈ⾰ࠍ᦭ߔࠆశࡔࡕ࡝࡯‛⾰ࠍត⚝࡮ഃ⵾ߔࠆ㧚(2) ഃ⵾ߐࠇߚశࡔࡕ࡝࡯‛⾰⴫㕙ߦᖱႎࠍㄭធ႐శቇ㗼ᓸⵝ⟎ߣ⚿วߐߖߡ㧘࠽ࡁࡔ㧙࠻࡞ࠨࠗ࠭ߩ⿒࡮㕍࡮⊕ઁߩࠞ࡜࡯࠼࠶࠻೉ߣߒߡᖱႎࠍ⿥㜞ኒᐲ⸥㍳ߔࠆᣇᴺࠍ⏕┙ߔࠆ㧚‫⎇ޣ‬ⓥ᭎ⷐ‫ޤ‬Ԙࠞ࡜࡯ࡈࠜ࠻ࡔࡕ࡝࡯‛⾰ߩត⚝࡮ഃ⵾⚡ᄖ࡟࡯ࠩ࡯శߢ㊄ዻࠗࠝࡦߩଔᢙᄌൻ࡮㉄⚛ᰳ㒱↢ᚑߢࡈࠜ࠻࡞ࡒࡀ࠶࠮ࡦࠬ⦡ߩᄌൻ߇ᦼᓙ
ߢ߈ࠆ㊄㊄ዻ㉄ൻ‛߿ߘߩⶄว૕㧘ጀ㑆ߦ⒳‫ߩޘ‬ಽሶጀࠍᝌ౉ߒߡ೙ᓮน⢻ߥᄙ᭽ߥࡈࠜ࠻࡞ࡒࡀ
࠶࠮ࡦࠬ․ᕈ߇ᦼᓙߢ߈ࠆጀ
ጀ⁁ࡍࡠࡉࠬࠞࠗ࠻ൻว‛㧘߹ߚ㧘㔚႐⊒శ߽ᦼᓙߢ߈ࠆ᦭
᦭ᯏൻว‛╬ߦ⌕⋡ߒߡ㧚࡟࡯ࠩ࡯శ⺃⿠⊒శ⦡ᄌൻ⃻⽎ࠍ⎇ⓥߔࠆ㧚ԙ࡟࡯ࠩ࡯శߦࠃࠆಽⵚ࡮Ⲣว࡮᭴ㅧᄌൻࠍ฽߻ォ⒖ߩㆊᷰ⃻⽎ࠍ⸘⸘▚ᯏࠪࡒࡘ࡟࡯࡚ࠪࡦߦࠃߞ
ߡ⸃᣿ߔࠆ㧚
Ԛㄭធ႐శቇ㗼ᓸಽశⵝ⟎ߣߩ⚿วߦࠃࠆ࠽ࡁࠨࠗ࠭ࠞ࡜࡯࠼࠶࠻೉ߩ⸥㍳࠽ࡁࡔ࡯࠻࡞ⓨ㑆ಽ⸃⢻ㄭធ႐శቇ㗼ᓸಽశⵝ⟎ߦࠃࠅ㧘ഃ⵾ߒߚ‛⾰ߩ⴫㕙ߦ࠽࠽ࡁࡔ㧙࠲㧙
ࠨࠗ࠭ߩ⿒࡮㕍࡮⊕ઁߩࠞ࡜࡯࠼࠶࠻೉ߣߒߡᖱႎࠍ⸥㍳ߔࠆታ㛎ࠍⴕ߁㧚ߘߩ⚿ᨐߪએਅߩߣ߅ࠅߢ޽ࠆ㧚(1)Ꮧ࿯㘃㊄ዻ㉄ൻ‛ߩࠞ࡜࡯ࡈࠜ࠻ࡔࡕ࡝࡯⃻⽎ߩ࠳ࠗ࠽ࡒࠢࠬߩ⎇ⓥ㉄ൻ࡙࡯ࡠࡇ࠙ࡓ Eu2O3㧘㉄ൻࠨࡑ࡝࠙ࡓ Sm2O3 ߦߟ޿ߡ㧘ߎࠇ߹ߢߩ⎇ⓥߦ⛯޿ߡࡈࠜ࠻࡞
ࡒࡀ࠶࠮ࡦࠬࠬࡍࠢ࠻࡞ߣ࡜ࡑࡦᢔੂࠬࡍࠢ࠻࡞ࠍ⹦⚦ߦ᷹ቯߒ㧘࡟࡯ࠩ࡯శሶ႐ߣ‛⾰ߣߩബ⿠ࠛࡀ࡞ࠡ࡯ߩ߿ࠅขࠅߣߒߡࠞ࡜࡯ࡈࠜ࠻ࡔࡕ࡝࡯⃻⽎ߩ࠳ࠗ࠽ࡒࠢࠬࠍ⎇ⓥߒߚ㧚ߎߩ⎇ⓥ
ᚑᨐߩ৻ㇱߪᧄᐕ㧡᦬ߦࡈ࡜ࡦࠬ࿖ Rennes University 1 ߢ㐿௅ߐࠇࠆశ⺃⿠⋧ォ⒖ߦ㑐ߔࠆ࿖㓙ળ⼏ the Second International Conference on Photo-Induced Phase Transitions Cooperative, non- linear
and functional properties ߢਅ⸥ߩ⺰ᢥߣߒߡ౏⴫ߔࠆ੍ቯߢ޽ࠆ㧚
Shosuke Mochizuki,Fumito Fujishiro,Ken’ichiro Ishiwata: Photo-induced valence-number changes anddefects in Eu2O3
߶߆ߦ㧘JAl2O3-Eu2O3, AgI-anatase TiO2, silica glass ߦߟ޿ߡ߽ࠞ࡜࡯ࡈࠜ࠻ࡔࡕ࡝࡯⃻⽎ࠍ⊒⷗ߒ㧘ߘߩ࠳ࠗ࠽ࡒࠢࠬࠍ⎇ⓥߒߚ㧚(2)శ⸅ᇦ࡮㊂ሶᏱ⺃㔚૕ SrTiO3 ߩࡈࠜ࠻ࡔࡕ࡝࡯⃻⽎ߩ⎇ⓥశ⸅ᇦᕈ߿㊂ሶᏱ⺃㔚ᕈߢ⥝๧߇ᜬߚࠇߡ޿ࠆ࠴࠲ࡦ㉄ࠬ࠻ࡠࡦ࠴࠙ࡓ SrTiO3 ߩࡈࠜ࠻ࡔࡕ
࡝࡯⃻⽎ߣࡈࠜ࠻࡞ࡒࡀ࠶࠮ࡦࠬ⃻⽎ߩ⿠Ḯࠍ⎇ⓥߒ㧘ߎߩ‛⾰ߦߟ޿ߡೋ߼ߡ⚡ᄖ࡟࡯ࠩ࡯శ
⺃⿠ࡈࠜ࠻ࡔࡕ࡝࡯⃻⽎ࠍ⊒⷗ߒߚ㧚ߘߒߡ㧘ᓥ᧪ߎߩ‛⾰ߩ㊂ሶᏱ⺃㔚ᕈߣ㑐ㅪઃߌߡ⼏⺰ߐࠇߡ޿ߚࡈࠜ࠻࡞ࡒࡀ࠶࠮ࡦࠬߪ㧘ߎߩ‛⾰․᦭ߩ⚿᥏ᰳ㒱᭴ㅧߣൻቇ⊛߳࠹ࡠᕈߦ⿠Ḯࠍᜬߟ⊒శߢ޽ࠆߎߣࠍ᣿ࠄ߆ߦߒߚ㧚ߎߩ⎇ⓥߩ᭎ⷐߪએਅߩߣ߅ࠅߢ޽ࠆ㧚ή⦡ㅘ᣿ߥ࠴࠲ࡦ㉄ࠬ࠻ࡠࡦ࠴࠙ࡓ SrTiO3 න⚿᥏⹜ᢱ߇ቶ᷷ߢ⌀ⓨਛߦߡ 325nm ࡟࡯ࠩ࡯శ
ߢᾖ኿ߐࠇࠆߣ㧘2.4eV ࠍਛᔃߣߔࠆ᏷ᐢ޿⊒శᏪ߇ߘߩ⊒శᒝᐲࠍჇᄢߐߖࠆ㧚ߎߩᒝ޿⊒శ⁁ᘒߪ࡟࡯ࠩ࡯శߩᾖ኿߇ήߌࠇ߫㧘ቶౝἮߩਅߢᅤ૗ߥࠆ㔓࿐᳇ߢ߽㐳ᦼ㑆଻ሽߐࠇࠆ㧚⹜ᢱ㔓࿐᳇ࠍ㉄⚛ࠟࠬߦߔࠆߣ㧘߆߆ࠆ⚡ᄖ࡟࡯ࠩ࡯శߩਅߢ⊒శᒝᐲࠍᒙ߼ߟߟ㧘రߩ⊒శ⁁ᘒߦᚯࠆ㧚ߎߩࠃ߁ߥ࡟࡯ࠩ࡯శ⺃⿠⃻⽎ߪૐ᷷ 13K ߢ߽᷹ⷰߐࠇ㧘⃻⽎ߪ⚐☴ߦ SrTiO3 ⚿᥏ߩ㔚ሶ♽ߩബ⿠ߦ㑐ଥߒߚ߽ߩߢ޽ࠆߎߣࠍ␜ߒߡ޿ࠆ㧚ߎߩታ㛎ߪ㧘SrTiO3 ߩ as-grown ⚿᥏߿ߎࠇࠍ Ar-H2 㔓࿐᳇ਛ㧘973K ߢࠕ࠾࡯࡞ߒߡ㧘ή⦡ㅘ᣿⚿᥏ߦ⥋ࠆㆊ⒟ߦߟ޿ߡ߽ⴕࠊࠇߚ㧚᷹ⷰߐࠇߚ⃻⽎ߪ SrTiO3 ⚿᥏⴫㕙ઃㄭߢߩ㧘⚡ᄖ࡟࡯ࠩ࡯శߦ⺃⿠ߐࠇߚశㆶర࡮శ㉄ൻߣߒߡℂ⸃ߐࠇࠆ㧚ߎࠇࠄߩ⎇ⓥᚑᨐࠃࠅ㧘ߎߩ SrTiO3 ⚿᥏ߩ⴫㕙ࠍ⊒శ‛⾰ߢୃ㘼ߔࠆߣ㧘ᣂߒ޿ࠞ࡜࡯ࡈࠜ࠻ࡔࡕ࡝࡯‛⾰߇ഃ⵾ߢ߈ࠆน⢻ᕈ߇޽ࠆߎߣ߇ಽ߆ߞߚ㧚ߎࠇ߹ߢ SrTiO3 ⚿᥏ߦߟ޿ߡߪ㧘2.4eV ⊒శ߇⍮ࠄࠇߡ޿ߚ߇㧘ᧄ⎇ⓥߢߪߎߩ‛⾰ߩࡃࡦ࠼ࠡࡖ࠶ࡊએ਄ߩࠛࡀ࡞ࠡ࡯ߩశ㧘Nd3+-YAG ࡟࡯ࠩ࡯ߩ╙㧟㜞⺞ᵄ 355nm ߩബ⿠ߢ 2.4eV ⊒శߩ߶߆ߦ 3.2eV ࠍਛᔃߣߔࠆࡃࡦ࠼┵ㄭற⊒శߣ 2.9eV ࠍਛᔃߣߔࠆ A ࡃࡦ࠼⊒శࠍ⊒⷗ߒߚ㧚
2.4eV ⊒శߪᢙ⑽߹ߢ⛯ߊ㐳ኼ๮ᱷశᕈ⊒శࠍ․ᓽߣߔࠆ߇㧘ᣂߒߊ⷗ߟߌࠄࠇߚੑߟߩ⊒శߪశബ⿠ᓟ 50ns એౝߦᶖṌߒߡߒ߹߁⍴ኼ๮ᅱ⊒శߢ޽ࠆߎߣ߇ಽ߆ߞߚ㧚߹ߚ㧘ࡈࠜ࠻࡞ࡒࡀ
࠶࠮ࡦࠬߩബ⿠࡟࡯ࠩ࡯శᒝᐲଐሽᕈࠍ᷹ቯߔࠆߣ㧘2.4eV ⊒శߪ 4mJ/cm2 ߢᦨᄢ୯ࠍߣࠅ㧘ߘࠇએ਄ߩኻബ⿠ߢߪ⊒శല₸ߪᷫዋߒᆎ߼ࠆ߇㧘 3.2eV ߅ࠃ߮ 2.9eV ⊒శߪߤߎ߹ߢ߽✢ᒻ⊛ߦჇ߃⛯ߌࠆߎߣ߇ಽ߆ߞߚ㧚ߎࠇߪ㧘2.4eV ⊒శ߇ߎࠇ߹ߢౝᄖߩ⎇ⓥ⠪߇ਥᒛߒߡ߈ߚ㊂ሶᏱ⺃㔚ᕈߦ⿠Ḯࠍᜬߟ⌀ᕈ⥄Ꮖ᧤❈ബ⿠ሶߢ޽ࠆߎߣࠍุቯߔࠆᣂ੐ታߢ޽ࠆ㧚(3)⿥ࠗࠝࡦዉ㔚૕ AgI-anatase TiO2, AgI-ZrO2 ߩࡈࠜ࠻ࡔࡕ࡝࡯⃻⽎ߩ⎇ⓥߔߢߦ⿥ࠗࠝࡦዉ㔚૕ AgI-anatase TiO2 ߩశ⺃⿠ࠬࡍࠢ࠻࡞ᄌൻߩ⊒⷗ߦߟ޿ߡߪ⺰ᢥߢ౏⴫ߒߡ޽ࠆ߇㧘੹ᐕᐲߩ⎇ⓥߢߪߎߩ⹦⚦ߦߟ޿ߡታ㛎ߔࠆߣ઻ߦ㧘หߓ⿥ࠗࠝࡦዉ㔚૕ߩ
AgI-ZrO2 ࠍ૞⵾ߒ㧘శࡔࡕ࡝࡯ᕈࠍ⎇ⓥߒߚ㧚ߎߩ‛⾰ߪቶ᷷ߢ⌀ⓨਛߦߡ 325nm ࡟࡯ࠩ࡯శ
ߢᾖ኿ߐࠇࠆߣ㧘2.6eV ࠍਛᔃߣߔࠆ᏷ᐢ޿⊒శᏪ߇ߘߩ⊒శ᏷ࠍჇᄢߐߖࠆ㧚ߎߩ᏷ᐢ޿⊒శ⁁ᘒߪ࡟࡯ࠩ࡯శߩᾖ኿߇ήߌࠇ߫㧘ቶౝἮߩਅߢᅤ૗ߥࠆ㔓࿐᳇ߢ߽㐳ᦼ㑆଻ሽߐࠇࠆ㧚⹜ᢱ㔓࿐᳇ࠍ㉄⚛ࠟࠬߦߔࠆߣ㧘߆߆ࠆ⚡ᄖ࡟࡯ࠩ࡯శߩਅߢ㧘రߩ⊒శ⁁ᘒߦᚯࠆ㧚ߎߩࠃ߁ߥ࡟
࡯ࠩ࡯శ⺃⿠⃻⽎ߪૐ᷷ 13K ߢ߽᷹ⷰߐࠇ㧘⃻⽎ߪ⚐☴ߦ ZrO2 ߩ㔚ሶ♽ߩബ⿠ߦ㑐ଥߒߚ߽ߩ
ߢ޽ࠆߎߣࠍ␜ߒߡ޿ࠆ㧚߹ߚ㧘ߎߩ⿥ࠗࠝࡦዉ㔚૕ߩၮ␆᧚ᢱߩ AgI ߩ⭯⤑ᚑ㐳ㆊ⒟ߦߟ޿ߡ㧘⫳⌕⤑ᒻᚑਛ߅ࠃ߮
ࠕ࠾࡯࡞ㆊ⒟ߢశๆ෼ࠬࡍࠢ࠻࡞࡮ࡈࠜ࠻࡞ࡒࡀ࠶࠮ࡦࠬࠬࡍࠢ࠻࡞ߩ᷹ቯࠍⴕߞߚ‫ޕ‬ߎߩ⎇ⓥ࠹࡯ࡑߦߟ޿ߡ⃻࿷㧘AgI-SrTiO3 ♽ߩ⹜ᢱࠍ૞⵾ߒ㧘ࡈࠜ࠻࡞ࡒࡀ࠶࠮ࡦࠬ․ᕈࠍ⺞ߴߡ޿ࠆ㧚‫ޤ߼ߣ߹ޣ‬ߎࠇ߹ߢߦ⊒⷗ߒߚ Eu2O3, Sm2O3, JAl2O3-Eu2O3, AgI-anatase TiO2, silica glass ╬ߩࡈࠜ࠻ࡔࡕ࡝
࡯‛⾰ߩశ⺃⿠⃻⽎ߩ࠳ࠗ࠽ࡒࠢࠬࠍ⎇ⓥߔࠆߣߣ߽ߦ㧘శ⸅ᇦ‛⾰ SrTiO3 ߣ⿥ࠗࠝࡦዉ㔚૕
AgI, AgI-ZrO2 ߩࡈࠜ࠻࡞ࡒࡀ࠶࠮ࡦࠬ․ᕈࠍ᣿ࠄ߆ߦߒ㧘ࡈࠜ࠻ࡔࡕ࡝࡯⃻⽎ࠍ⊒⷗ߒߚ㧚㩷‫⎇ޣ‬ⓥᬺ❣╬‫ޤ‬࡮⊒⴫⺰ᢥ(1) S. Mochizuki, F. Fujishiro and S. Minami "Photoluminescence and reversible photo-induced spectralchange of SrTiO3" Journal of Physics: Condensed Matter 17 (2005) 923-948.
(2) F. Fujishiro and S. Mochizuki "Photoluminescence studies on AgI-ZrO2 composites" Journal of
Luminescence 112 (2005) 71-74.
(3) S. Mochizuki, S. Minami and F. Fujishiro "The reversible UV-laser-light-induced spectral change andorigin of the 2.4 eV luminescence band in SrTiO3" Journal of Luminescence 112 (2005) 267-270.
(4) S. Mochizuki and F. Fujishiro "In situ real time optical study of films grown on low temperature
substrates by vacuum evaporation of iodine and silver iodide: spectral transitions during deposition andannealing processes" Journal of Physics: Condensed Matter 16 (2004) 3239-3256.
(5) Y. Ishida, Y. Mita, M. Kobayashi, S. Endo and S. Mochizuki "Pressure-induced phase transition in
D-MnS" Journal of Magnetism and Magnetic Materials 272-276 (2004) 428-429.
(6) F. Fujishiro and S. Mochizuki "Excitons in AgI-based-glasses and -composites" (accepted).
(7) F. Fujishiro and S. Mochizuki "Structural- and optical studies on mesoscopic defect structure in highly
conductive AgI-ZnO composites" Physica B 340-342 (2003) 216-220.
(8) S. Mochizuki and H. Araki "Reversible photoinduced spectral transition in Eu2O3-JAl2O3 composites atroom temperature" Physica B 340-342 (2003) 913-917.
(9) S. Mochizuki "Intense white-luminescence of Sm2O3 irradiated with ultraviolet laser light undervacuum" Physica B 340-342 (2003) 944-948.
(10) S. Mochizuki, T. Shimizu and F. Fujishiro "Photoluminescence study on defects in pristine anatase andanatase-based composites" Physica B 340-342 (2003) 956-959.
(11) S. Mochizuki and H. Araki "UV-laser-light-produced defects and reversible blue-whitephotoluminescence change in silica" Physica B 340-342 (2003) 969-973.
(12) S. Mochizuki and F. Fujishiro "Structural, electrical and optical studies on AgI-anatase composites"
Journal of Physics: Condensed Matter 15 (2003) 5057-5072.
(13) S. Mochizuki and F. Fujishiro "Shallow and deep excited states of mesoscopic structure in AgI-JAl2O3
composites" physica status solidi (c) 0 (2002) 763-766.
(14) S. Mochizuki and F. Fujishiro "Shallow- and deep- luminescence centers in AgI- based superionic
conductor glass" physica status solidi (c) 0 (2002) 767-770.
(15) F. Fujishiro and S. Mochizuki "The photoexcitation and relaxation of high ionicconductivity glasses(AgI)x(AgPO3)1-x" Nonlinear Optics 29 (2002) 443-449.
(16) T. Nakanishi and S. Mochizuki "Exhanced photoluminescence of C70 porous glass composite" Nonlinear
Optics 29 (2002) 603-608.
(17) Y. Mita, Y. Sakai, M. Kobayashi, S. Endo and S. Mochizuki "Magnetic phase transition in
antiferromagnetic materials under high pressure" physica status solidi (a) 189 (2002) 935-938.
(18) S. Mochizuki, T. Nakanishi, Y. Suzuki and K. Ishi "Reversible photoinduced spectral change in Eu2O3 atroom temperature" Applied Physics Letters 79 (2001) 3785-3787.
(19) S. Mochizuki "Stacking faults and excitons in AgI" Physica B 308-310 (2001) 1042-1045.
(20) S. Mochizuki, Y. Suzuki, T. Nakanishi and K. Ishi "Valence-change and defect-induced whiteluminescence of Eu2O3" Physica B 308-310 (2001) 1046-1049(21) Y. Mita, Y. Sakai, D. Izaki, M. Kobayashi, S. Endo and S. Mochizuki "Optical study of MnO under highpressure" physica status solidi (b) 223 (2001) 247-251.
(22) T. Asaji and T. Ishizaka; "Effect of Deuteration on Ammonium Motion and Structural Phase Transition
Studied by Nuclear Quadrupole Resonance", Z.Naturforsch., 55a, 2000, 83-89(23) H. Fujimori and T. Asaji; "35Cl NQR in Glassy Crystal of 2-chlorothiophene", Z.Naturforsch., 55a,
2000, 183-185(24) Y. Kume and T. Asaji; "Hydrogen Isotope Effect on the Temperature Dependence of the 35Cl-NQR
Frequency in (NH4)2IrCl6", Z.Naturforsch., 57a, 2002, 504-508(25) H. Fujimori, T. Jinbo, and T. Asaji; "13C NMR Study of Liquid Crystals CBOOA", Proc. of Institute of
Natural Sci., Nihon Univ., 39, 2004, 405-409(26) T. Asaji, E. Akiyama, F. Tajima, K. Eda, M. Hashimoto, and Y. Furukawa; "Salts of tetrachloroauric acidwith pyridine N-oxide having various base/acid ratios of 1/1, 4/3, 3/2 and 2/1. Crystal structures, 35Cl
NQR and phase transitions", Polyhedron, 23, 2003, 1605-1611࡮ቇળ⊒⴫ญ㗡⊒⴫(ᣣᧄ‛ℂቇળ)(1) ᦸ᦬┨੺㧘⮮ઍผ㧘྾ኅ੗ජ⧷㧘ධ⡛ሶ‫ޟ‬SrTiO3 ࡌ࡞࠿ࠗන⚿᥏ߩశࠬࡍࠢ࠻࡞㧦Ớ㕍⦡ as-grown⚿᥏߆ࠄㅘ᣿⚿᥏ߦ⥋ࠆ߹ߢ‫ ޠ‬ᣣᧄ‛ℂቇળ ╙ 60 ࿁ᐕᰴᄢળ 㧔᧲੩ℂ⑼ᄢቇ㧕(2) ⮮ઍผ㧘ᦸ᦬┨੺‫ޟ‬SrTiO3 ߩ⚡ᄖ࡮นⷞ㗔ၞߩࡈࠜ࠻࡞ࡒࡀ࠮ࡦࠬ㧦⴫㕙ലᨐ㧘ᰳ㒱ലᨐ‫ ޠ‬ᣣᧄ‛ℂቇળ ╙ 60 ࿁ᐕᰴᄢળ 㧔᧲੩ℂ⑼ᄢቇ㧕(3) ᦸ᦬┨੺㧘ධ⡛ሶ‫ޟ‬SrTiO3 න⚿᥏ߩ࡟࡯ࠩ࡯శ⺃⿠ࠬࡍࠢ࠻࡞ᄌൻ⃻⽎Σ‫ޠ‬ᣣᧄ‛ℂቇળ 2004
ᐕ⑺ቄᄢળ 㧔㕍᫪ᄢቇ㧕ᐔᚑ 16 ᐕ 9 ᦬(4) ධ⡛ሶ㧘ᦸ᦬┨੺‫ޟ‬SrTiO3 න⚿᥏ߩࡈࠜ࠻࡞ࡒࡀ࠶࠮ࡦࠬΣ‫ޠ‬ᣣᧄ‛ℂቇળ 2004 ᐕ⑺ቄᄢળ
㧔㕍᫪ᄢቇ㧕ᐔᚑ 16 ᐕ 9 ᦬(5) ᦸ᦬┨੺㧘⮮ઍผ‫ޟ‬ZrO2 ߩࡈࠜ࠻࡞ࡒࡀ࠶࠮ࡦࠬ‫ޠ‬ᣣᧄ‛ℂቇળ 2004 ᐕ⑺ቄᄢળ 㧔㕍᫪ᄢቇ㧕ᐔᚑ 16 ᐕ 9 ᦬(6) ⮮ઍผ㧘ᦸ᦬┨੺‫ޟ‬AgI-ZrO2 ⶄว૕ߩ㔚᳇‛ᕈ࡮శ‛ᕈ‫ޠ‬ᣣᧄ‛ℂቇળ 2004 ᐕ⑺ቄᄢળ 㧔㕍᫪ᄢቇ㧕ᐔᚑ 16 ᐕ 9 ᦬(7) ⮮ઍผ㧘ᦸ᦬┨੺‫ޟ‬AgI ߩബ⿠ሶ㧦ㆊ೾࡛࠙⚛ലᨐ㧘ⶄว૕ൻലᨐ㧘Ⓧጀᰳ㒱ലᨐ߶߆‫ޠ‬ᣣᧄ
‛ℂቇળ╙ 59 ࿁ᐕᰴᄢળ 㧔਻Ꮊᄢቇ㧕ᐔᚑ 16 ᐕ 3 ᦬(8) ᦸ᦬┨੺㧘⮮ઍผ‫ޟ‬Ag㧘I㧘AgI ߩᾲ⊛⫳⊒ㆊ⒟ߣ⭯⤑ᚑ㐳ㆊ⒟ߩಽశቇ⊛⎇ⓥ‫ޠ‬ᣣᧄ‛ℂቇળ
╙ 59 ࿁ᐕᰴᄢળ 㧔਻Ꮊᄢቇ㧕ᐔᚑ 16 ᐕ 3 ᦬(9) ⮮ઍผ㧘ᦸ᦬┨੺‫ޟ‬AgI/㉄ൻ‛ⶄว‛⾰ߩ᭴ㅧߣశ‛ᕈ࡮㔚᳇‛ᕈ‫ޠ‬ᣣᧄ‛ℂቇળ 2003 ᐕ⑺ቄ
ᄢળ 㧔ጟጊᄢቇ㧕ᐔᚑ 15 ᐕ 9 ᦬(10) ᦸ᦬┨੺㧘⮮ઍผ‫ޟ‬AgI-anatase ⶄว૕ߩนㅒߥ࡟࡯ࠩ࡯శ⺃⿠ࠬࡍࠢ࠻࡞ᄌൻ⃻⽎‫ޠ‬ᣣᧄ‛ℂ
ቇળ 2003 ᐕ⑺ቄᄢળ 㧔ጟጊᄢቇ㧕ᐔᚑ 15 ᐕ 9 ᦬(11) ⨹ᧁኡਯ㧘ᦸ᦬┨੺‫㉄ޟ‬ൻ‛ߩ࡟࡯ࠩ࡯శ⺃⿠శࠬࡍࠢ࠻࡞ᄌൻ⃻⽎Σ‫ޠ‬ᣣᧄ‛ℂቇળ 2003
ᐕ⑺ቄᄢળ 㧔ጟጊᄢቇ㧕ᐔᚑ 15 ᐕ 9 ᦬(12) ᷡ᳓⾆༹㧘⮮ઍผ㧘ᦸ᦬┨੺‫ࠍ࠯࡯࠲࠽ࠕޟ‬฽߻㜞޿ࠗࠝࡦዉ㔚ᕈⶄว૕ߩ࡜ࡑࡦᢔੂΤ‫ޠ‬ᣣᧄ‛ℂቇળ 2003 ᐕ⑺ቄᄢળ 㧔ጟጊᄢቇ㧕ᐔᚑ 15 ᐕ 9 ᦬(13) ⮮ઍผ㧘㙃↰⋥ੱ㧘૒㊁ା੺㧘ᦸ᦬┨੺‫ޟ‬AgI/㕖ㆫ⒖㊄ዻ㉄ൻ‛ⶄว‛⾰ߩේሶ⊛᭴ㅧߣశ࡮
㔚᳇‛ᕈ‫ޠ‬ᣣᧄ‛ℂቇળ╙ 58 ࿁ᐕᰴᄢળ 㧔᧲ർᄢቇ㧕ᐔᚑ 15 ᐕ 3 ᦬

(14) ᦸ᦬┨੺㧘⮮ઍผ‫ޟ‬AgI/ㆫ⒖㊄ዻ㉄ൻ‛ⶄว‛⾰ߩේሶ⊛᭴ㅧߣశ࡮㔚᳇‛ᕈ‫ޠ‬ᣣᧄ‛ℂቇળ
╙ 58 ࿁ᐕᰴᄢળ 㧔᧲ർᄢቇ㧕ᐔᚑ 15 ᐕ 3 ᦬(15) ⨹ᧁኡਯ㧘ᦸ᦬┨੺㧘ᷰཅᢝᤊม㧘ਛ⷏ᄥቝੱ㧘દᏧำ‫ޟ‬Eu2O3࡮⴫㕙ᵴᕈ㉄ൻ‛ⶄว૕ߩน
ㅒ⊛శ⺃⿠శࠬࡍࠢ࠻࡞ᄌൻ⃻⽎ II‫ޠ‬ᣣᧄ‛ℂቇળ ╙ 58 ࿁ᐕᰴᄢળ 㧔᧲ർᄢቇ㧕ᐔᚑ 15
ᐕ3᦬(16) ᷡ᳓⾆༹㧘ᦸ᦬┨੺‫ࠍ࠯࡯࠲࠽ࠕޟ‬฽߻㜞޿ࠗࠝࡦዉ㔚ᕈⶄว૕ߩ࡜ࡑࡦᢔੂ‫ޠ‬ᣣᧄ‛ℂቇળ
╙ 58 ࿁ᐕᰴᄢળ 㧔᧲ർᄢቇ㧕ᐔᚑ 15 ᐕ 3 ᦬(17) ⮮ઍผ㧘ᦸ᦬┨੺‫ޟ‬AgI ߩ㜞Ớᐲᭂ㒢ㄭறߢߩ AgI ♽ዉ㔚ᕈࠟ࡜ࠬߩේሶ⊛᭴ㅧߣశࠬࡍࠢ࠻
࡞㧦(AgI)x(AgPO3)1-x‫ޠ‬ᣣᧄ‛ℂቇળ 2002 ᐕ⑺ቄᄢળ 㧔ਛㇱᄢቇ㧕ᐔᚑ 14 ᐕ 9 ᦬(18) ᦸ᦬┨੺㧘⮮ઍผ‫ޟ‬AgI-JAl2O3 ⶄว‛⾰ߩේሶ⊛᭴ㅧߣశࠬࡍࠢ࠻࡞‫ޠ‬ᣣᧄ‛ℂቇળ 2002 ᐕ⑺ቄᄢળ 㧔ਛㇱᄢቇ㧕ᐔᚑ 14 ᐕ 9 ᦬(19) ⨹ᧁኡਯ㧘ᦸ᦬┨੺‫ޟ‬Eu2O3࡮⴫㕙ᵴᕈ㉄ൻ‛ⶄว૕ߩนㅒ⊛శ⺃⿠శࠬࡍࠢ࠻࡞ᄌൻ⃻⽎ I‫ޠ‬ᣣᧄ‛ℂቇળ 2002 ᐕ⑺ቄᄢળ 㧔ਛㇱᄢቇ㧕ᐔᚑ 14 ᐕ 9 ᦬(20) ᦸ᦬┨੺㧘⮮ઍผ‫ޟ‬AgI ߣ AgI ♽ࠗࠝࡦዉ㔚ᕈࠟ࡜ࠬߩశࠬࡍࠢ࠻࡞Τ‫ޠ‬ᣣᧄ‛ℂቇળ╙ 57
࿁ᐕᰴᄢળ 㧔┙๮㙚ᄢቇ㧕ᐔᚑ 14 ᐕ 3 ᦬(21) ⮮ઍผ㧘ᦸ᦬┨੺‫ޟ‬㜞ࠗࠝࡦዉ㔚ᕈࠟ࡜ࠬ(AgI)x(AgPO3)1-x ߩࡈࠜ࠻࡞ࡒࡀ࠶࠮ࡦࠬߣવዉΣ‫ޠ‬ᣣᧄ‛ℂቇળ╙ 57 ࿁ᐕᰴᄢળ 㧔┙๮㙚ᄢቇ㧕ᐔᚑ 14 ᐕ 3 ᦬(22) ਎⦟ᄥ㇢㧘Raphael RUPPIN㧘ᦸ᦬┨੺‫ޟ‬Ꮧ࿯㘃㊄ዻߩࠟࠬਛ⫳⊒ߣశࠬࡍࠢ࠻࡞ II -Sm㧘Eu㧘
Yb ࠢ࡜ࠬ࠲࡯ߩᚑ㐳ߣ㔚ሶ⁁ᘒ-‫ޠ‬ᣣᧄ‛ℂቇળ╙ 57 ࿁ᐕᰴᄢળ 㧔┙๮㙚ᄢቇ㧕ᐔᚑ 14 ᐕ 3᦬(23) ⮮ઍผ㧘ቝ㊁⦟᥍㧘ᦸ᦬┨੺‫(ޟ‬AgI)x(AgPO3)1-x ♽ࠗࠝࡦዉ㔚ᕈࠟ࡜ࠬߩశࠬࡍࠢ࠻࡞Σ‫ޠ‬ᣣᧄ
‛ℂቇળ 2001 ᐕ⑺ቄᄢળ 㧔ᓼፉᢥℂᄢቇ㧕ᐔᚑ 13 ᐕ 9 ᦬(24) ᦸ᦬┨੺㧘⮮ઍผ‫ޟ‬AgI ߣ AgI ♽ࠗࠝࡦዉ㔚ᕈࠟ࡜ࠬߩశࠬࡍࠢ࠻࡞Σ‫ޠ‬ᣣᧄ‛ℂቇળ 2001
ᐕ⑺ቄᄢળ 㧔ᓼፉᢥℂᄢቇ㧕ᐔᚑ 13 ᐕ 9 ᦬(25) ㋈ᧁ␭਽㧘ਛ⷏ᄥቝੱ㧘㗇⽴ብⴕ㧘દᏧำᒄ㧘ᦸ᦬┨੺‫ޟ‬శᖱႎࡔࡕ࡝࡯‛⾰ Eu2O3 ߩశࠬࡍࠢ࠻࡞‫ޠ‬ᣣᧄ‛ℂቇળ 2001 ᐕ⑺ቄᄢળ 㧔ᓼፉᢥℂᄢቇ㧕ᐔᚑ 13 ᐕ 9 ᦬(26) ਛ⷏ᄥቝੱ㧘⨹ᧁኡਯ㧘ᦸ᦬┨੺‫ޟ‬ᄙሹࠟ࡜ࠬਛߦಽᢔߒߚ C60 ࠢ࡜ࠬ࠲࡯ߩశቇࠬࡍࠢ࠻࡞‫ޠ‬
ᣣᧄ‛ℂቇળ 2001 ᐕ⑺ቄᄢળ 㧔ᓼፉᢥℂᄢቇ㧕ᐔᚑ 13 ᐕ 9 ᦬(27) ਛ⷏ᄥቝੱ㧘⨹ᧁኡਯ㧘ᦸ᦬┨੺‫ޟ‬ᄙሹࠟ࡜ࠬਛߦಽᢔߒߚ C70 ࠢ࡜ࠬ࠲࡯ߩ⊒శᯏ᭴ II‫ޠ‬ᣣᧄ‛ℂቇળ 2001 ᐕ⑺ቄᄢળ 㧔ᓼፉᢥℂᄢቇ㧕ᐔᚑ 13 ᐕ 9 ᦬(28) ਎⦟ᄥ㇢㧘Raphael RUPPIN㧘ᦸ᦬┨੺‫ޟ‬Ꮧ࿯㘃㊄ዻߩࠟࠬਛ⫳⊒ߣశࠬࡍࠢ࠻࡞ - Eu㧘Yb ࠢ࡜ࠬ࠲࡯ߩᚑ㐳ߣ㔚ሶ⁁ᘒ - ‫ޠ‬ᣣᧄ‛ℂቇળ 2001 ᐕ⑺ቄᄢળ 㧔ᓼፉᢥℂᄢቇ㧕ᐔᚑ 13 ᐕ 9᦬(29) ᦸ᦬┨੺‫ޟ‬AgI ߩബ⿠ሶ‫ޠ‬ᣣᧄ‛ℂቇળ ╙ 56 ࿁ᐕᰴᄢળ 㧔ਛᄩᄢቇ㧕ᐔᚑ 13 ᐕ 3 ᦬(30) ㋈ᧁ␭਽㧘ᦸ᦬┨੺㧘ᦦᩮ↰ᔘ๺㧘ਛ⷏ᄥቝੱ㧘ᐔ㊁᤯ม㧘ቝ㊁⦟ᷡ㧘દᏧำᒄ㧘Michele FAUCHER㧘
Oon-Kyoung MOUNE㧘Jeannette DEXPERT-GHYS㧘Yolande KIHN‫ޟ‬Eu2O3㧘Sm2O3 ߩࡈࠜ࠻࡞ࡒࡀ
࠶࠮ࡦࠬߩశ⺃⿠นㅒ⊛ᄌൻ⃻⽎‫ޠ‬ᣣᧄ‛ℂቇળ ╙ 56 ࿁ᐕᰴᄢળ 㧔ਛᄩᄢቇ㧕ᐔᚑ 13 ᐕ 3᦬(31) ᦸ᦬┨੺㧘ቝ㊁⦟ᷡ㧘㋈ᧁ␭਽㧘Michele Faucher㧘O. Kiyoung Moune㧘Jeannette Dexpert-Ghys㧘
Yolande Zihn‫ޟ‬Eu-O ಝ❗♽ߦ߅ߌࠆ Eu3+ࠗࠝࡦߩశቇㆫ⒖ߩ⚿᥏᭴ㅧߣߩ⋧㑐Σ‫ޠ‬ᣣᧄ‛ℂቇળ
╙ 55 ࿁ᐕᰴᄢળ 㧔ᣂẟᄢቇ㧕ᐔᚑ 12 ᐕ 9 ᦬(32) ㋈ᧁ␭਽㧘ᦸ᦬┨੺㧘ਛ⷏ᄥቝੱ㧘દᏧำᒄ‫ޟ‬Eu2O3 ߩนㅒ⊛శ⺃⿠ଔᢙᄌൻ⃻⽎‫ޠ‬ᣣᧄ‛ℂቇળ ╙ 55 ࿁ᐕᰴᄢળ 㧔ᣂẟᄢቇ㧕ᐔᚑ 12 ᐕ 9 ᦬(33) ᄢ↰᦭૞㧘ᦸ᦬┨੺‫ޟ‬AgI ߩ⿥ࠗࠝࡦዉ㔚⋧ߩశࠬࡍࠢ࠻࡞‫ޠ‬ᣣᧄ‛ℂቇળ ╙ 55 ࿁ᐕᰴᄢળ 㧔ᣂẟᄢቇ㧕ᐔᚑ 12 ᐕ 9 ᦬

(34) ਛ⷏ᄥቝੱ㧘㜞ᯅ▵ᄦ㧘ᦸ᦬┨੺‫ޟ‬ᄙሹࠟ࡜ࠬਛߦಽᢔߒߚ C70 ࠢ࡜ࠬ࠲࡯ߩ⊒శᯏ᭴‫ޠ‬ᣣᧄ
‛ℂቇળ ╙ 55 ࿁ᐕᰴᄢળ 㧔ᣂẟᄢቇ㧕ᐔᚑ 12 ᐕ 9 ᦬ญ㗡⊒⴫(╙ 6 ࿁⿥ࠗࠝࡦዉ㔚૕‛ᕈ⎇ⓥળ)(1) ⮮ઍผ㧘ᦸ᦬┨੺‫ޟ‬㜞Ớᐲ AgI ߩ⿥ࠗࠝࡦዉ㔚ࠟ࡜ࠬߩ᭴ㅧߣశ‛ᕈ‫ ╙ޠ‬6 ࿁⿥ࠗࠝࡦዉ㔚૕‛ᕈ⎇ⓥળ 㧔߬ࠆࠆࡊ࡜ࠩ੩ㇺ㧕ᐔᚑ 14 ᐕ 5 ᦬(2) ᦸ᦬┨੺㧘⮮ઍผ‫ޟ‬AgI ߩේሶ⊛᭴ㅧߣബ⿠ሶ᭴ㅧ‫ ╙ޠ‬6 ࿁⿥ࠗࠝࡦዉ㔚૕‛ᕈ⎇ⓥળ 㧔߬ࠆࠆࡊ࡜ࠩ੩ㇺ㧕ᐔᚑ 14 ᐕ 5 ᦬ญ㗡⊒⴫(࿖㓙ળ⼏)(1) ⮮ઍผ㧘ᦸ᦬┨੺‫ޟ‬Excitons in AgI-based-glasses and -composites‫ޠ‬The 1st International Discussion
Meeting of Superionic Conductor Physics 㧔หᔒ␠ᄢቇ㧕ᐔᚑ 15 ᐕ 9 ᦬ࡐࠬ࠲࡯⊒⴫(࿖㓙ળ⼏)(1) ᦸ᦬┨੺㧘ධ⡛ሶ㧘⮮ઍผ‫ޟ‬The reversible UV-laser-light-induced spectral change and origin of the 2.4
eV luminescence band in SrTiO3‫ޠ‬6th International Conference on Excitonic Processes in Condenesd
Matter 㧔Cracow㧕ᐔᚑ 16 ᐕ 7 ᦬(2) ⮮ઍผ㧘ᦸ᦬┨੺‫ޟ‬Photoluminescence studies on AgI-ZrO2 composites‫ޠ‬6th International Conference on
Excitonic Processes in Condenesd Matter 㧔Cracow㧕ᐔᚑ 16 ᐕ 7 ᦬(3) ⮮ઍผ㧘ᦸ᦬┨੺‫ޟ‬Structural- and optical studies on mesoscopic defect structure in highly conductive
AgI-ZnO composites‫ޠ‬the 22nd International conference on defects in semiconductors 㧔Aarhus㧕ᐔᚑ
15 ᐕ 7 ᦬(4) ᦸ᦬┨੺㧘⨹ᧁኡਯ‫ޟ‬Reversible photoinduced spectral transition in Eu2O3–JAl2O3 composites at room
temperature‫ޠ‬the 22nd International conference on defects in semiconductors 㧔Aarhus㧕ᐔᚑ 15 ᐕ 7᦬(5) ᦸ᦬┨੺‫ޟ‬Intense white-luminescence of Sm2O3 irradiated with ultraviolet laser light under vacuum‫ޠ‬the22nd International conference on defects in semiconductors 㧔Aarhus㧕ᐔᚑ 15 ᐕ 7 ᦬(6) ᦸ᦬┨੺㧘ᷡ᳓⾆༹㧘⮮ઍผ‫ޟ‬Photoluminescence study on defects in pristine anatase and anatase-based
composites‫ޠ‬the 22nd International conference on defects in semiconductors 㧔Aarhus㧕ᐔᚑ 15 ᐕ 7 ᦬(7) ᦸ᦬┨੺㧘⨹ᧁኡਯ‫ޟ‬UV-laser-light-produced defects and reversible blue-white photoluminescencechange in silica‫ޠ‬the 22nd International conference on defects in semiconductors 㧔Aarhus㧕ᐔᚑ 15 ᐕ
7᦬(8) ᦸ᦬┨੺㧘 ⮮ઍผ‫ޟ‬Shallow and deep excited states of mesoscopic structure in AgI–JAl2O3
composites‫ޠ‬10th International Conference on Shallow Level Centers in Semiconductors 㧔Warsaw㧕ᐔᚑ 14 ᐕ 7 ᦬(9) ᦸ᦬┨੺㧘⮮ઍผ‫ޟ‬Shallow- and deep- luminescence centers in AgI- based superionic conductor glass‫ޠ‬
10th International Conference on Shallow Level Centers in Semiconductors 㧔Warsaw㧕ᐔᚑ 14 ᐕ 7 ᦬(10) ⮮ઍผ㧘ᦸ᦬┨੺‫ޟ‬The photoexcitation and relaxation of high ionicconductivity glasses(AgI)x(AgPO3)1-x‫ޠ‬5th International Conference on Excitonic Processes in Condenesd Matter 㧔Darwin㧕ᐔᚑ 14 ᐕ 7 ᦬(11) ਛ⷏ᄥቝੱ㧘ᦸ᦬┨੺‫ޟ‬Exhanced photoluminescence of C70 porous glass composite‫ޠ‬5th International
Conference on Excitonic Processes in Condenesd Matter 㧔Darwin㧕ᐔᚑ 14 ᐕ 7 ᦬

APPLIED PHYSICS LETTERSVOLUME 79, NUMBER 23

3 DECEMBER 2001

Reversible photoinduced spectral change in Eu2O3 at room temperature
Shosuke Mochizuki,a) Tauto Nakanishi, and Yuya Suzuki
Department of Physics, College of Humanities and Sciences, Nihon University, Tokyo 156-8550, Japan

Kimihiro Ishi
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan

共Received 5 February 2001; accepted for publication 1 October 2001兲
When Eu2O3 powder compact and ﬁlm are irradiated with ultraviolet 共UV兲 laser light in a vacuum,
their photoluminescence 共PL兲 spectra change from a red sharp-line structure to a white broad band,
which can be clearly seen with the naked eye. After removing the UV laser light, the white PLcontinues for more than several months at room temperature under room light, in spite of any
changes of atmosphere. By irradiating with the same UV laser light at room temperature under O2
gas atmosphere, the original red PL state reappears. Such a reversible phenomenon may well yieldmaterials for white-light-emitting devices and erasable optical storage. © 2001 American Instituteof Physics. 关DOI: 10.1063/1.1425064兴

Recently, considerable attention has been devoted to research for new materials to be used for erasable optical storage and for laser media for tunable wide-band lasers. Theinterest in photoluminescent materials showing photoinduced
reversible spectral change at room temperature has signiﬁcantly grown, because a combination of such reversible phenomenon and the near-ﬁeld optical microscope1 is very
promising when applied to high-density optical storage withnanometer resolution. It is a hope that we can ﬁnd whitephotoluminescence 共PL兲 material, which is useful for thewide-band tunable-laser medium. Hitherto, X-ray- andphotoinduced spectral change of rare-earth ions has been observed mainly on Eu-doped ionic crystals and glasses.2– 6
However, there is a limitation to dopant concentration. The
storage density of such doped crystals cannot, in principle,
prevail against that of rare-earth metal compounds 共for example, Eu2O3 and Sm2O3兲. We have observed recently thereversible photoinduced phenomenon and white PL state in
Eu2O3 powder compact and ﬁlm. In this letter, we report theexperimental results in detail.
Powder compact specimens are prepared by pressing
Eu2O3 powder 99.98% in purity under a pressure of 0.2 GPa
for 1 h at room temperature. They are then sintered at 1273
K in air for 24 h. X-ray diffraction analysis of every compact
shows that they are cubic-phase Eu2O3. 7 Eu2O3 ﬁlms aregrown on a silica glass substrate in a planar diode of theradio-frequency-excited sputter deposition system. X-ray diffraction analysis of the ﬁlms shows that they are thehexagonal-phase Eu2O3. 7 Irradiation is carried out with acontinuous wave 共cw兲 of He–Cd laser line (␭⫽325 nm)with power densities between 31 and 840 kW/m2. The illumination optics area is between 2.54⫻10⫺6 and 1.13
⫻10⫺8 m2. The same He–Cd laser line excites luminescence. Emitted light is dispersed and detected using a grating
spectrograph equipped with a multichannel photodetection
system. Excitation spectra are recorded in an apparatus consisting of a 150 W xenon lamp, a mechanical light chopper,a兲

Author to whom correspondence should be addressed; electronic mail:motizuki@physics.chs.nihon-u.ac.jp

two grating monochromators and a synchronous light detection system. The PL and excitation spectra are corrected,
respectively, for the response of the optical measurement
systems and the emission intensity spectrum of the lightsource. The values of the PL intensity are given in arbitrary
units. In the ﬁlm measurements, the sensitivity of the detections was twice that in the powder compact measurements.
Figures 1共a兲, 1共b兲, and 1共c兲 show the reversible spectralchange of the Eu2O3 powder compact sintered at 1273 K inair. Measurements are carried out in alphabetical order:
共a兲→共b兲→共c兲. Irradiation is carried out with a 325 nm laserline at room temperature. Irradiation time t ir under a given
atmosphere and kind of atmosphere are indicated by each
spectrum. The spectrum shown in Fig. 1共a兲 corresponds tothat of the as sintered Eu2O3 specimen, which is measured inO2 gas of 1.01⫻105 Pa. Clear red emission can be seen with
the naked eye. The specimen chamber is then evacuated below 1.33⫻10⫺4 Pa under UV light irradiation. With increasing t ir , all the Eu3⫹ 关 5 D 0 → 7 F J (J⫽0,1,2,3,4) 兴 emission
lines become weak, while a very broad emission band appears and spreads all over the visible light region, as seen in
the spectrum shown in Fig. 1共b兲 as observed at 90 min later
after the evacuation. At this stage, the bright white PL can beseen with the naked eye. Oxygen gas is again introduced in
the chamber. The change from white PL emission to redemission can be seen with the naked eye. The spectrum asshown in Fig. 1共c兲 is obtained 150 min later after introducing
the O2 gas. It is found through many successive experimentsthat the spectral changes as shown in Figs. 1共b兲 and 1共c兲 arereversible. Similar experiments were carried out by changing
the wavelength of the irradiating laser light. For example, a
cw He–Cd laser (␭⫽442 nm), a cw Ar⫹ laser 共␭⫽528.7,
514.5, 501.7, 496.5, 488.0, 476.5, 472.2, 465.8, and 457.9
nm兲, and a pulsed Nd3⫹:YAG laser 共␭⫽266, 354.7, 532, and1064 nm兲 were used. No spectral change was observed withthese lasers, which indicates that the cw UV light of the
wavelength 325 nm is required to induce the spectral change.
The speed of the change accelerates by increasing the power
density of the activating light. The change to the white PL
emission can also be observed at low temperatures down to 7

0003-6951/2001/79(23)/3785/3/$18.00
3785© 2001 American Institute of Physics
Downloaded 31 Jan 2005 to 133.43.174.230. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

3786

Appl. Phys. Lett., Vol. 79, No. 23, 3 December 2001

FIG. 1. Reversible photoinduced PL spectral change in Eu2O3 powder compact at room temperature. The powder compact was sintered at 1273 K inair. The atmosphere and irradiation time t ir at 325 nm are indicated in theplots. Experiments were carried out in alphabetical order: 共a兲→共b兲→共c兲.

K, which indicates that the phenomenon does not arise froma phonon assisted process and that it is a purely electronicmechanism. In spite of the changing atmosphere 共for example, O2 gas and air exposures兲, the white PL state lasts formore than several months at room temperature after the removal of the laser light under room light, and the red PL
state reappears only by irradiating with the same UV laser
light under O2 gas atmosphere.
Figures 2共a兲, 2共b兲, and 2共c兲 show the reversible spectralchange of the Eu2O3 ﬁlm under 325 nm irradiation at room
temperature. Measurements are carried out in alphabeticalorder: 共a兲→共b兲→共c兲. The results are quite similar to those of
the powder compact specimen obtained both at room temperature and 7 K. The wavelength, however, at the intensity
peak for the ﬁlm specimen is at about 550 nm which isblueshifted by about 60 nm from that observed for the powder compact specimen. The observed spectral structure forthe 5 D 0 → 7 F 2 emission, incidentally, differs from that for the
powder compact specimen 共C phase Eu2O3兲 in which each
Eu3⫹ ion is surrounded by six oxygen ions. Seven coordinate
Eu3⫹ ions account for the spectral difference.8
The excitation 共PL yield兲 spectra of the Eu2O3 powder
compact and ﬁlm were measured at room temperature for the5
D 0 → 7 F 2 emission and the white emission band. The results
show that the 325 nm laser photons are slightly absorbed by
the 7 F 0 → 5 L J transitions.9 The excitation spectrum for thewhite band does not show any yield peak due to intra-4 f

Mochizuki et al.

FIG. 2. Reversible photoinduced spectral change in Eu2O3 ﬁlm at room
temperature. The ﬁlm was produced by the rf-sputtering method. The atmosphere and irradiation time t ir at 325 nm are indicated in the plots. Experiments were carried out in alphabetical order: 共a兲→共b兲→共c兲.

transitions from the excited 5 D 0 level of Eu3⫹ ions. The
yield at the CT band is depressed considerably which is due
to decreased number of Eu3⫹ ions. Excitation spectra quite
similar to the powder compact are also observed in the ﬁlm
specimen.
The present experimental results of the Eu2O3 powder
compact and ﬁlm specimens may be summarized as follows:
共1兲 Photoinduced reversible spectral change between red
and white PL can be observed at different temperatures from
7 K to room temperature. The change is induced only by a
325 nm cw laser line. The 325 nm corresponds to transition
onset wavelength for the charge transfer excitation and for
excitation of the 7 F 0 → 5 L J transitions. After removing the
UV laser light irradiation, each PL state continues for morethan several months at room temperature under room light,
in spite of any change of atmospheres 共1.01⫻105 Pa O2 gas
and air兲.
共2兲 The change to white PL occurs only as a result of the
325 nm cw laser light irradiation under vacuum, while wehave not observed under any other reducing atmosphere; forexample, Ar–H2 mixture gas.
共3兲 The change to the white PL is accompanied by reduction of the 5 D 0 → 7 F J (J⫽0,1,2,3,4) emissions of Eu3⫹
ions.
The result 共1兲 indicates that the observed photoinduced
spectral changes are purely electronic phenomena and are notphonon assisted. The result 共2兲 indicates that the phenom-

Downloaded 31 Jan 2005 to 133.43.174.230. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Mochizuki et al.

Appl. Phys. Lett., Vol. 79, No. 23, 3 December 2001

enon is a photoinduced associative detachment of O2 near the
specimen surface.
The results above summarize how spectral change arises
naturally from photoactivated oxidation and reduction. The
reduction is accompanied both by a valence-number changeof europium ions (Eu3⫹→Eu2⫹) and by oxygen defect formation. Broadband emission is connected with Eu2⫹ ionsand local structural changes arising from oxygen defects. It
should be noted that EuO shows an absorption edge at about
1 eV and a photoluminescence band centered at 1.19 eV.8,10
These energies are far away from those of the observed white
luminescence. Therefore, the photogenerated Eu2⫹ ions andoxygen defects do not form any structure based on EuO crystal. Then, we examine other types of Eu2⫹ luminescent center as follows. First, we assume that the Eu2⫹ ions in Eu2O3
constitute different types of luminescent cluster with surrounding ions. The broad white PL band indicates that theenergies of emitting centers are slightly different from eachother, which may arise from the diversity of the cluster size,
shape, and structure. Such diversity may also give the difference 共about 60 nm兲 of the white PL intensity peak wavelength between the powder 共cubic crystal structure兲 and theﬁlm 共hexagonal one兲 specimens. Second, we assume that theEu2⫹ ions are well isolated from each other in Eu2O3. They
can be luminescent centers and also give rise to the broad PL
band. The 4 f 6 5d→4 f 7 transition gives a PL band at wavelengths from 400 to 520 nm, which has been observed in
Eu2⫹-doped crystals.11–13 The oxygen vacancies are also responsible for the broad PL band of metal oxides.14 Further
studies, for example, the measurements on the electronic andcrystal structures of Eu2O3 surface, are required in order toascertain the validity of the cluster model and in order toclarify which mechanism is responsible for the observed
broad white PL. Now, we will return to discussion of theresult 共2兲. Since the UV light 共325 nm兲 cannot dissociate
directly free O2 as atmospheric gas, the photoinduced oxidation indicates that the photodissociation energy of O2 is considerably decreased at the specimen surface by the interactions between O2 and Eu2O3, as known since 1932 as

3787

dissociative adsorption of molecules on a metal surface.15
The result 共2兲 indicates that the UV light with shorter wavelength is needed for photoreduction in H2 gas atmosphere.
Incidentally, the photodissociation energy of free H2 is about
two times that of free O2. 16
To clarify the mechanism of the observed spectralchange in detail, simultaneous measurements of the Raman
scattering and PL spectra of Eu2O3 during UV light irradiation, together with the studies on surface electronic states,
are now in progress.
This work is supported by Interdisciplinary General
Joint Research Grant for Nihon University, and by Project
Research Grant from The Institute of Information Sciences,
College of Humanities and Sciences, Nihon University. This
work is also partially supported by a Grant-in-Aid for Scientiﬁc Research from the Ministry of Education, Science,Sports, Culture and Technology, Japan.
E. Bezig and J. K. Trautman, Science 257, 189 共1992兲.
B. Welber, J. Appl. Phys. 36, 2744 共1965兲.
3
W. Beck, A. Ya. Karasik, J. Arvanitidis, and D. Richard, Eur. Phys. J. D
10, 131 共2000兲.
4K. Miura, J. Qui, T. Mitsuyu, and K. Hirao, Proc. SPIE 3618, 141 共1999兲.
5
V. Barbin, J. P. Jouart, and Th. D’Almeida, Chem. Geol. 130, 77 共1996兲.
6
K. Fujita, K. Tanaka, K. Yamashita, and K. Hirao, J. Lumin. 87–89, 682
共2000兲.
7
K. A. Gschneid, Jr. and L. R. Eyring, Handbook on the Physics andChemistry on Rare Earths 共North-Holland, Amsterdam, 1979兲, Vol. 3.
8
S. Mochizuki, Y. Suzuki, R. Uno, M. Faucher, O. K. Moune, J. DexpertGhys, and Y. Zihn, Meeting Abstracts Phys. Soc. Jpn. 55, 627 共2000兲.
9G. Wakeﬁeld, H. A. Keron, P. J. Dobson, and J. L. Hutchison, J. Colloid
Interface Sci. 215, 179 共1999兲.
10
G. Guntherodt and P. Wachter, AIP Conf. Proc. 10, 1284 共1973兲.
11
M. Akiyama, C. Xu, K. Nonaka, and T. Watanabe, Appl. Phys. Lett. 73,
3046 共1998兲.
12
T. Takasaki, S. Tanabe, and T. Harada, J. Ceram. Soc. Jpn. 104, 322
共1996兲.
13
T. Matsuzawa, Y. Aoki, N. Takeuchi, and Y. Murayama, J. Electrochem.
Soc. 143, 2670 共1996兲.
14
Y. Kawabe, A. Yamanaka, E. Hanamura, T. Kimura, Y. Takiguchi, H. Kan,and Y. Tokura, J. Appl. Phys. 88, 1175 共2000兲.
15J. E. Lennard-Jones, Trans. Faraday Soc. 28, 333 共1932兲.
16
G. Herzberg, I. Spectra of Diatomic Molecules, p. 351, p. 446, 1989.
1
2

Downloaded 31 Jan 2005 to 133.43.174.230. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Physica B 308–310 (2001) 1042–1045

Stacking faults and excitons in AgI
S. Mochizuki*
Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan

Abstract
AgI powder is gradually evaporated in vacuum onto a substrate whose temperature is higher than the superionictransition point TC : During the evaporation, the optical absorption spectrum of the ﬁlm is measured as a function oftime-elapsed after the beginning of evaporation. By increasing the time (i.e., with increasing ﬁlm thickness), the shifts of
the absorption edge and exciton energy are observed, which are due to exciton or carrier conﬁnement eﬀects in the
averaged structure of a-AgI. Information as to the electronic structure of the averaged structure is deduced fromanalyzing the observed spectra. After stopping evaporation, the ﬁlm is gradually cooled down to 131 K. Through thecooling process, the absorption spectrum is also measured as a function of temperature. New exciton absorption bands(H1, H2 and H3) appear, together with the Z1, 2 and Z3 excitons in the g-AgI. The H1, H2 and H3 excitons are assigned toa metastable disordered polytype structure of b-AgI, in the light of the X-ray diﬀraction data. r 2001 Elsevier Science
B.V. All rights reserved.
Keywords: Polytypism; Averaged structures; Excitons; Superionic conductor

1. Introduction
Silver iodide AgI is well known as a solid electrolyte
and has three phases designated as a; b and g at normalpressure in the order of decreasing temperature with thefollowing properties [1]. At superionic transition point
TC (419 K), the superionic conductor a-phase transforms into semiconductor b-phase (wurtzite lattice). Thea-phase has a body-centered cubic arrangement of I
ion with highly mobile Ag+ ions randomly distributed
through the equivalent interstices, which has been
known as the averaged structure. Extensive attention
has been paid to the mechanisms of the superionicmotion and to the electron–phonon interactions in the
averaged structure for many years [2,3]. However, an insitu optical study on the electron states of a-AgI islimited in number and many basic problems remain tobe solved. Moreover, the interest in AgI-based superionic conducting glasses and AgI : metal oxide composites has signiﬁcantly grown and extreme conductivity
enhancement has been observed at room temperature[4]. For this reason, also, it is necessary to clarify the*Tel.:+81-3-5317-9733 ; fax: +81-3-5317-9771.
E-mail address: motizuki@physics.chs.nihon-u.ac.jp(S. Mochizuki).

electronic and ionic structures of a-AgI. At 408 K, the bphase transforms mostly into the second semiconductorg-phase (zinc blende lattice) with a small amount of theb-phase [5]. However, at temperatures lower than TC ;the eﬀects of stacking faults and metastable structuresneed to be pointed out [6]. It is said that, if annealed
and aged, the structures may convert into the wurtzite
lattice. Therefore, in order to clarify the details of such atransition between the wurtzite and zinc blende structures, any measurement must be carried out in situ. The
exciton absorption spectrum of AgI is very sensitive tosuch a crystal modiﬁcation and local structural change[7,8]. While high-temperature a-AgI is cooled down to
low temperature, it is useful to measure spectral
transition in the exciton absorption.

2. Experimental
Vacuum evaporation experiments are carried out
under a base pressure of 104 Pa. Optically ﬂat-platesof silica glass, sapphire (R-cut) and MgO (1 0 0) are usedas substrates. The surface temperature of the substrate ismonitored with an alumel–chromel thermocouple. A
platinum crucible containing nominally pure AgI

0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 9 1 9 - X

S. Mochizuki / Physica B 308–310 (2001) 1042–1045

powder was gradually heated indirectly in an alumina
crucible on which a tungsten-wire heater was wound.
The thickness of AgI ﬁlm on the substrate increases withtime. During ﬁlm deposition, the substrate temperature
was maintained above TC : The optical density spectrumof the ﬁlm was measured as a function of the time td
elapsed after the beginning of the evaporation with an
optical multichannel analyzer system. The experimental
procedure is almost the same as the experiments for
microcrystals, except for introducing noble gas stream,
which has been already reported [9]. After stopping theevaporation, the ﬁlm is gradually cooled down to 131 K.
Through the cooling process, the absorption spectrumof the ﬁlm was also measured as a function oftemperature.

3. Results and discussion
3.1. Optical absorption of a-AgI ﬁlm
Fig. 1 shows the time evolution of the optical density
spectrum of a-AgI. The substrate temperature is 453 K.
Although one time-resolved spectrum consists of successive sixty-four spectra in this ﬁgure, only eight spectra
are selected and presented in order to show clearly thetime evolution. The time td is indicated in the plots. The
ﬁnal thickness of the ﬁlm was about 300 nm. At theinitial stage of the evaporation, a kink and anabsorption edge appear at about 2.8 eV and at about
2.6 eV, respectively. With the progressing evaporation,
the absorptions increase and the kink and the edge shift
to lower energies, together with the growth of anabsorption tail below 2.5 eV. In order to observe theFig. 1. Time evolution of the optical density spectrum of a-AgI
ﬁlm on a silica glass at 453 K during vacuum evaporation.

1043

detail, as shown in Fig. 2, we compared the spectrum
measured at td ¼ 48:88 s (curve A) with that measured at
td ¼ 123:63 s (curve B), by scaling the curve A, so thatthe optical density at 3 eV coincides with that of curve B.
The curve C obtained by such scaling coincides with thecurve B not only at the energy region between 3 and
4 eV, but also at the lowest photon energy (1.549 eV).
The result shows that, with progressing evaporation,
namely, the increasing ﬁlm thickness, the steepness ofthe optical density curve in the neighborhood of theabsorption edge decreases and the absorption-edge shifts
from 2.57 eV to a lower energy 2.52 eV. The red shift
nature observed arises from a decrease in the quantum
conﬁnement of carriers or excitons in a-AgI ﬁlm. We
investigated the absorption tail below 2.5 eV by replacing the silica glass substrate with a MgO (1 0 0) plate
and a R-cut sapphire plate, and changing the evaporation speed. The results can be summarized as follows:(1) With the increasing evaporation speed, the fabricated ﬁlms become somewhat cloudy.
(2) Films on the MgO- and sapphire single-crystal
substrates do not show a prominent tail below
2.5 eV.
Therefore, the absorption tail observed below 2.5 eV isclosely connected with surface roughness, and withcrystalline defects at the ﬁlm/substrate interface.

3.2. Optical absorption of AgI ﬁlm below TC
After stopping the evaporation, the specimen was
gradually cooled to 131 K with an average cooling rate

Fig. 2. Comparison of the optical density spectra of AgI ﬁlm
between the early and middle stages of the vacuum evaporation.

1044

S. Mochizuki / Physica B 308–310 (2001) 1042–1045

of 0.03 K/s. During the cooling process, the opticaldensity spectrum was measured as a function oftemperature, at temperature intervals of 5 K. Only thefour spectra are presented in order to show clearly thetemperature dependence. The result is shown in Fig. 3.
During cooling from 453 to 398 K, the optical density
spectrum is almost independent of temperature. Urbach
rule does not hold in a-AgI. This indicates that thecontribution from the electron–phonon interaction
characteristic of the a-phase counterbalances that fromthe temperature-shift of the band gap energy due tothermal contraction of a-AgI. No remarkable spectralchange was observed near TC : This is due to thehysteresis of the superionic conduction transition [8]. At
393 K, several weak exciton absorption peaks and ashoulder appear faintly at 2.934, 2.975 (shoulder), 3.058,
3.110, 3.766 and 3.977 eV. On further cooling, theseabsorption peaks become prominent. In order to show
the detailed spectral structure, the spectrum measured at
131 K is shown, together with that measured at 453 K, inFig. 4. The X-ray diﬀraction (XRD) pattern of the AgI
ﬁlm was measured at room temperature. The XRD
result shows a considerable amount of g-AgI and thebroadening of the (h 0 l) diﬀraction lines of b-AgI. The
broadening arises from the stacking disorder in b-AgI
hexagonal stacking sequence (abababy), as pointed out
by Lee et al. [10]. Taking into account the X-ray
diﬀraction data, as indicated in the ﬁgure, the peaks, theabsorption peaks and the shoulder observed at 2.934,
2.975 (shoulder) and 3.766 eV are assigned to the Z1 -,Z2 - and Z3 - exciton absorptions of g-AgI [7,8], while thepeaks observed at 3.058, 3.110 and 3.977 eV are assigned
to the H1 -, H2 - and H3 -exciton absorptions of someFig. 3. Optical density spectra of AgI ﬁlm on a silica glass at
diﬀerent temperatures during cooling process.

Fig. 4. Optical density spectra of AgI ﬁlm at 131 and 453 K.

stacking-disorder-induced polytype structure of AgI.
Quite a similar exciton absorption due to the polytype
structure was found in CuI ﬁlm evaporated onto aroom-temperature substrate [7].
3.3. Miscellaneous
In Fig. 5, the absorption intensity peak energy andphotoluminescence intensity peak energy of AgI ﬁlm at
9 K are plotted as a function of the substrate temperature during vacuum evaporation. The observed absorption and photoluminescence (PL) peaks are due to theZ1;2 exciton. The PL intensity peak appears several tensof meV lower than the absorption one, which indicatesthe existence of shallow exciton traps due to electron–
phonon interactions or defects. With the increasing
substrate temperature above about 450 K, the redshift
becomes prominent. Silica glass substrate, which has anextremely small thermal expansion coeﬃcient, gives alarger shift than that of sapphire. The results are
qualitatively explained by the asymmetric stress at theinterface coming from thermal shrinking with diﬀerent
thermal expansion coeﬃcients between AgI and substrate material. The stress aﬀects the electronic structureof AgI and induces an additional shift of the Z1;2 exciton
energy. This thermal stress may be one of the candidates
for the cause of polytype structure formation. Our
experiments show that the H1 ; H2 and H3 absorptions
are not observed in thick AgI ﬁlms which were producedby the vacuum evaporation with a small evaporation
rate. Thinner sample tends to be inﬂuenced by thethermal stress and, therefore, the H1 ; H2 and H3
absorptions become more prominent. In addition tothis stress eﬀect, the eﬀect of the deviation from

S. Mochizuki / Physica B 308–310 (2001) 1042–1045

1045

by the stress at the nanocrystal/matrix interface similar
to CuI nanocrystals [13].

Acknowledgements
This work is supported by Interdisciplinary General
Joint Research Grant for Nihon University and by
Project Research Grant from The Institute of Information Sciences, College of Humanities and Sciences,
Nihon University. This work is also partially supported
by a Grant-in-Aid for Scientiﬁc Research from the
Ministry of Education, Science, Sports, Culture andTechnology, Japan.

References
Fig. 5. Substrate temperature dependence of the absorption
intensity peak and PL intensity peak energies at 9 K for thesilica glass and sapphire substrates.

stoichiometry, Ag1I1x, should also be considered at ahigh temperture for further discussion.
Eﬀect of the stacking disorder observed in ﬁlm
specimens appears frequently even in the absorption
spectra of nanocrystal specimens. Nanocrystals both ona substrate [11] and in a matrix [12] show faintabsorption bands between 3.0 and 3.2 eV, which are in
the H1 - and H2 exciton energy region. This indicatesthat AgI nanocrystals contain a stacking disorderinduced by some reconstruction of surface atoms and[1] G. Burley, Amer. Min. 48 (1963) 1266.
[2] G. Eckold, K. Funke, J. Klaus, R.E. Lechner, J. Phys.
Chem. Solids 37 (1976) 1097.
[3] A. Rakitin, M. Kobayashi, Phys. Rev. B 53 (1996) 3088.
[4] St. Adams, J. Maier, Sol. Stat. Ionics 105 (1998) 67.
[5] R. Bloch, H. Moller, Z. Phys. Chem. A125 (1931) 245.
[6] P.R. Prager, Acta. Cryst. A30 (1974) 369.
[7] M. Cardona, Phys. Rev. 129 (1963) 69.
[8] S. Mochizuki, Y. Ohta, J. Lumin. 87–89 (2000) 299.
[9] S. Mochizuki, J. Lumin. 70 (1996) 60.
[10] J.–S. Lee, S. Adams, J. Maier, J. Phys. Chem. Solids 61(2000) 1607.
[11] S. Mochizuki, K. Umezawa, Phys. Lett. A 228 (1997) 111.
[12] G. Mshvelidse, et al., Phys. Stat. Sol. (b) 207 (1998) 369.
[13] A. Tanji, I. Akai, K. Kojima, T. Karasawa, T. Komatsu,
J. Lumin. 87–89 (2000) 516.

Physica B 308–310 (2001) 1046–1049

Valence-change- and defect-induced white luminescenceof Eu2O3
S. Mochizukia,*, Y. Suzukia, T. Nakanishia, K. IshibaDepartment of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui, Setagaya-ku,
Tokyo 156-8550, Japanb
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan

Abstract
Under irradiating with a CW UV laser light (l ¼ 325 nm) at room temperature in vacuum and oxygen gas
atmosphere, the ﬁlm, microcrystals and powder compacts of Eu2O3 exhibit a reversible photoluminescence (PL)spectral change between a red sharp-line structure and a white broad one. After stopping the UV irradiation, the abilityof the white PL lasts for more than several months at room temperature under room light, in spite of changes ofatmosphere. The reversible phenomena to be observed are interpreted as the results of both the valence-number changeof europium ions (Eu3+-Eu2+) and the oxygen vacancy formation. r 2001 Elsevier Science B.V. All rights reserved.
Keywords: Defect structures; Valence change; White-light emission

1. Introduction
Eu2O3 is stable in air and has ﬁve phases designatedby X, H, A, B and C at atmospheric pressure in order ofdecreasing temperature as follows [1]. At about 2553 K,
the X phase transforms into the H phase whichtransforms to the A phase at 2413 K. At 2313 K, the A
phase (hexagonal structure: space group D3d) transforms into the B phase (monoclinic structure: spacegroup C32h). In the A phase, the europium atoms areseven-coordinate with four oxygen atoms closer than theother three, while, in the B phase, the europium atoms
are six- and seven-coordinate. At about 1373 K, the Bphase transforms into the C phase (cubic structure:space group T7h) in which the europium atoms are sixcoordinate. At room temperature, the present authors
have recently observed the reversible UV-laser-lightinduced spectral transitions between the red- and whiteluminescence states in Eu2O3-powder compacts andFﬁlms produced by the radio-frequency sputtering*Corresponding author. Tel.:+81-3-5317-9733; fax: +81-35317-9771.
E-mail address: motizuki@physics.chs.nihon-u.ac.jp(S. Mochizuki).

method [2,3]. The interest in photoluminescent materials
showing such photo-induced reversible spectral change
at room temperature has signiﬁcantly grown, because acombination of the reversible phenomenon and the nearﬁeld optical microscope [4] is very promising whenapplied to high-density optical storage with nanometer
resolution. It is also the hope that we can ﬁnd white PLmaterial, which is useful for the wide-band tunable-laser
medium. Very recently, we have also observed thesimilar photoinduced spectral transition in Eu2O3 ﬁlms
produced by the pulsed laser ablation method in vacuum
and Eu2O3 microcrystal ﬁlms produced by the laser
ablation in low-pressure oxygen gas atmosphere.
In this paper, an overview of our results obtained in
the past three years will be given, together with aphenomenal model of the photoinduced spectral transition.

2. Experimental
All specimens were made from Eu2O3 powder of
99.98% purity. Powder compact specimens were prepared by pressing the Eu2O3 powder under a pressure of
0.2 GPa for 1 h at room temperature. They were then

0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 9 2 1 - 8

S. Mochizuki et al. / Physica B 308–310 (2001) 1046–1049

sintered at 1273 K in air for 24 h. First types of Eu2O3
ﬁlms were grown on a silica glass substrate by the radiofrequency sputtering (RFS) method. They are named
RFS ﬁlms. Second-type of Eu2O3 ﬁlms were grown on asilica glass substrate by the pulsed laser ablation (PLA)
method in vacuum of 1.33  104 Pa. They are named
PLA-ﬁlms. Eu2O3 microcrystal-accumulated ﬁlms weregrown on a silica glass substrate by the PLA method in
oxygen gas of the pressure lower than 100 Pa. They are
named PLA-MC ﬁlm. They are characterized by the Xray diﬀraction (XRD) method and the scanning electron
microscopy (SEM). Irradiations were carried out with a
CW He–Cd laser line (l ¼ 325 nm) with power densities
between 31438 and 840000 W/m2. The same He–Cd laserline excites luminescence. Emitted light is dispersed anddetected using a grating spectrograph equipped with amultichannel photodetection system.

3. Results and discussion
The XRD analysis of the produced specimens
indicates that the powder compact, RFS ﬁlm, PLA ﬁlm,
and PLA-MC ﬁlm are respectively, the C phase, the Hphase [1], a mixture of predominantly C phase with alittle B phase, and a mixture of predominantly B phasewith a little C phase of Eu2O3. The specimens were also
examined by the SEM. The powder compacts consist of
randomly oriented and connected rectangular-parallelepiped grains, as shown in Fig. 1(a). The RFS-ﬁlms andthe PLA ones have smooth surfaces and are opticallytransparent. The PLA-MC ﬁlms consist of a lot of small
particles whose sizes is approximately several hundred
nanometers and little aggregates whose sizes can beseveral thousand nanometers, as shown in Fig. 1(b).
Figs. 2–5 show the photoinduced spectral changes atroom temperature of the Eu2O3-powder compact,

1047

Eu2O3-RFS ﬁlm, Eu2O3-PLA ﬁlm and Eu2O3-PLAMC ﬁlm, respectively. In the measurements of thepowder compact, the RFS ﬁlms, the PLA ﬁlms and the
PLA-MC ﬁlms, their relative sensitivities of the detections are 1, 8, 170 and 34, respectively. Experiments werecarried out in alphabetical order: (a)-(b)-(c). Irradiation time tir under a given atmosphere and the kind of

Fig. 2. Reversible spectral change of Eu2O3 powder compact.

Fig. 3. Reversible spectral change of Eu2O3 ﬁlm produce by the
RF-sputtering method.

Fig. 1. SEM photographs of the Eu2O3-powder compact (a) and Eu2O3-PLA-microcrystal ﬁlm (b).

1048

S. Mochizuki et al. / Physica B 308–310 (2001) 1046–1049

atmosphere are indicated in each plot, together with therelative sensitivity of the detection, SD. First, these asproduced Eu2O3 specimens are irradiated with a 325 nm
laser line at room temperature under oxygen atmosphereof a pressure 1.01  105 Pa. Under 325 nm excitation,these specimens show the red sharp 5D0-7FJ(J ¼ 0; 1; 2; 3; 4) emissions [5,6]. Depending on the
crystal structure of the specimens, the detailed spectral
structure of the 5D0-7F2 emission diﬀers each other[7,8]. Next, the specimen chamber is evacuated to1.33  104 Pa. Under 325 nm irradiation in this vacuum, quite similar photoinduced spectral changes wereobserved in all the specimens. Namely, with increasing
tir ; the (5D0-7FJ (J ¼ 0; 1; 2; 3; 4)) emission intensity of
Eu3+ ions becomes weak, while a very broad emission
band appears and spreads all over the visible lightregion. The wavelength of the intensity peak of the white
PL band observed in all the specimens is between 540
and 620 nm. Especially, the relative intensity decreasesof the 5D0-7F2 emission of the powder compact, RFS
ﬁlm, PLA ﬁlm and PLA-MC ﬁlm are 75.3%, 15.6%,
34.5% and 15.4%, respectively. The intensity decreaseindicates the decrease of number of Eu3+ ion, which isFig. 4. Reversible spectral change of Eu2O3 ﬁlm produced by
the laser ablation method in vacuum.

Fig. 5. Reversible spectral change of Eu2O3 microcrystal ﬁlm
by the laser ablation method in low-pressure oxygen gas
atmosphere.

due to photoreduction (Eu3+-Eu2+). At this stage,
the bright white PL can be seen with the naked eye. The
whitening speed of the PLA ﬁlm specimen was thehighest among all specimens. In addition, the wavelength of their intensity maximum of the white PL bandis specimen-dependent. The spectral structures to beobserved in the 5D0-7F2 emission in the RFS ﬁlm, PLA
ﬁlm and PLA-MC ﬁlm, incidentally, diﬀer from that forthe powder compact specimen (C-phase Eu2O3) in which
each Eu3+ ion is surrounded by six oxygen ions. Sevencoordinate Eu3+ ions account for the spectral diﬀerence[2,3,7,8].
Oxygen gas is again introduced into the chamber. Thechange from white PL emission to red emission can beseen with the naked eye. It is found through manysuccessive experiments that the spectral changes asshown in these ﬁgures are reversible. Similar experiments were carried out by changing the wavelength of
the irradiating laser light. For example, a CW He–Cdlaser (l ¼ 442 nm), a CW Ar+ laser (l ¼ 528:7; 514.5,
501.7, 496.5, 488.0, 476.5, 472.2, 465.8, and 457.9 nm),and a pulsed Nd3+ : YAG laser (l ¼ 266; 354.7, 532,
and 1064 nm) were used. No spectral change wasobserved with these lasers, which indicates that theCW UV laser light of the wavelength 325 nm is required
to induce the spectral change. The speed of the spectralchange is accelerated by increasing the power density of
the activating light. The change to the white PL emission
can also be observed at low temperatures down to 7 K,
which indicates that the phenomenon does not arise
from a phonon-assisted process and that it is a purely
electronic mechanism. In spite of the changing atmosphere (for example, O2 gas and air exposures), the white
PL state lasts for more than several months at room
temperature after the removal of the laser light underroom light, and the red PL state re-appears only byirradiating with the same UV laser light under O2 gas
atmosphere of a pressure 1.01  105 Pa.
Finally, we turn to the observed photoinduced
spectral change between the red- and white PL. Noting
the result that the 325 nm photons are essential to induce
the spectral change. The 325 nm irradiation in vacuum
produces both valence change (Eu3+-Eu2+) andoxygen vacancies. They then induce local structural
relaxation (distortion) around photoexcited ions to forma metastable state. A simpliﬁed scheme is drawn in
Fig. 6, omitting the detailed structure of the 4f electronic
states of Eu3+ ions. The 7F0, MS and X represent the
ground state of Eu3+ ions, the metastable state, and oneof some 4f excited energy level of Eu3+ ions or chargetransfer state, respectively. The coordinate Q expresses
some local structural change due to both the valence
number change of Eu3+ ions and the oxygen vacancy
formation. The potential barrier of the height DE
separates the metastable state from the ground state.
In oxidizing atmosphere, the 325 nm radiation produces

S. Mochizuki et al. / Physica B 308–310 (2001) 1046–1049

1049

edge of the charge-transfer (CT) absorption band
(Eu3++O2-Eu2++O1) and the transition energy
to higher excited states of Eu3+ ions; for example,
7
F0-5LJ transitions. Therefore, the CT and 5LJ states
are candidates for the origin of the X state. However, theobserved spectral change is thought to be a surface
phenomenon. The detailed surface electronic structure
should be clariﬁed for further discussion.

Acknowledgements
Fig. 6. Schematic energy diagram of the photoinduced reversible spectral change of Eu2O3.

the localized excitation of Eu3+ ions and the deexcitation of the photoexcited Eu3+ ions provides the redluminescence due to the 5D0-7FJ transitions, via thenonradiative transitions (NR) to the 5L6, 5DJ(J ¼ 3; 2; 1; 0) states. When the photoexcitation iscarried out in vacuum, the excited state is relaxedthrough both a radiative recombination and the local
structural relaxation to the metastable state. The 325 nm
excitation of the metastable state induces both the
backward transition and the transition to the localized
electronic state (LES) originating from the Eu2+ andoxygen vacancy formations. Since the photoproduced
metastable state is extremely stable even at room
temperature in oxygen, air and vacuum atmospheres
after stopping 325 nm irradiation as described above, thepotential barrier separating the ground state (7F0) from
the MS must be high enough to suppress the thermalforward and -backward transitions between the 7F0 stateand MS. The LES is a luminescence center strongly
coupled with lattice vibrations and, therefore, gives a
broadband luminescence as observed. Our PL-excitation
spectra measurements on all the specimens show that thelaser wavelength 325 nm corresponds the lower-energy

This work is supported by Interdisciplinary General
Joint Research Grant for Nihon University, and by
Project Research Grant from The Institute of Information Sciences, College of Humanities and Sciences,
Nihon University. This work is also partially supported
by a Grant-in-Aid for Scientiﬁc Research from the
Ministry of Education, Science, Sports, Culture andTechnology, Japan.

References[1] K.A. Gschneid Jr., L.R. Eyring, in: Handbook on the
Physics and Chemistry on Rare Earths, Vol. 3, NorthHolland, Amsterdam, 1979.
[2] S. Mochizuki, T. Nakanishi, Y. Suzuki, K. Ishi, Book of
Abstracts, International Chemical Congress of Paciﬁc Basin
Societies, Part 2, Honolulu, Hawai, 2000.
[3] S. Mochizuki, T. Nakanishi, Y. Suzuki, K. Ishi, Meeting
Abstr. Phys. Soc. Jpn, Part 4, 55 (2) (2000) 627.
[4] E. Bezig, J.K. Trautman, Science 257 (1992) 189.
[5] G.H. Dieke, Spectra and Energy Levels of Rare Earth Ionsin Crystal, Interscience, New York, 1968.
[6] H.M. Crosswhite, H.W. Moos (Eds.), Optical Properties ofIons in Crystals, Interscience, New York, 1967, p. vi.
[7] J. Dexpert-Ghys, M. Faucher, P. Caro, Phys. Rev. B 23
(1981) 607.
[8] G. Wakeﬁeld, H.A. Keron, P.J. Dobson, J.L. Hutchison, J.
Colloid Interface Sci. 215 (1999) 179.

Y. Mita et al.: Optical Study of MnO under High Pressure247

phys. stat. sol. (b) 223, 247 (2001)
Subject classification: 78.30.Hv; S10.1

Optical Study of MnO under High Pressure
Y. Mita1) (a), Y. Sakai (a), D. Izaki (a), M. Kobayashi (a, c), S. Endo (b, c),and S. Mochizuki (d)(a) Graduate School of Engineering Science, Osaka University, Osaka 560-8531, Japan(b) Research Center for Materials Science at Extreme Conditions, Osaka University,
Osaka 560-8531, Japan(c) CREST, Japan Science and Technology Corporation, Kawaguchi, Saitama 332-0012,
Japan(d) College of Humanities and Sciences, Nihon University, Tokyo 156-8550, Japan(Received October 10, 2000)
Raman and optical absorption studies of the antiferromagnetic insulator MnO were performed atseveral temperatures under high pressure. In Raman scattering at 2 K, it was observed that thetwo-magnon peak shifts to higher energy with increasing pressure and then disappears at around
20 GPa. This suggests that a change in spin arrangement occurs at that pressure. At 77 K thepeak-vanishing pressure increases to 25 GPa. At room temperature, a jump of the Raman peak
position was observed at 30 GPa and the peak vanishes at 89 GPa. These results are consistent
with previous X-ray diffraction studies which showed phase transitions at 30 and 90 GPa. The
vanishing of the Raman peak suggests a metallization transition. The pressure dependence of the
Mn2+ d–d transition observed in optical absorptions was consistent with the crystal field theory up
to around 25 GPa at room temperature.

Introduction MnO is well known as a Mott insulator and has antiferromagnetic character at temperatures lower than TN = 118 K. It has type-II spin structure in which
ferromagnetically ordered (111) sheets are stacked antiferromagnetically in the [111]direction. This causes the rock-salt (B1) structure to undergo a small trigonal lattice
distortion below TN, contracting along a [111] direction. Recently new phase boundaries
have been observed at 30, 90 and 120 GPa by X-ray diffraction performed at room
temperature [1] and theoretical works about the phase transitions have been reported[2]. There have been few experimental studies of the magnetic properties at low temperature and high pressure [3–5] and the highest reported value of applied pressure is
only 3.3 GPa [4]. It is difficult to study aniferromagnetic materials by conventional
magnetization measurement under high pressure, both because the magnetization signal
is very weak on account of the anti-parallel spin configuration in antiferromagnetic
materials and because of the small sample volume needed in the pressure cell. On theother hand, only 10 ––8 cm3 of crystal is required for accurate Raman detection, whichenables us to use a diamond anvil cell (DAC). Furthermore, the Raman shift due to theantiferromagnetic magnon is larger than that of a ferromagnetic one [6]. Since the posi1) Corresponding author; Tel.: +81-6-6850-6417; Fax: +81-6-6845-4632;
e-mail: mita@mp.es.osaka-u.ac.jp
17*

248

Y. Mita et al.

tion and intensity of the magnon Raman peak are strongly affected by the spin configuration, drastic changes of the magnon Raman peak indicate changes in the spinarrangement. Thus magnon Raman scattering is one of the best ways to study magnetic
phase transitions of antiferromagnetic materials under high pressure [7].
The localized d-states of Mn2+ ions are split into de and dg states by the octahedral
crystal field created by surrounding O2–– ions. The validity of this crystal field model
can be investigated from the effect of pressure on the d–d transition energy [8, 9]. In
this paper we report studies of magnetic phase transitions and Mn2+ d–d transition of
MnO under very high pressure by Raman scattering and absorption measurements.
Experiment Raman and absorption measurements were performed. The Raman study
was carried out in back scattering geometry on single crystals of MnO in the pressure
range 0–90 GPa at 2, 77 and 300 K. Samples were pressurized using a DAC. Liquid
helium or a methanol–ethanol (4 : 1) solution was used as a pressure medium as occasion demands. For pressure calibration, the standard ruby fluorescence method wasused. The excitation source was the 514.5 nm line of an argon ion laser. Scattered light
was detected by a Jobin-Yvon triple monochromator T64000 and a Jobin-Yvon Spex
Spectrum-one CCD (charge coupled device) system. Visible and near-infrared (IR) absorption measurements were also performed up to 80 GPa on powdered samples atroom temperature. No pressure medium was used. Jobin-Yvon types H20 and H20IR
monochromators were used in visible and near-infrared regions, respectively.
Results and Discussion Figure 1 shows pressure dependence of the Raman spectrum
measured at 2 K. In the spectrum obtained at 1 GPa, two peaks are observed clearlyaround 500 and 550 cm ––1. These are a two-magnon (2M) and a two-TO phonon Raman peak, respectively [10, 11]. With increasing pressure, the 2M and 2TO peaks shift
toward higher energy. The 2M peak
becomes weak and disappears around19 GPa. A relation between appliedpressure and the peak position is
illustrated in Fig. 2, here open circles
and full squares represent 2M and
2TO Raman peaks, respectively. Up to
20 GPa, pressure coefficients of 2M
and 2TO are 1.1 and 2.4 cm ––1/GPa,

Fig. 1. Raman spectra of MnO under several pressures obtained at 2 K. A two-magnon (2M) and a 2TO phonon peak areseen. With increasing pressure, the former
becomes weak and vanishes around 19 GPa

Optical Study of MnO under High Pressure249
Fig. 2. Pressure dependence of Raman
peak positions at 2 K. Full squares andopen circles represent 2TO phonon andtwo-magnon (2M) Raman peaks, respectively. At around 20 GPa, the 2M
peak vanishes and a change in pressure
coefficient of the 2TO peak occurs
from 2.4 to 1.3 cm ––1/GPa

respectively. Above 20 GPa, the 2M peak disappears and the pressure coefficient of
the 2TO peak becomes 1.3 cm ––1/GPa. These results suggest that the antiferromagnetic
spin configuration exists up to 20 GPa and a change in spin arrangement (magnetic
phase transition) occurs around 20 GPa. However, we are unable to determine theprecise spin arrangement in the higher pressure region. At 77 K, the 2M Raman peak
disappears around 25 GPa. At room temperature, only the 2TO peak is observed, because the temperature is higher thanTN (Fig. 3). A small peak around670 cm ––1 is attributed to scattering
associated with electronic excitationof impurities [10]. The 2TO peakvanishes at 89 GPa and we observedthat the sample becomes highly reflective around the pressure. The
metallic nature around 90 GPa has
observed previously by Kondo et al.
[1]. The pressure dependence of the
2TO peak position is illustrated inFig. 4. A slight jump of the peak toward lower energy is seen aroundFig. 3. Raman spectra at room temperature. The peak disappears at 89 GPa.
Vanishing of the Raman peak suggests
metallization of the sample

250

Y. Mita et al.
Fig. 4. Relation between Raman peak position and applied pressure at room temperature. A jump of the Raman peak toward
lower energy is observed at 30 GPa, where
the phase transition occurs

30 GPa. These changes around 30 and 90 GPa may be due to the phase transition
observed by X-ray diffraction [1].
Figure 5 shows typical absorption spectra at room temperature under three differentpressures. The observed peak, marked by arrows, is ascribed to the 3d crystal field
splitting transition from the ground state 6A1 to the excited state 4T1 [12]. With increasing pressure, the peak shifts to lower energy which can be explained by the increase of
the crystal field formed by O2–– ions. According to the crystal field theory [8], thecrystal field splitting 10Dq is proportional to a ––5, where a is the lattice constant, andthe 6A1 ! 4T1 transition energy (E) normalized to the Racah parameter B is given by
E/B ¼ 32.5, which is a constant. Figure 6 shows the relation between E and (a/a0) ––5,
where a0 is the lattice parameter at ambient pressure, and the values of a were obtained from a pressure–volume relation obtained by X-ray analysis [1]. The highest
pressure of the data point corresponds to 78 GPa. The slanting line in the figure is
calculated by crystal field theory [8]. Up to (a/a0) ––5 ¼ 1.24 (26.1 GPa), the agreement
between theory and experimental result is good. However, the experimental data begin

Fig. 5. Absorption spectra at room temperature under three different pressures

Optical Study of MnO under High Pressure251
Fig. 6. Relation between (a/a0) ––5 and 3d transition(6A1 ! 4T1) energies of the Mn2+ ion

to deviate from the theoretical line with increasing pressure. This deviation suggests that
Mn2+ ions are no longer behaving as isolated
impurities and the effect of formation of anenergy band is no longer ignorable.
Conclusions Raman scattering and absorption measurements of MnO were performed
under high pressure. In Raman scattering at 2 K, it was observed that the two-magnon
peak shifts toward higher energy with increasing pressure and disappears around
20 GPa. This suggests that a change in spin arrangement occurs at that pressure. At
room temperature, a phase transition around 30 GPa is observed, as has previously
been reported by X-ray diffraction experiments. Furthermore, vanishing of the 2TO Raman peak is observed around 90 GPa. It seems to correspond to the metallization transition reported previously. From absorption measurements, it appears that the crystal
field theory which treats the Mn2+ ions as isolated impurities is valid up to around
25 GPa.
Acknowledgements The authors thank Dr. H. Furuta of Research Center for Material
Science at Extreme Conditions of Osaka University for technical support, and Dr.
G. J. Ackland of Edinburgh University for help with writing the manuscript.

References[1] T. Kondo, T. Yagi, Y. Shono, T. Kikegawa, and O. Shimomura, Rev. High Press. Sci. Technol.
7, 148 (1998).
[2] Z. Fang, I. V. Solovyev, H. Sawada, and K. Terakura, Phys. Rev. B 59, 762 (1999).
[3] H. Barholin, D. Bloch, and R. Geoges, C.R. Acad. Sci. 264, 360 (1967).
[4] D. Bloch and C. Vettier, J. Magn. Magn. Mater. 15/18, 589 (1980).
[5] S. Tamura, High Temp. –– High Press. 19, 657 (1987).
[6] P. A. Fleury and R. Loudon, Phys. Rev. 166, 514 (1968).
[7] Y. Mita, M. Kobayashi, and S. Endo, Phys. Rev. B 62, 8891 (2000).
[8] Y. Tanabe and S. Sugano, J. Phys. Soc. Jpn. 9, 766 (1954).
[9] M. Kobayashi, T. Nakai, S. Mochizuki, and N. Takayama, J. Phys. Chem. Solids 56, 341(1995).
[10] H.-H. Chou and H. Y. Fan, Phys. Rev. B 13, 3924 (1976).
[11] H. Y. Fan, Indian J. Phys. A 55, 218 (1981).
[12] D. R. Huffman, R. L. Wild, and M. Shinmei, J. Chem. Phys. 50, 4092 (1969).

phys. stat. sol. (c) 0, No. 2, 763– 766 (2003) / DOI 10.1002/pssc.200306199

Shallow and deep excited states of mesoscopic structure
in AgI–γAl2O3 composites
Shosuke Mochizuki* and Fumito Fujishiro
Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui,
Setagaya-ku, Tokyo 156-8550, Japan
Received 24 July 2002, accepted 27 July 2002
Published online 30 January 2003
PACS 61.72.Nn, 68.35.Fx, 71.35.Cc, 78.55.Qr
The photoluminescence (PL) and PL excitation (PLE) spectra of superionic conducting AgI – γAl2O3 composites have been measured at different temperatures between 7 K and room temperature. The X-ray diffraction patterns of these composites have also been measured at room temperature. The PL intensity peakobserved at 426 nm is closely connected with radiative decay of free excitons in AgI, while other emission bands are connected with shallow excited states in AgI and deep excited states at the AgI/γAl2O3 interface. With increasing excitation light intensity, the PL efficiencies of several emission bands become
saturated, except for the free exciton band. These results may give important information about the originsof the high ionic motion in these composites.

1. Introduction Recently, there has been growing interest in fast ion conduction in composites andglasses because these may yield new solid electrolytes. They are also useful materials for studying cooperative phenomena between mobile ions and electrons in solids. Moreover, it is hoped that transparent
electrical conductors can be found, which are useful for optoelectronic devices. Shahi and Wagner firstreported the extremely high absolute values of electrical conductivity in AgI –γAl2O3 composites [1].
Hitherto, considerable work to clarify the mechanism of high ionic conductivity in the AgI–γAl2O3 system has been carried out by many workers [2, 3]. Many models to explain the high ionic conductivity
have been proposed by noting the high ionic conductivity at the AgI/γAl2O3 boundary. However, a basic
problem arises. What is the atomic structure of the AgI/γAl2O3 interface?
The AgI constituent material shows clear optical spectra due to excitons in the bulk [4], film [5–7]
and microcrystals [8]. The exciton behavior is sensitive to the environment in which the excitons are
moving, and, therefore, the optical spectra due to excitons may provide useful information about the
AgI/γAl2O3 interface. In this paper, we report in detail on the optical and structural studies that we haveconducted on AgI– γAl2O3 composites and propose an atomic structural model.
2. Experimental AgI – γAl2O3 composites were prepared by mixing AgI and wet γAl2O3 of particle
size 60 nm, heating the mixture at about 873 K for 12 h in air and annealing at 373 K for 12 h. The
atomic structural data of the composites were gathered using an X-ray diffraction (XRD) system with
Cu Kα1 radiation. Photoluminescence (PL) spectra were measured using an optical multichannel analyzer. A N2 laser (wavelength λ = 337.1 nm, pulse width <1 ns), a Nd3+:YAG laser (λ = 355 nm, pulse*

Corresponding author: e-mail: motizuki@physics.chs.nihon-u.ac.jp, Tel./Fax: +81 3 5317 9771

© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1610-1634/03/00002-0763 $ 17.50+.50/0

β(210)β(211)

Fig. 1 XRD patterns for different (AgI)x(γAl2O3)1−x.

(2010)(2011)β(112)β(110)(108)(1010)(1011)(103)

β(100)β(002)

Intensity (arb. scale)x = 0.73

(104)
(106)

S. Mochizuki and F. Fujishiro: Shallow and deep excited states in AgI – γAl2O3 composites764

x = 0.93
x = 0.87
x = 0.73
x = 0.57
x = 0.23
x = 0.20
γ-Al2O3

10°

20°

30°

40°

2θ

50°

60°

70°

width <5 ns) and a monochromatic light source consisting of a 150 W xenon lamp and a grating monochromator were used as the excitation source. PL excitation (PLE) spectra were recorded using an apparatus consisting of the same monochromatic light source described above, a grating monochromator anda synchronous light detection system.

PL intensity (arb. scale)

3. Results and discussion The XRD patterns of different (AgI)x(γAl2O3)1−x composites were measured
at room temperature and the results for x = 0, 0.20, 0.23, 0.57, 0.73, 0.87 and 0.93 are shown in Fig. 1.
With increasing x, some broadening of the peaks and weakening of the (h0l) peaks of βAgI become
pronounced at 2θ = 25.4°, 32.8°, 42.6°, 47.2°, 52.0° and 66.5°. However, new diffraction lines can beobserved near reduced (h0l) peaks, as reported by Lee et al. [2]. They can be referred to a new stacking

x = 0.57

x = 0.93400

500

x = 0.20

x = 0.87

γ-Al2O3x = 0.73600

700 400

500

Wavelength (nm)

600

700

Fig. 2 PL spectra for different(AgI)x(γAl2O3)1−x at 10 K.

phys. stat. sol. (c) 0, No. 2 (2003)

765

PL intensity (arb. scale)

PL intensity (arb. scale)

PL intensity (arb. scale)

PL intensity (arb. scale)sequence of the close-packed planes in the wurtzite structure of AgI. They also explain the XRD featuresof AgI– γAl2O3 composites by taking account of seven-hexagonal-layer polytype AgI (7H-AgI) with thestacking sequence ABCBCAC [2, 9, 10]. The new diffraction lines at 24.5°, 25.8°, 30.7°, 34.8°, 41.5°,
43.9°, 58.2° and 60.2° seem to correspond to the (103), (104), (106), (108), (1010), (1011), (2010) and(2011) peaks of the polytype structure, respectively. On further increasing x, the XRD pattern becomesclose to that of the Type II AgI specimen [9], which has the 7H structure.
The PL spectra obtained for (AgI)x(γAl2O3)1−x
with different compositions at 10 K are comparedin Fig. 2. The PL was excited by the 337.1 nm(d)laser line of the N2 laser. Pure γAl2O3 shows a
T=9K
broad PL band centered at about 448 nm. Addition of a small amount of AgI, x = 0.2, leads to a
I0
0.03I0decrease of the PL intensity at shorter waveXelengths than about 425 nm which corresponds tothe Z1,2 exciton energy of pristine AgI. The lightabsorption by environmental AgI may decrease
the intensity of light emitted from γAl2O3 parti400
420
440
460
480
500
520
cles.
A very broad PL band centered at 720 nm
Wavelength (nm)appears, which may arise from the AgI/γAl2O3(c)interface. This broad band may be assigned to
PLE
PLradiative decay of AgI excitons trapped by a deep
λob
λex
T=9K
potential at the AgI/γAl2O3 interface. On further
450nm
337nm
700nm
increasing x, the PL intensity of the very broadband decreases and several PL bands around 440
nm become prominent. Since the Z1,2 free-exciton
energy is 2.915 eV (= 425 nm), the bands around440 nm may be assigned to radiative decay of
AgI excitons trapped by a shallow potential in300
400
500
600
700
800
Wavelength (nm)side AgI domains in the AgI–γAl2O3 composite.
At the higher x region (x ≥ 0.87), the 425 nm(b)emission due to free excitons in AgI appears
clearly.
T(K)9Figure 3a shows the temperature dependence
148of the PL spectrum of (AgI)x(γAl2O3)1−x for x =0.87. With increasing temperature, free-exciton
emission prevails against the shallow- and deeplevel emissions. In Fig. 3b the PL spectra observed at 9 and 148 K are compared, which may
400
420
440
460
480
500
be due to thermal activations from shallow andWavelength (nm)deep excited states to the free-exciton state. Figure 3c shows the PL and PLE spectra at 9 K. This
(a)

400

λex=337.1nm
T(K)
9
37
58
98
148

420

440

460

Wavelength (nm)

480

500

Fig. 3 PL spectra for (AgI)0.87(γAl2O3)0.13: a, b) temperature dependence; c) PL and PLE spectra; d) excitation light intensity dependence.

S. Mochizuki and F. Fujishiro: Shallow and deep excited states in AgI – γAl2O3 composites

766

indicates that the shallow and deep levels are below 0.16 and 0.56 eV from the bottom of the freeexciton band, respectively. The PLE peak at 413 nm is assigned to free-exciton absorption in polytypestructured AgI [7].
The excitation light intensity (Iex) dependence of the PL spectrum was also studied. The results are
summarized as follows. The free-exciton emission increases with Iex, while the shallow and deep levelemissions have higher yields than that of the free exciton, but they become saturated under intense excitation. These saturations arise from the finite number of shallow and deep level PL centers. In Fig. 3d the
PL spectra under intense I0 and weak 0.03I0 (peak power I0 = 200 kW) excitations at 9 K are typicallycompared.
Finally, on the basis of the results described above, we propose a structural model for AgI– γAl2O3
composites as follows. γAl2O3 particles are enveloped with the hexagonal-layer polytype AgI, which has
high ionic conductivity. Both the interface and the polytype structure of AgI may play important roles infast ion conduction. Through this interface, the mobile Ag+ – Ag+ ion exchange responsible for fast ion
conduction may occur. Free excitons are trapped by a shallow potential inside AgI domains and a deep
potential at the AgI/γAl2O3 interface. Since the mean free path of free excitons is finite, the effects of theinterface on PL become obscured by the contribution from shallowly trapped excitons inside AgI domains with increasing AgI content (i.e. AgI domain size), as shown in Fig. 2.
Acknowledgements This work was supported in part by a Project Research Grant from the Institute of Information Sciences of the College of Humanities and Sciences (Nihon University), and by a Cooperative Research Grant
from the Institute of Natural Sciences (Nihon University). This work was also partially supported by a Grant-in-Aid
for Scientific Research from the Ministry of Education, Science, Sports, Culture and Technology, Japan.

References[1] K. Shahi and J. B. Wagner Jr., J. Electrochem. Soc. 128, 6 (1981).
[2] J.-S. Lee, S. Adams, and J. Maier, Solid State Ionics 136 – 137, 1261 (2000).
[3] M. Nagai and T. Nishino, Solid State Ionics 117, 317 (1999).
[4] S. Mochizuki and F. Fujishiro, in: Proc. 6th Forum Superionic Conductor Physics, Kyoto, Japan, 24 – 25 May
2002 (The Solid State Ionics Society of Japan, Kyoto, 2002), pp. 41 – 46.
[5] M. Cardona, Phys. Rev. 129, 69 (1963).
[6] S. Mochizuki and Y. Ohta, J. Lumin. 87 – 89, 299 (2000).
[7] S. Mochizuki, Physica B 308 – 310, 1042 (2001).
[8] S. Mochizuki and K. Umezawa, Phys. Lett. A 228, 111 (1997).
[9] J.-S. Lee, S. Adams and J. Maier, J. Phys. Chem. Solids 61, 1607 (2000).
[10] B. L. Davis and L. R. Johnson, Cryst. Latt. Defects 5, 235 (1974).

phys. stat. sol. (c) 0, No. 2, 767– 770 (2003) / DOI 10.1002/pssc.200306200

Shallow- and deep-luminescence centers
in AgI-based superionic conductor glass
Shosuke Mochizuki* and Fumito Fujishiro
Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui,
Setagaya-ku, Tokyo 156-8550, Japan
Received 24 July 2002, accepted 27 July 2002
Published online 30 January 2003
PACS 61.72.Nn, 68.35.Fx, 71.35.Cc, 78.55.Qr
The photoluminescence (PL), time-resolved photoluminescence (TRPL) and PL-excitation (PLE) spectraof superionic conductor glass (AgI)0.85(Ag2WO4)0.15 have been measured at different temperatures between
8 K and room temperature. The PL intensity peak is observed at 426 nm which corresponds to free exciton wavelength of pristine AgI. There are also different PL bands well related to the internal and interface
states of mesoscopic AgI particles in Ag2WO4 glass matrix. This may give important information about
the origins of superionic conduction. With increasing excitation light intensity, the PL efficiencies of several emission bands become saturated, except for the free exciton band.

1. Introduction In recent years, a growing interest has arisen in the study of highly ion-conducting
glasses because the glasses may yield new solid electrolytes and are useful specimens for studying theinteractions between mobile ions and electrons in solids. Progress in this research field on the high ionic
conductor has been mainly achieved by studying the electrical and thermal properties [1, 2]. However,
several fundamental problems arise. What are the atomic structures of glasses? What delocalizes someof the ions in glasses to exhibit fast ion conductions? In order to elucidate these problems,(AgI)x(Ag2WO4)1–x glasses are chosen for the present study because they have high ionic conductivity
due to Ag+ ions. The AgI which is a constituent material shows clear optical spectra due to excitons in
the bulk [3], film [4–6] and microcrystals [7]. The exciton behavior is sensitive to the environment inwhich excitons are moving, and therefore the optical spectra may provide useful information about glass
structure. In the present paper, we report on the experimental results of the optical studies on(AgI)0.85(Ag2WO4)0.15 glasses in detail, and we deduce the glass structure by comparing the optical datawith the XRD patterns.
2. Experiments (AgI)x(Ag2WO4)1–x glasses were prepared by mixing AgI, Ag2O, and WO3 in air, by
heating the mixture at about 873 K for 12 h and by cooling rapidly down to room temperature with atwin-roller. The glass structural data were gathered by a XRD system using Cu Kα1 line.
The PL and TRPL spectra were measured by an optical multichannel analyzer system. A N2-laser(wavelength λ = 337.1 nm, pulse width <1 ns), a Nd3+ :YAG laser (λ = 355 nm, pulse width <5 ns) and a
monochromatic light source consisting of a 150 W xenon lamp and a grating monochromator were usedas the excitation source. The PLE spectra were recorded in an apparatus consisting of the same monochromatic light source, a grating monochromator and a synchronous light detection system.
*

Corresponding author: e-mail: motizuki@physics.chs.nihon-u.ac.jp, Phone/Fax: +81-3-5317-9771© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1610-1634/03/00002-0767 $ 17.50+.50/0

768

S. Mochizuki and F. Fujishiro: Shallow- and deep-luminescence centersα -211

α -200γ -111, β -002

Intensity (arb. scale)

α -110Fig. 1 X-ray diffraction patterns of different(AgI)0.85(Ag2WO4)0.15 glasses.

Type-αα glass

β -110α -110β -100γ -111, β -002

3. Results and discussion Depending on
the cooling speed of the melt, two different
types of specimen were obtained. As shown in
Fig. 1, under rapid cooling condition, glass
predominantly showing the XRD pattern of
αAgI phase (Type-α glass) was obtained,
while under less rapid cooling condition, glass
predominantly exhibiting the XRD pattern of
low-temperature AgI phases (Type-β glass)
was obtained. It has been elsewhere reported
Type-ββ glassthat AgI precipitated in glass are particles
whose sizes are several ten nanometers [8].
Figure 2 shows the PL and PLE spectra of
20°
30°
40°
50°
60°
the
type-α glass at 9 K. The spectra are nor2θ
malized in intensity. We have found that the
PL for the type-α glass cannot be excited withany ultraviolet light, but with green light. The PL has very broad bandwidth and the intensity peak of the
PLE spectra appears near the absorption edge wavelength (483 nm) [6] of αAgI. This indicates that theobserved PL is due to excitons of αAgI particles precipitated in glass matrix. The PL spectrum is shiftedby about 0.6 eV from the intensity peak of the PLE spectra. The shift may arise from deeply trapped
exciton states due to strong electron–phonon interaction in αAgI particle or deeply trapping potential at
αAgI/matrix glass interface. The observation wavelength dependence of PLE spectra may arise fromdifferent trapped states due to different shapes and sizes of the αAgI particles.
Figure 3 shows the PL spectrum of the type-β glass at different temperatures. The intensity peak at
about 425 nm is assigned to radiative decay of free excitons in AgI particles [4, 5, 7], while the sideband
at about 436 nm is assigned to radiative decay of excitons trapped by lattice defects and impurities. The
broadband peak at about 484 nm is assigned to radiative decay of excitons trapped at β or αAgI/matrix
glass interface. The TRPL measurements show that the PL decay times of the free exciton and board

Intensity (arb. scale)

Type-αα glass (T=9K)
λ ob
586nm
630nm
659nm
701nm
722nm

PL

PLE

λ ex
500nm

Fig. 2 PL and PLE spectra(AgI)0.85(Ag2WO4)0.15 glass at 9 K.
400

500

600

Wavelength (nm)

700

800

of

type-α

400

T(K)
8
47
98
148
198

769

Type-ββ glass

λ ex=337.1nm

Fig. 3 (online colour at:
www.interscience.wiley.com) Temperaturedependence of the PL spectrum of type-β(AgI)0.85(Ag2WO4)0.15 glass.

T (K)
8
198

PL intensity

PL intensity (arb. scale)phys. stat. sol. (c) 0, No. 2 (2003)

400

500

600

700

Wavelength (nm)

450

500

550

600

650

Wavelength (nm)700

bands are shorter than 10 ns, that is about 25 ns. As shown in the inset, the PL except for free excitons
are weakened and only free exciton PL becomes prominent with increasing temperature, which may be
due to a thermally activated reverse process from trapped states to free exciton state.
Figure 4 shows the excitation-light-intensity dependence of the PL spectrum of the type-β glass at
10 K. With increasing laser fluence (peak power I0 = 200 kW), the intensity of PL due to free excitons
increases linearly, while the intensity of PL due to trapped excitons increases and then becomes saturated
at higher fluences. In the inset figure, the spectra for intense and weak-excitation limits are compared,
which correspond to curves S and W, respectively. This excitation-light-intensity dependence shows that
the PL centers due to trapped excitons have higher quantum yields than free excitons, but the number of
the PL center is limited.
Finally, we apply the results of the present optical study to the ion conduction. Since the chemical
potentials of Ag+ in AgI and Ag2WO4 glass differ, a transition layer is formed at the interface between
AgI particle and Ag2WO4 glass matrix. Many crystalline defects may be also contained at the interface,which gives rise to deep trap for free exciton and enhances ionic conductivity. The deeply trapped exciton states can be connected to the transition layer. Through this layer, the mobile Ag+–Ag+ ion exchange
responsible for high ionic conduction occurs in the type-α and type-β glasses.

Type-ββ glass (Τ=10Κ)400

λ ex=337.1nm

S W

PL intensity

PL intensity (arb. scale)

I0
0.33I00.18I00.10I00.06I0

400

500

600

700

Wavelength (nm)Fig. 4 Excitation-light-intensity dependence of the PL spectrum of type-β(AgI)0.85(Ag2WO4)0.15 glass at 10 K.
450

500

550

600

Wavelength (nm)

650

700

770

S. Mochizuki and F. Fujishiro: Shallow- and deep-luminescence centers

Acknowledgements This work was partially supported by Project Research Grant from The Institute of Information Sciences of College of Humanities and Sciences (Nihon University) and by Cooperative Research Grant fromThe Institute of Natural Sciences (Nihon University). This work was also partially supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Science, Sports, Culture and Technology, Japan.

References[1] B. Roling and M. D. Ingram, Phys. Rev. B 56, 13619 (1997).
[2] M. Hosono, J. Kawamura, H. Itoigawa, N. Kuwata, T. Kamiyama, and Y. Nakamura, J. Non-Cryst. Solids 244,
81(1999).
[3] S. Mochizuki and F. Fujishiro, in: Proc. 6th Forum Superionic Conductor Physics, 24–25 May 2002, Kyoto,
Japan (The Solid State Ionics Society of Japan, Kyoto, 2002), pp. 41–46.
[4] M. Cardona, Phys. Rev. 129, 69 (1963).
[5] S. Mochizuki and Y. Ohta, J. Lumin. 87/89, 299 (2000).
[6] S. Mochizuki, Physica B 308/310, 1042 (2001).
[7] S. Mochizuki and K. Umezawa, Phys. Lett. A 228, 111(1997).
[8] M. Tatsumisago, N. Itakura, and T. Minami, J. Non-Cryst. Solids 232/234, 267 (1998).

phys. stat. sol. (a) 189, No. 3, 935–938 (2002)

Magnetic Phase Transition in Antiferromagnetic Materials
under High Pressure
Y. Mita1) (a), Y. Sakai (a), M. Kobayashi (a, c), S. Endo (b, c),and S. Mochizuki (d)(a) Graduate School of Engineering Science, Osaka University, Osaka 560-8531, Japan(b) Research Center for Materials Science at Extreme Conditions, Osaka University,
Osaka 560-8531, Japan(c) CREST, Japan Science and Technology Corporation, Kawaguchi, Saitama 332-0012,
Japan(d) College of Humanities and Sciences, Nihon University, Tokyo 156-8550, Japan(Received May 1, 2001; accepted September 30, 2001)
Subject classification: 75.30.Kz; 75.50.Ee; S5.2; S10.1
Pressure dependence of magnon Raman scattering was studied at 1.8 K up to 4 and 60 GPa forsolid oxygen and MnO, respectively. In both cases, magnon peaks shift to the higher energy sidewith increasing pressure. In solid oxygen, the pressure coefficients and intensity of magnon Raman
peak are observed to change simultaneously around 0.1 GPa and the peak vanishes at 2.9 GPa. In
MnO, the two-magnon Raman peak vanishes around 20 GPa. These results mean that changes inspin configurations (i.e. magnetic phase transitions) occur at these pressures.

1. Introduction It is difficult to study aniferromagnetic materials by conventional magnetization measurement under high pressure, both because the magnetization signal isvery weak on account of anti-parallel spin configuration in antiferromagnetic materials
and because of the small sample volume possible to be loaded in a pressure cell. On
the other hand, only 10 ––8 or 10 ––9 cm3 of sample volume is required for the accurate
Raman detection, which enables us to use a diamond anvil cell (DAC). Furthermore,
the Raman shift due to the antiferromagnetic magnon is larger than that of a ferromagnetic one [1]. Since the position and intensity of magnon Raman peak is strongly affected by spin configuration, drastic changes of magnon Raman peak indicate changes
in spin arrangement. Thus, magnon Raman scattering is one of the best ways to study
magnetic phase transition of antiferromagnetic materials under high pressure. We applied the method to detect magnetic phase transitions of two important materials (solid
oxygen and MnO) under high pressure.
Solid oxygen is one of the most interesting molecular crystals because of its behavior
as a magnetic material. From many magnetic experiments, it is clarified that the lowest
temperature phase (a-phase) is antiferromagnetic with TN (Néel temperature) = 24 K
[2]. However, the highest pressure at which magnetization measurements have been
performed is only 0.6 GPa [3]. On the other hand, MnO exhibits antiferromagnetism attemperatures lower than TN = 118 K. It has type II spin structure in which ferromagnetically ordered (111) sheets are stacked antiferromagnetically in the [111] direction.
1) Corresponding author; Tel.: +81-6-6850-6417; Fax: +81-6-6845-4632;
e-mail: mita@mp.es.osaka-u.ac.jp# WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 2002

0031-8965/02/18902-0935 $ 17.50þ.50/0

936

Y. Mita et al.: Magnetic Phase Transition in AF Materials under High PressureThis causes the rock-salt (B1) structure to accompany a small trigonal lattice distortion
below TN, contracting along a [111] direction. No magnetization measurement under
low temperature and high pressure conditions has been reported previously.
2. Experiments Raman scattering measurements were performed in backscattering geometry at 1.8 K in solid oxygen and MnO in the pressure range of 0–4 and 0–60 GPa,
respectively. Samples were pressurized using a DAC. Solid oxygen was compressed directly and liquid helium was used as a pressure medium for MnO. For pressure calibration, the standard ruby fluorescence method was used. The excitation source was a514.5 nm line of argon ion laser. Scattered light was detected by a Jobin-Yvon triple
monochromator T64000 and a Jobin-Yvon Spex Spectrum-one CCD (charge coupleddevice) system. In solid oxygen, the measurements were performed at both librational(0–300 cm ––1) and vibrational (1500–1600 cm ––1) regions.
3. Results and Discussion Figure 1 shows the Raman spectra in librational region under various pressures. Besides two strong libron peaks, a peak due to the one-magnon
scattering [4] is also seen around 30–40 cm ––1 marked by arrows. The relations between
applied pressure and Raman peak positions of the vibron (V), librons (L1, L2) andmagnon (M) are illustrated in Fig. 2. The pressure coefficients of the L1, L2 and M
peaks change around 0.1–0.2 GPa, on the other hand, in vibron (V), the change occurs
around 0.4 GPa and the influence remains up to 0.6 GPa. Until now the reason of thisdiscrepancy is unknown. Furthermore, the change in pressure coefficient of vibron isobserved also at 2.3 GPa. The pressure dependences of the magnon peak position(open circles) and its peak intensity normalized by L2 (solid circles) are shown in
Fig. 3. With increasing pressure the intensity decreases drastically around 0.1 GPa andthe change in the pressure coefficient ofpeak position occurs at nearly the samepressure. The peak intensity decreases withincreasing pressure and disappears around
2.9 GPa. The fact that change in magnon
peak position and intensity occurs nearly at
the same pressure (at 0.1–0.2 GPa) indicates that the change in spin arrangement(i.e. magnetic phase transition) occurs
around there. On the other hand, jump in
magnon peak energy at 0.1 GPa is not large
and the peak shows nearly continuous highFig. 1. Raman spectra of solid oxygen at 1.8 K
under various pressuresphys. stat. sol. (a) 189, No. 3 (2002)

937
Fig. 2. Pressure dependence of Raman
peak positions of the vibron (V), librons(L1 and L2) and magnon (M) in solid
oxygen. Lines seen in the upper figure
are guides for eyeenergy shift with pressure. Therefore, it is deduced that the antiferromagnetic character basically continues at least up to 2.9 GPa andphase transition at 0.1 GPa may be
due to a slight change in spin arrangements in the antiferromagnetic
phase. Since one-magnon Raman
signal reflects the long-range magnetic order (k = 0), vanishing of themagnon peak at 2.9 GPa probably
means that this antiferromagnetic
spin order becomes short range.
However, it must be emphasizedthat the phase starts from 2.9 GPa
does not necessarily mean that thephase is no more antiferromagnetic.
It is also possible the spins align indifferent type of an antiferromagnetic order.
Figure 4 shows the pressure dependence of Raman spectrum of
MnO. In the spectrum obtained at
1 GPa, two peaks are observed
clearly around 500 and 540 cm ––1.
These are a two-magnon (2M) and

Fig. 3. Relative intensity (solid circles)
and peak position (open circles) of magnon Raman peak at various pressures insolid oxygen

938

Y. Mita et al.: Magnetic Phase Transition in AF Materials under High Pressure
Fig. 4. Raman spectra of MnO at 1.8 K
under several pressures

a 2-TO phonon Raman peaks, respectively [5, 6]. With increasingpressure, the 2M and 2TO peaks
shift toward higher energy. The 2M
peak becomes weak and disappears
around 20 GPa. Therefore, the antiferromagnetic spin configuration exists up to 20 GPa and a change inspin arrangement (magnetic phasetransition) occurs around 20 GPa.
No further phase transition is detected in the pressure region up to
60 GPa.
4. Conclusions Magnetic
phasetransitions were detected by magnon Raman scattering under highpressure at 1.8 K. In solid oxygen,change in peak intensity and jumpof peak position of antiferromagnetic magnon are observed around 0.1 GPa andvanishes at 2.9 GPa. In MnO also, the two-magnon peak shifts toward higher energy
with increasing pressure and disappears around 20 GPa. These facts reflect the change
in spin arrangements which occurs around the corresponding pressures.
Acknowledgement The authors thank Dr. H. Furuta of Research Center for Material
Science at Extreme Conditions of Osaka University for technical support.

References[1] P. A. Fleury and R. Loudon, Phys. Rev. 166, 514 (1968).
[2] E. Kanda, T. Haseda, and A. Otsubo, Physica 20, 131 (1954).
[3] R. J. Meier, C. J. Schinkel, and A. De Visser, J. Phys. C 15, 1015 (1982).
[4] P. M. Mathai and E. J. Allin, Can. J. Phys. 48, 1518 (1970).
[5] H.-H. Chou and H. Y. Fan, Phys. Rev. B 13, 3924 (1976).
[6] H. Y. Fan, Indian J. Phys. A 55, 218 (1981).

ARTICLE IN PRESS

Physica B 340–342 (2003) 216–220

Structural and optical studies on mesoscopic defect structure
in highly conductive AgI–ZnO composites
Fumito Fujishiro*, Shosuke Mochizuki
Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan

Abstract
The electrical conductivity of (x)AgI–(1x)ZnO (0pxp1) composites at room temperature increases with increasing
AgI content and reaches a maximum at about 50% AgI. The results obtained by the scanning electron microscopy,
X-ray diffractometry and photoluminescence spectroscopy have clariﬁed high-ionic-conduction pathways related tomesoscopic defect structure at AgI/ZnO interfaces and mesoscopically disordered structure in AgI domain. We haveobserved also new optical phenomenon, which may arise from excitation energy transfer between AgI-exciton andphotoinduced oxygen vacancy at the AgI/ZnO interface.
r 2003 Elsevier B.V. All rights reserved.
PACS: 72.80.Tm; 61.72.Nn; 68.35.Fx; 71.35.Cc; 78.55.Hx
Keywords: Defects in oxides; Defects at/near surfaces and interfaces

1. Introduction
The ionic conduction in a composite consistingof AgI and oxide ﬁne particles has received muchattention as a model material for searching new
solid electrolyte [1,2]. In the composites, electricalconductivity s was enhanced by a few orders ofmagnitude higher than that of pristine AgI. Such s
variation is initially explained by taking into
account a space charge layer around nucleophilic
surface [3]. However, several basic problems wereraised about AgI-based composites. How does AgI
attach to oxide ﬁne particles? What are the atomic
and electronic structures of the AgI/oxide ﬁne*Corresponding author. Tel./fax: +81-3-5317-9771.
E-mail address: fumito@phys.chs.nihon-u.ac.jp(F. Fujishiro).

particle interface? Why oxide ﬁne particles enhance conductivity? Constituent material AgI
shows clear optical spectra due to excitons in thebulk [4], ﬁlms [5,6] and microcrystals [7]. Besides
AgI, most oxides also show exciton spectra.
Generally, exciton spectra are very sensitive tothe environment in which excitons are moving
and, therefore, the optical spectra due to excitons
may provide useful information about the AgI/
oxide ﬁne particle interfaces. Recently, we havereported the structural, electrical and opticalproperties of AgI-anatase composites [8]. Veryrecently, we have fabricated also (x)AgI–(1x)ZnO (0pxp1) composites and observed senhancement. Besides this, we have found new
photofunction in the composites. In this paper, we
report the structural, electrical and opticalstudies of AgI–ZnO composites and discuss the0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2003.09.025

ARTICLE IN PRESS
F. Fujishiro, S. Mochizuki / Physica B 340–342 (2003) 216–220

3. Results and discussion
The SEM observations and atom-distribution
maps indicate that ZnO particles less than 1 mm
in size are densely dispersed in AgI matrix andthe surroundings of ZnO particles are silver rich.
Fig. 1 shows the XRD patterns of (x)AgI–(1x)ZnO composites at 300 K. All XRD patterns

E(112),
J(311)

E(110),J(220)

E(002),J(111)

E(102)

E(103)

E(100)

x= 1.0x= 0.5

20

25

30

35

(102)

(101)

(002)

x= 0.2(100)

AgI–ZnO composites were prepared by mixing
AgI powder and ZnO ﬁne particles (with anaverage diameter of 20 nm), heating the mixture
at 873 K for 12 h and then annealing at 373 K for
12 h to remove the stress introduced at the phasetransition point of AgI, 420 K. For the smeasurements, the composite powder was uniaxially compressed into pellets under 0.2 GPa for
30 min at room temperature. To avoid the socalled memory effect [9], the pellets were heattreated at 473 K in air for 12 h and then annealed
at 373 K for 12 h, in a lidded crucible. Afterevaporating silver onto both sides of the pellet, thes measurements were performed by the impedance
method. The composites were characterized by
X-ray diffraction (XRD) analysis with Cu Ka1
radiation. The morphologies and distributions ofAg, I, Zn and O atoms in the composites werestudied by a scanning electron microscope (SEM)(Shimadzu SSX-550). A pulsed Nd3+:YAG laser(wavelength l ¼ 355 nm) was used as the excitation source. The photoluminescence (PL) spectra
were measured by an optical multichannel analyzer (OMA) consisting of a grating monochromatorand an intensiﬁed diode-array detector.
Photoinduced-spectral-change experiments werecarried out with a continuous wave of He–Cd laserline (l=325 nm) with a power density of 5.4
kW/m2. The same He–Cd laser line was used toexcite luminescence. Emitted light is measured as afunction of irradiation time tir with the same
OMA.

E(101)

2. Experimental

XRD intensity (arb. scale)effects of defects at AgI/ZnO interface in thesecomposites.

217

40

45

x= 0.050

55

2T (degree)
Fig. 1. The XRD patterns of (x)AgI–(1x)ZnO composites (at300 K).

in this ﬁgure are normalized at the peak intensity.
With increasing x, some diffraction lines related to
AgI become pronounced and all diffraction lines
can be almost assigned to g-AgI (zincblende
structure), b-AgI (wurtzite structure) and ZnO(wurtzite structure), as shown in Fig. 1. The
intensity ratio of the b-AgI (0 0 2) line, whichcoincides with the g-AgI (1 1 1) peak, to the b-AgI(1 0 0) line tends to increase from about 4 to about
8 while increasing x from 0.1 to 1.0. Such anincrease may be explained by either a preferredorientation of b-AgI crystallites or an increase ofg-AgI component in the composites, accompaniedby stacking disorder in hexagonal b-AgI. However, it is noted that the b-AgI (1 0 1), b-AgI (1 0 2)
and b-AgI (1 0 3) lines become weak in the large x
region (x>0.5), which may be connected tostacking disorder in b-AgI lattice [8,10]. Referringto the known result that a silver-deﬁcient AgI
solution crystallizes in the g phase and silver-rich
AgI solutions do so in the b phase, this isinterpreted as follows. Preferential Ag+ ion
adsorption on ZnO surfaces (silver-rich) induces
a concentration gradient of Ag+ ions in the AgI
domain and oxygen vacancies on ZnO particle
surfaces. The gradient produces then g-AgI
crystallites and b-AgI ones in the AgI domain.
The degree of the gradient is dependent on theZnO contents and, therefore, the diffraction
intensity ratio of the g-AgI line to the b-AgI line
varies with x.

ARTICLE IN PRESS
F. Fujishiro, S. Mochizuki / Physica B 340–342 (2003) 216–220

x= 1.00-3

10x= 0.9

-4

10

-5

10

40
30
20
10
0

-6

10

0.0 0.2 0.4 0.6 0.8 1.0

AgI content, x

0.0

0.2

0.4

0.6

0.8

1.0

AgI content, x

Fig. 2. The s variation of (x)AgI–(1x)ZnO composites at
300 K. The porosity variation with x is shown in the inset.

PL intensity (arb. scale)0
Porosity (%)

Conductivity V (S/m)

218

x= 0.80x= 0.70x= 0.60x= 0.50x= 0.40x= 0.30x= 0.20x= 0.00

The s at 1 kHz for (x)AgI–(1x)ZnO composites at 300 K is plotted against x in Fig. 2. No
frequency dependence of s was observed between
42 Hz and 100 kHz. As seen in this ﬁgure, s
increase is abrupt against x. On further increasingx, s reaches a maximum (about 2.3  103 S/m) at
x ¼ 0:5: Since the s for a pristine AgI specimen
prepared in the same way is 4.5  105 S/m, the sis enhanced by about 50 times. In the x region
larger than 0.5, s falls gradually toward the s ofpristine AgI. In the light of the porosity data
shown in the inset of Fig. 2, the s variation in thesmall x region may be mainly caused by insulating
pore effect. On the other hand, the s enhancement
observed in the intermediate region of x may arise
from some composite effects, since the starting
materials, pristine AgI and ZnO, have low s. The
composite effects may arise from highly ionicconductive regions at the AgI/ZnO interfaces. The
g-AgI and b-AgI crystallites may act as Ag+ ion
donor and acceptor, respectively. Thus, we cananticipate high Ag+ ion conductivity not only at
the AgI/ZnO interface but also at the g-AgI/b-AgI
interface. With decreasing x from 1 to 0.5,
dispersed ZnO particles approach each other andthen the space charge effect spreads over the AgI
domain to display a maximum in the s–x curve.
We have measured the PL spectra of (x)AgI–(1x)ZnO composites at different temperatures
between 9 and 288 K. Of them, the PL spectra
obtained by 355 nm laser excitation at 9 K arecompared in Fig. 3. All PL spectra in this ﬁgure

420

440

460

480

500

Wavelength (nm)

520

540

Fig. 3. The PL spectra of different (x)AgI–(1x)ZnO composites at 9 K.

are normalized at the peak intensity. Pristine ZnO
shows broad PL band ranging from 400 to 700 nm.
The PL has been fully studied by many workers andhas been attributed to oxygen vacancies [11–13]. The
addition of small amount of AgI, x ¼ 0:2; leads to aconsiderable decrease of this ZnO-related PL andgives a sharp PL band at 428 nm and a broad PL
band centered at 440 nm. The addition of AgI to
ZnO particle may produce oxygen vacancies at the
AgI/ZnO interfaces. This may act on the oxygen
vacancies to decrease luminescence related topristine ZnO and may give pathway of Ag+ ionsto enhance s. Since AgI domain size is small
compared with mean free path of free exciton in
the small x region, free excitons tend to be trappedat the AgI/ZnO interface. The excitation-laserlight absorption by added AgI should be also
taken into account for the decreased luminescence.
These PL bands at 428 nm, centered at 440 nm arethus assigned to radiative decay of bound excitonin AgI domain and bound exciton trapped at the
AgI/ZnO interface, respectively. Up to the x ¼ 0:5;
the broad PL band centered at 440 nm increases
with increasing x. On further increasing x, from
0.5 to 0.9, the broad PL band centered at the
440 nm comes to disappear and three bands grow

ARTICLE IN PRESS
F. Fujishiro, S. Mochizuki / Physica B 340–342 (2003) 216–220

at about 422, 427 and 433 nm. These 422, 427 and433 nm bands are attributed to radiative decay of
free exciton of AgI, bound exciton in AgI domain
and another type bound exciton, respectively.
Since AgI domain size increases with increasingx, free excitons move with larger mean free path.
Some of such excitons are trapped inside AgI
domain and then radiatively decayed to increase
luminescence intensity, before reaching at the AgI/ZnO interface.
We have found also a reversible photoinduced
PL spectral change in the present (0.5)AgI–(0.5)ZnO composite at room temperature, as
shown in Fig. 4(a). The arrows indicate the orderof the experiments. After fully irradiating with325 nm laser light in an evacuated specimen
chamber for 30 min, the specimen shows a broad
PL band centered at 520 nm due to trapped
excitons of ZnO at oxygen vacancies, as indicatedby curve 1. Oxygen gas is then introduced into thespecimen chamber and, with increasing tir, thebroad PL band decreases in intensity and a sharp
PL due to AgI-excitons appears at 426 nm, as
indicated by curve 2. Oxygen gas is then evacuatedfrom the specimen chamber and, with increasingtir, the broad PL band becomes prominent, while asharp PL band due to AgI-excitons degrades as
indicated by curve 3. Through many successive

219

experiments, it is found that such spectral changes(curve 22curve 3) appear repeatedly. The observed spectral changes may originate fromexcitation energy transfer between excitons in
AgI domain and photoinduced oxygen vacancies
at the surface of ZnO particle. The same experiment is also carried out for pristine ZnO. The
results are given in Fig. 4(b). The PL intensitychange by replacing atmosphere is very similar tothat for (0.5)AgI–(0.5)ZnO. In the present experiments, the 325 nm laser light irradiation in vacuum
and in oxygen gas may correspond to photoreduction and photooxidation, respectively. Therefore,
the PL intensity increase under laser light irradiation in vacuum is thought to be due to the increasein number of oxygen vacancies. The PL band
centered at 480 nm and the PL band tail shorter
than 410 nm have been assigned to trapped
excitons at oxygen vacancies and neutral donor
bound excitons, respectively [11–13]. The present
photoreduction experiments indicate that both socalled near-ultraviolet PL band and green one of
ZnO closely relate to oxygen vacancies.

Acknowledgements
This work is partially supported by a Grant-inAid for Scientiﬁc Research from the Ministry of
Education, Science, Sports, Culture and Technology, Japan. This work is supported by Interdisciplinary General Joint Research Grant for Nihon
University. This work is also partially supportedby Project Research Grant from The Institute ofInformation Sciences of College of Humanities
and Sciences (Nihon University) and by Cooperative Research Grant from The Institute of Natural
Sciences (Nihon University).

References

Fig. 4. The photoinduced spectral changes at 300 K in (a)(0.5)AgI–(0.5)ZnO and (b) in pristine ZnO.

[1] K. Shahi, J.B. Wagner Jr., Solid State Ionics 3–4 (1981)295.
[2] M.C.R. Shastry, K.J. Rao, Solid State Ionics 51 (1992)
311.
[3] T. Jow, J.B. Wagner Jr., J. Electrochem. Soc. 126 (1979)
1963.

ARTICLE IN PRESS
220

F. Fujishiro, S. Mochizuki / Physica B 340–342 (2003) 216–220[4] S. Mochizuki, F. Fujishiro, Proceeding of the 6th Forumon Superionic Conductor Physics, The Japanese Society of
Ion Transport, Kyoto; 2002, pp. 41–46.
[5] M. Cardona, Phys. Rev. 129 (1963) 69.
[6] S. Mochizuki, Y. Ohta, J. Lumin. 87–89 (2000) 299.
[7] S. Mochizuki, K. Umezawa, Phys. Lett. A 228 (1997) 111.
[8] S. Mochizuki, F. Fujishiro, J. Phys.: Condens. Matter 15(2003) 5057.
[9] G. Burley, Acta Crystallogr. 23 (1967) 1.

[10] J.-S. Lee, S. Adams, J. Maier, J. Phys. Chem. Solids 61(2000) 1607.
[11] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant,
J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996)
7983.
[12] S.H. Bae, S.Y. Lee, H.Y. Kim, S. Im, Opt. Mater. 17(2001) 327.
[13] A. van Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, A.
Meijerink, J. Lumin. 87–89 (2000) 454.

ARTICLE IN PRESS

Physica B 340–342 (2003) 913–917

Reversible photoinduced spectral transition in Eu2O3–gAl2O3
composites at room temperature
Shosuke Mochizuki*, Hiroyuki Araki
Department of Physics, College of Humanities and Sciences, Nihon University 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan

Abstract
The photoluminescence (PL) intensity of Eu2O3–gAl2O3 composite has been found to decrease at room temperature
with 325 nm laser light irradiation in vacuum, without any accompanying white luminescence. Then, under the samelaser light irradiation in oxygen gas atmosphere, the PL intensity recovers. Through many successive experiments, it isfound that the spectral change is nearly reversible. In spite of the changing atmosphere (for example, oxygen gas and air
exposures), each PL state is stored for a long time at room temperature after the removal of the laser light under room
light. This phenomenon is interpreted as results of photoinduced valence-number change of europium ions, the oxygen
vacancy formation and some excitation energy transfer. Such a reversible phenomenon may well yield materials forerasable optical storage and oxygen sensor.
r 2003 Elsevier B.V. All rights reserved.
Keywords: Defects in oxides; Defects at/near surfaces and interfaces
PACS: 73.20.H; 82.50.F; 42.79.V; 78.55

1. Introduction
There has been a great deal of interest in the
study of materials displaying photoinduced reversible spectral change at room temperature [1],because a combination of such reversible phenomenon and the near-ﬁeld optical microscope [2] isvery promising when applied to a high-density
optical storage with nanometer resolution. Recently, we have observed a new-type erasable
optical memory effects accompanied by intensewhite light emission in Eu2O3 [3,4], at room
temperature under 325 nm laser light irradiation.
Such optical memory effects arise from photo*Corresponding author. Tel./fax: +81-3-5317-9771.
E-mail address: motizuki@physics.chs.nihon-u.ac.jp(S. Mochizuki).

induced valence-number change, photoinduced
oxygen vacancies or both. It is a hope that we
can ﬁnd new materials having a photomemory
function by combining Eu2O3 and a photocatalyst
oxide (for example, anatase and gAl2O3). Veryrecently, we have fabricated Eu2O3–gAl2O3 composites and found an erasable photomemory
phenomenon without any accompanying whiteband emission. In this paper, we report theexperiments obtained for the (0.2)Eu2O3–(0.8)gAl2O3 composite, as a typical example.

2. Experimental
Eu2O3–gAl2O3 composite powder was preparedby mixing Eu2O3 with particle size less than

0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2003.09.204

ARTICLE IN PRESS
914

S. Mochizuki, H. Araki / Physica B 340–342 (2003) 913–917

5000 nm and gA12O3 ﬁne particles (with anaverage diameter of 60 nm) in air, by heating the
mixture in the lidded crucible at 873 K for 69 h inair. Powder compact specimens are prepared bycompressing the powder mixture under a pressureof 0.2 GPa for 2 h at room temperature. Microwave (frequency=2450 MHz, power=500 W) irradiation was carried out for 50 min. Suchmicrowave treatment not only removes somestrain induced by compaction but also induces
effective sintering at low temperature [5]. If the
microwave power and frequency are set appropriately, sintering may proceed at low temperature
without any particle grain growth and, therefore,
without losing surface activity of gA12O3 ﬁneparticles.
The composites were characterized at 300 K by
X-ray diffraction (XRD) analysis with CuKa1
radiation. The composite morphology was analyzed by a scanning electron microscope and an
optical microscope with a charge-coupled device
camera system.
Irradiation is carried out with a continuous
wave (CW) of He–Cd laser line (wavelengthl=325 nm) with a power density of 318 W/m2,
which is less than 1% of that for previous works inpristine Eu2O3 [3,4]. The same He–Cd laser line
excites luminescence. Emitted light is dispersedand detected by using a grating spectrographequipped with a multichannel photodetection
system. PLE spectra were recorded by varying
the excitation-light wavelength lex with an apparatus consisting of a monochromatic light sourceand detecting the emission light intensity at theﬁxed observation-light wavelength lob as a function of lex with a grating monochromator and asynchronous light detection system. All the measurements were carried out at room temperature.

3. Results and discussion
The apparent density of the (0.2)Eu2O3–(0.8)gAl2O3 composite is 1780 kg/m3 and theporosity is 59% at room temperature. The X-ray
analysis indicates that the fabricated powder
compact is a mere mixture of the C-type (cubic)
Eu2O3 [6] and gAl2O3.

Figs. 1(a) and (b) show the photoinduced PL
spectral changes of (0.2)Eu2O3–(0.8)gAl2O3 composite under CW 325 nm laser light irradiation.
The experiments are carried out in numericalorder: 1-2-3-4. The irradiation time tir under
given atmosphere and the kind of atmosphere
are given on each curve. When (0.2)Eu2O3–(0.8)gAl2O3 composite is irradiated with 325 nm
laser light at room temperature in vacuum of
about 1  104 Pa, the PL intensity decreases withincreasing tir. The PL spectrum of the composite
shows red Eu3+ emission (curve 1) due to the5
D0-7FJ (J=0, 1, 2, 3, 4) transitions [7]. The
pristine gAl2O3 displays a broad PL band, as
shown in the inset of Fig. 1(a), and pristine Eu2O3
displays both sharp PL line and a broad PL band[4], while the broad PL bands due to pristinegAl2O3 and Eu2O3 disappear in (0.2)Eu2O3–(0.8)gAl2O3 composite. This indicates that there
are many nonradiative traps (NRT) for photoexcited oxygen vacancies at gAl2O3/Eu2O3 interfaces. The specimen chamber is then ﬁlled withoxygen gas of about 0.1 MPa under the same
325 nm laser light irradiation. With increasing tir,
PL intensities due to the 5D0-7FJ (J=0, 1, 2, 4)transitions increases. This change is not accompanied by white light emission, which may be
simple materials for photomemory and optoelectronic in comparison with pristine Eu2O3 [3,4].
Oxygen gas is again evacuated from the chamber.
The red emission becomes weak. The spectralchange observed for (0.2)Eu2O3–(0.8)gAl2O3 composite appears reversible for replacing kind of
atmosphere. In Fig. 1(c), the PL intensity of theline due to the 5D0-7F2 transition is plotted as afunction of tir for each specimen atmosphere. It isalso noted that the spectral change becomes more
prominent for more specimen-atmosphere change.
After removing the 325 nm laser light, both theintense- and weak-red emission states are stored
for a long time at room temperature in air.
Figs. 2(a) and (b) show the PLE spectra of(0.2)Eu2O3–(0.8)gAl2O3 composite and pristine
Eu2O3 powder compact. Fig. 2(c) shows the PLE
spectrum (solid curve) and PL one (broken curve)of pristine gAl2O3 powder compact. In the PLE
spectrum of pristine gAl2O3 powder compact, twobroad absorption bands named X and Y are seen

ARTICLE IN PRESS
S. Mochizuki, H. Araki / Physica B 340–342 (2003) 913–917

915

Fig. 1. Reversible photoinduced spectral change of (0.2)Eu2O3–(0.8)gAl2O3 composite by replacing specimen atmosphere at room
temperature: (a) in vacuum, (b) in oxygen gas, (c) irradiation time dependence of the PL due to 5D0-7F2 transition. In the inset of (a),the PL spectrum of pristine gAl2O3 is compared with that of (0.2)Eu2O3–(0.8)gAl2O3.

at 210 nm (=5.90 eV) and 370 nm (=3.35 eV),
respectively, and a shoulder band named S band isseen at 261 nm (=4.74 eV). The full-width at halfmaximum (FWHM) of the X and Y bands are 1.24
and 0.57 eV, respectively. The Y band centered at
3.35 eV corresponds to deep electronic levels in theenergy gap (8.7 eV) of solid gAl2O3. The PL
spectrum is shifted toward longer wavelength todisplay quasi-mirror relation with the PLE one.
Similar relation has been frequently observed fordeeply trapped and spacially localized electrons
around oxygen vacancies in metal oxide crystals
and in imperfect ionic crystals. We name such
localized electronic states due to oxygen vacancies
LESOV. In such case, the magnitude of the
FWHM depends on the electron–phonon interactions and defect structure. Surely, the PL intensityof the X and Y bands in the pristine gAl2O3
powder compact has been found in the present
experiments to increase with increasing tir under
325 nm light irradiation in vacuum. Since the
325 nm laser light irradiation in vacuum corresponds to photoinduced associative detachment of
O2 molecules near the specimen surfaces [4], the X
and Y bands are assigned to oxygen vacancies at
gAl2O3 surfaces. The energy diagram of Eu3+ ionsis thought to be insensitive to oxygen defects, since
the extent of 4f-electron wave functions is extremely small. We note especially the following
three of the obtained results.
(1) Eu2O3 attachment to gAl2O3 creates nonradiative traps to annihilate the broad PL banddue to gAl2O3 and the white PL due to Eu2O3,

ARTICLE IN PRESS
916

S. Mochizuki, H. Araki / Physica B 340–342 (2003) 913–917

Fig. 2. PLE spectra at room temperature: (a) (0.2)Eu2O3–(0.8)gAl2O3 composite, (b) pristine Eu2O3, (c) pristine gA12O3. The PL
spectrum of the pristine gAl2O3 is shown in the inset of (c).

(2) 325 nm laser light irradiation in vacuum
decreases the PL intensity of Eu2O3, while
the same laser light irradiation in oxygen gas
recovers the PL intensity of Eu2O3,(3) After removing the 325 nm laser light, boththe intense- and weak-red emission states are
stored for a long time at room temperature inair.
On the basis of present PL and PLE spectra forpristine gAl2O3, pristine Eu2O3 and (0.2)Eu2O3–(0.8)gAl2O3 composite, we have drawn an energy
diagram to explain the spectral transitions observed for Eu2O3–gAl2O3 composite in Fig. 3.
Solid arrows and dotted arrows indicate radiativetransitions and non-radiative ones, respectively.
The 325 nm (=3.81 eV) laser light wavelength
corresponds to the onset wavelength for the
charge-transfer (CT) excitation, the wavelength
for the 7F0-5LJ excitation and oxygen defectexcitation at Eu2O3 surfaces. Gathering up these

excitations, they are indicated as a transition from
7
F0 to Z. Also, the LESOV in both gAl2O3 andEu2O3 can absorb incident 325 nm light and decay
nonradiatively to the NRT at gAl2O3/Eu2O3
interfaces. Under CW 325 nm laser light irradiation in vacuum, some of Eu3+ ions are excited tohigher-energy Z state and others transform toEu2+ ions with creating oxygen vacancies at
Eu2O3 particle surfaces. In the energy diagram,the electronic states of Eu2+ ion, the higher energy
levels of Eu3+ ion and the LESOV in Eu2O3
particle surfaces are not shown for simplicity. The
CW 325 nm laser light creates also oxygen
vacancies at gAl2O3- and Eu2O3-particle surfaces
in vacuum atmosphere. The valence-number
change (Eu3+-Eu2+) depresses the 5D0-7F2
emission and the excitation energy of Eu2O3 isalso transferred to unphotoexcited oxygen vacancies at gAl2O3 surfaces, which depress the5
D0-7F2 emission of Eu3+ ions. Since theoxygen-vacancy-related PL has not been observed

ARTICLE IN PRESS
S. Mochizuki, H. Araki / Physica B 340–342 (2003) 913–917

917

faces, the annihilation of oxygen vacancies occurs
to decrease the number of oxygen vacancies. In the
result, the 5D0-7F2 emission of Eu3+ ionsrecovers in oxygen atmosphere.

Acknowledgements
This work is partially supported by a Grant-inAid for Scientiﬁc Research from the Ministry of
Education, Science, Sports, Culture and Technology, Japan. This work is supported by Interdisciplinary General Joint Research Grant for Nihon
University. This work is also partially supportedby Project Research Grant from The Institute ofInformation Sciences of College of Humanities
and Sciences (Nihon University) and by Cooperative Research Grant from The Institute of Natural
Sciences (Nihon University).

References

Fig. 3. Energy diagram of Eu2O3–gAl2O3 composite.

for the present composite, the deexcitation of theoxygen vacancies is accomplished with nonradiative trapping (LESOV-NRT). Under CW 325 nm
laser light irradiation in oxygen gas atmosphere,some of Eu2+ ions transform to Eu3+ ions withannihilating oxygen vacancies at Eu2O3 particle
surfaces. Simultaneously, at gAl2O3 particle sur-[1] K. Tanaka, S. Okamoto, Y. Kanemitsu, T. Kushida,
J. Lumin. 94–95 (2001) 519.
[2] E. Bezig, J.K. Trautman, Science 257 (1992) 189.
[3] S. Mochizuki, Y. Suzuki, T. Nakanishi, K. Ishi, Physica B
308–310 (2001) 1046.
[4] S. Mochizuki, T. Nakanishi, Y. Suzuki, K. Ishi, Appl. Phys.
70 (2001) 3785.
[5] M.A. Janney, H.D. Kimrey, M.A. Schmidt, J.O. Kiggans,
J. Am. Ceram. Soc. 74 (1991) 1675.
[6] G.H. Dicke, Spectra and Energy Levels of Rare Earth Ionsin Crystal, Interscience, New York, 1968.
[7] K.A. Gschneid Jr, L.R. Eyring, in: Handbook on the
Physics and Chemistry on Rare Earths, Vol. 3, NorthHolland, Amsterdam, 1979.

ARTICLE IN PRESS

Physica B 340-342 (2003) 944–948

Intense white luminescence of Sm2O3 irradiated withultraviolet laser light under vacuum
Shosuke Mochizuki*
Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan

Abstract
When Sm2O3-powder compact and -microcrystal ﬁlms are irradiated with 325 nm laser light at room temperature inan evacuated chamber, their photoluminescence (PL) intensities are enhanced enormously. Then, introducing oxygengas into the specimen chamber, the intense PL state returns to the original weak one under the same laser lightirradiation. Through many successive experiments, it is found that the spectral change is reversible under 325 nm laser
light irradiation. After removing the 325 nm laser light irradiation, each PL-state is stored for a long time at room
temperature under room light, in spite of any change of atmosphere. The PL excitation spectra indicate that theobserved phenomenon may arise from photoinduced valence-number change of samarium ions and photoinduced
oxygen defects.
r 2003 Elsevier B.V. All rights reserved.
PACS: 73.20.H; 82.50.F; 42.79.V; 78.55
Keywords: Defects in oxides; Defects at/near surfaces; White luminescence

1. Introduction
A purpose of the present work is ﬁnding newmaterials which display persistent optical spectralchanges at room temperature by laser lightirradiation under speciﬁc atmospheres (this corresponds to optical recording of information) andthen returns to the original optical spectra by laser
light irradiation under a different atmosphere (thiscorresponds to optical erasing of the recorded
information). Recently, we have found the reversible change in the photoluminescence (PL) spectraand intense white PL in different Eu2O3-powder*Tel./fax: +81-3-5317-9771.
E-mail address: motizuki@physics.chs.nihon-u.ac.jp(S. Mochizuki).

compacts, -ﬁlms and -microcrystals at room
temperature by changing the specimen atmosphere
under 325 nm laser light irradiation [1,2]. The
spectral change is closely related to the photooxidation and photoreduction, which induces thevalence number change of europium ions andoxygen defects. Very recently, we have also foundsimilar photoinduced PL spectral change andintense white PL in Sm2O3-powder compact and-microcrystals.
Sm2O3 is stable in air and has ﬁve phases
designated by X, H, A, B and C at atmosphericpressure in order of decreasing temperature [3].
At room temperature, the C type (T7h spacegroup) is thought to be the most stable phase.
However, the B type appears also frequently
as a metastable phase. The stability of the B-type

0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2003.09.253

ARTICLE IN PRESS
S. Mochizuki / Physica B 340-342 (2003) 944–948(C32h space group) phase may arise from slightnonstoichiometries and other defects. Owing tothis quasi-bistability of the C and B types, we
can anticipate some photoinduced-structuraland -valence changes in Sm2O3 under lightirradiation. In the present paper, we report thereversible photoinduced spectral change (PISC) of
Sm2O3 microcrystals, together with that of Sm2O3
powder compact, in detail.
2. Experimental
Sm2O3 powder compact (PC-Sm2O3) was prepared by compressing Sm2O3 powder of 99.9%
purity under a pressure of 0.2 GPa for 1 h at room
temperature and they were then sintered at 1273 K
in air for 24 h. The laser ablation target was thesame PC-Sm2O3 pellet. Sm2O3 microcrystal ﬁlm(mcF-Sm2O3) production and deposition on a silica
glass substrate (Toshiba Ceramics Co., T-1030)
were carried out in oxygen gas of differentpressures between 0 and 4 Pa, with pulsed
Nd3+:YAG laser (wavelength l ¼ 355 nm). The
PISC experiments were carried out with a continuous wave (CW) of He–Cd laser line
(l ¼ 325 nm) with a power density of 5.4 kW/m2.
The same He–Cd laser line excites luminescence.
Emitted light is dispersed and detected using a
grating spectrograph equipped with a multichannel photodetection system. The PL excitation(PLE) spectra were recorded by varying theexcitation-light wavelength lex with an apparatus
consisting of a monochromatic light source anddetecting the emission light intensity at the ﬁxed
observation-light wavelength lob as a function oflex with a grating monochromator and a synchronous light detection system. All the experiments
were carried out at room temperature. Vacuum of
about 104 Pa and oxygen gas of about 0.1 MPa
were used for the atmosphere around the specimen. Evacuation was carried out by a turbomolecular pump (oil free) system.
3. Results and discussion
The mcF-Sm2O3 was characterized, together
with the PC-Sm2O3 as a laser-ablation target, by945

the X-ray diffraction (XRD) method at room
temperature. The results are shown in Fig. 1,
together with XRD histograms for the C- and Btype Sm2O3 [4]. All the XRD lines of the PCSm2O3 are identiﬁed with both the B- and C-typelines given in Figs. 1(d) and (e). As shown in
Fig. 1(b), the XRD pattern of the as-deposited
mcF-Sm2O3 consists of a halo-like pattern ganging
from 25 to 35 and most of the sharp diffractionlines assigned to the B-type (monoclinic) Sm2O3.
The halo-pattern may be ascribed to amorphousdomains or nanometer-size crystallites in the ﬁlms.
After annealing at 873 K in air for 12 h, the halopatterns disappear and the C-type lines dominate
the B-type ones, as shown in Fig. 1(c). Anyhow,
the sintered PC-Sm2O3 specimen and different
annealed mcF-Sm2O3 specimens are the mixture of
the C-type Sm2O3 and the B-type one.
Preceding showing the spectral data, we report
the photoinduced color change in Sm2O3. Under
325 nm laser light irradiation in oxygen atmosphere at room temperature, the PC-Sm2O3 andFig. 1. The XRD patterns of different Sm2O3 specimens atroom temperature: (a) sintered powder compact (PC), (b) asdeposited microcrystal ﬁlm (mcF) and (c) annealed microcrystal
ﬁlm (mcF). The diffraction line diagrams of (d) C-type Sm2O3
and (e) B-type Sm2O3.

ARTICLE IN PRESS
946

S. Mochizuki / Physica B 340-342 (2003) 944–948

the annealed mcF-Sm2O3 display weak blue PL
color which can be seen with the naked eye. As atypical example, this PL color of the PC-Sm2O3 isshown in Fig. 2(a). The specimen chamber is thenevacuated under 325 nm laser light irradiation.
With increasing irradiation time tir ; the PC-Sm2O3
and mcF-Sm2O3 show white PL and increase theintensity. At 90 min later, after the evacuation, abright white PL can be easily seen with the naked
eye. As a typical example, this PL color of the PCSm2O3 is shown in Fig. 2(b). Through manysuccessive experiments, it is found that the PLcolor changes appear repeatedly at different
temperatures from 7 K to room temperature. This
result indicates that the observed photoinduced
spectral changes are purely electronic phenomena
and are not phonon assisted. The color change is
induced effectively only by a 325 nm CW laser
light, which cannot dissociate directly free O2
molecules. This result indicates that the phenomenon arises from the photoinduced associative
detachment and photoinduced dissociative adsorption of O2 molecules near the specimen surfaces[1,2]. The results also summarize how color changearises naturally from photoactivated oxidation
and reduction. After removal of the laser light,
such a white PL state is stored for a long time atroom temperature in air under room light, since 3
June 2003. This result indicates that the groundstate of the photoreduced state (GSPR) is separated from ground state of Sm2O3 by the potential
barrier of which height is higher than roomtemperature thermal energy to suppress thethermal-forward and -backward transitions be-

tween ground state of Sm2O3 and that of the
GSPR.
Next, we show the spectral data corresponding
to the above photoinduced color change. As
typical examples, the obtained PISC for the PCSm2O3 and the annealed mcF-Sm2O3 fabricated at
4 Pa are shown in Fig. 3. Experiments are carriedout in alphabetical order: (a)-(b) for the sintered
PC-Sm2O3, (c)-(d) for annealed mcF-Sm2O3.
Irradiation time tir under a given atmosphere andkind of atmosphere are indicated by each curve.
To display the spectral structure in detail, thespectra are appropriately rescaled. The rescaling
factor is given by each curve. Since mcF-Sm2O3 isvery thin, several hundred nanometers in thickness, the apparent PL intensity is weaker than thatof the PC-Sm2O3 of 2 mm in thickness. These PL
spectra of the specimens fully irradiated in vacuum
are quite similar to those of Sm2+-doped materials [5]. The PL contribution from the silica glass
substrate is about one-tenth of that of the mcFSm2O3. As the common feature of the sintered PCSm2O3 and the annealed mcF-Sm2O3, it has beenfound that the 325 nm laser light irradiation in
vacuum increases considerably the PL intensity ofboth the specimens, while that in oxygen gas
decreases the PL intensity to recover. The maximum PL enhancement factor, and the ratio of theintegrated PL intensities after evacuation to that
before evacuation, are 100 and 17 for the mcFSm2O3 and the PC-Sm2O3, respectively. The
spectra shown in Fig. 3(a) exhibit many indistinct
structures between 400 and 650 nm. It is known
that most of the Sm3+-doped materials exhibit theFig. 2. The PL color change in the sintered Sm2O3 powder compact: (a) in oxygen gas, (b) in vacuum.

ARTICLE IN PRESS
S. Mochizuki / Physica B 340-342 (2003) 944–948

Fig. 3. The PL spectral transition under 325 nm laser-lightirradiation: (a) sintered Sm2O3 powder compact (PC) in oxygengas, (b) sintered Sm2O3 powder compact (PC), (c) annealed
Sm2O3 powder microcrystal ﬁlm (mcF) in O2 gas and (d) annealed Sm2O3 microcrystal ﬁlm (mcF) in vacuum. The PLE
spectra of the intense white light emitting part is also given in (e).

947

of the mcF-Sm2O3 and the PC-Sm2O3. As a typicalresult, the PLE spectra of PC-Sm2O3 are shown in
Fig. 3(e). These PLE spectra are very similar tothat of Sm2+-doped materials [7,8]. The tail a isassigned to the charge transfer absorption, while
the bands b; g1 ; g2 ; d1 ; d2 ; d3 and d4 between 330
and 500 nm are attributable to the partiallyallowed 4d–5f transitions of Sm2+ ion. The band
Z may be assigned to the F centers, i.e., electron
captured by anion (oxygen) vacancies [9]. The
close similarity between the present results andthose of Sm2+ ion-doped materials indicate thatthe intense white luminescence closely related tophotoinduced valence-number change (Sm3+Sm2+) and photoinduced oxygen vacancies.
Finally, the reduction of Sm2O3 surface under
325 nm laser light irradiation in vacuum may be
accompanied by creation of both valence-number
change of samarium ion (Sm3+-Sm2+) and aelectron-captured oxygen defects, which can explain the observed white PL. Since microcrystals
have larger speciﬁc surface than the sinteredpowder compact, the photoreduction may occur
with high yield. This may be a reason why the
microcrystal ﬁlm specimen has higher enhancement factor for the photoinduced PL spectralchange.

Acknowledgements
K11/2-6HJ, 4G7/2-6HJ, 4H3/2-6HJ, 4G5/2-6HJ(J ¼ ð2n þ 1Þ=2) in this wavelength region, while
C-type Sm2O3 powder do indistinct PL spectral
structure [6,7]. Since the energy level structure of
4f electrons depends on the crystal ﬁeld [7], such
indistinct PL spectrum is ﬁrst explained by the
mixture of the B-type Sm2O3 in the C-type one,as observed in the XRD data shown in Fig. 1.
The defects and the reconstruction of atomic
arrangement at surfaces affect also the crystal
ﬁeld, namely, the energy level structure of 4f
electrons in microcrystal specimen. Besides them,reabsorption effect is also important, which gives
several dips in the PL spectra to become moreindistinct [6].
The PLE spectra of the white band have been
also measured at different lob in the white PL band
4

This work is partially supported by a Grant-inAid for Scientiﬁc Research from the Ministry of
Education, Science, Sports, Culture and Technology, Japan. This work is supported by Interdisciplinary General Joint Research Grant for Nihon
University. This work is also partially supportedby Project Research Grant from The Institute ofInformation Sciences of College of Humanities
and Sciences (Nihon University) and by Cooperative Research Grant from The Institute of Natural
Sciences (Nihon University).

References[1] S. Mochizuki, Y. Suzuki, T. Nakanishi, K. Ishi, Physica B
308–310 (2001) 1046.

ARTICLE IN PRESS
948

S. Mochizuki / Physica B 340-342 (2003) 944–948[2] S. Mochizuki, T. Nakanishi, Y. Suzuki, K. Ishi, Appl. Phys.
Lett. 70 (2001) 3785.
[3] K.A. Gschneid Jr, L.R. Eyring, Handbook on the Physics
and Chemistry on Rare Earths, Vol. 3, North-Holland,
Amsterdam, 1979.
[4] Powder Diffraction File: Inorganic Phases Sets 1-50,
International Center for Diffraction Data, Pennsylvania,
2000.

[5] Q. Zeng, N. Kilah, M. Rieley, J. Lumin. 101 (2003) 167.
[6] J.-F. Martel, S. Jandl, A.M. Lejus, B. Viana, D. Vivien,
J. Alloys and Compounds 275–277 (1998) 353.
[7] J.-F. Martel, S. Jandl, B. Viana, D. Vivien, J. Phys. Chem.
Solid 61 (2000) 1455.
[8] M. Nogami, J. Non-Cryst. Solids 259 (1999) 170.
[9] K. Tanaka, S. Okamoto, Y. Kanemitu, T. Kushida,
J. Lumin. 94–95 (2001) 519.

ARTICLE IN PRESS

Physica B 340–342 (2003) 956–959

Photoluminescence study on defects in pristine anatase andanatase-based composites
Shosuke Mochizuki*, Takashi Shimizu, Fumito Fujishiro
Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan

Abstract
Under 325 nm laser light irradiation at room temperature in oxygen gas, the photoluminescence (PL) intensity ofpristine anatase TiO2 decreases considerably and then it recovers reversibly with the same laser light irradiation at room
temperature in vacuum. This result gives important information on the oxygen defects at anatase particle surfaces andmay also clarify the origin of largely Stokes shifted broad PL band in pristine anatase. New reversible photoinduced
spectral change has been also found in anatase–AgI composites for replacing atmosphere (vacuum2oxygen gas) under
325 nm laser light irradiation at room temperature. This spectral change may arise from the excitation energy transfer
between excitons in AgI domain and photoinduced oxygen defects at the anatase particle surfaces.
r 2003 Elsevier B.V. All rights reserved.
PACS: 68.35.Fx; 71.35.Cc; 78.55.Hx; 78.68.+m
Keywords: Defects in oxides; Defects at/near surfaces and interfaces

1. Introduction
The physical and chemical properties of pristine
anatase TiO2 and anatase-based composites havereceived a much attention as novel materials forphotocatalyst [1]. Despite much effort, the photoabsorption and photoluminescence (PL) in anatase
are still unclear. Pristine anatase single crystal iseasily grown by the chemical vapor transport
reaction (CVTR) method using halogen as thetransport agent and it has been frequently used as
the specimen for PL studies. Hitherto, the maininterest of physicists was concentrated on the*Corresponding author. Tel./fax.: +81-3-5317-9771.
E-mail address: motizuki@physics.chs.nihon-u.ac.jp(S. Mochizuki).

origin of considerably Stokes shifted broad PL
band observed for the single crystal grown by the
CVTR method [2]. To explain such PL spectrumof pristine anatase, the self-trapped exciton model
has been frequently used [3]. However, anatase, as
well as rutile, is well known as an oxygen-deﬁcientoxide and the crystal grown by the CVTR
method tends to contain impurities and defects
related to the agent. Therefore, one should pay
attention to defect structures both at the surfaces
and on the inside of pristine anatase crystal,for the interpretation of the obtained spectra. It
is desired to know how oxidation and reduction
affect the optical properties of pristine anatase,except for impurity effects. For reply to these
problems, it is better to use powder specimen
than bulk specimen, since the powder specimen

0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2003.09.172

ARTICLE IN PRESS
S. Mochizuki et al. / Physica B 340–342 (2003) 956–959

has large speciﬁc surface, and, therefore, it iseasily oxidized and reduced to show sensitively
the effects related to surface defects. Recently,
a broad PL band has been induced in simple
oxides and rare-earth metal oxides [4–7] byphotoreduction at room temperature. The broad
PL band appears at the same wavelength region
where anatase displays, and it can be recovered
reversibly by photooxidation at room temperature. This photoreduction–photooxidation experiment may promise to clarify effects of oxygen
defects on the optical properties of pristine
anatase.
In the present paper, we report the photoinduced spectral change in pristine anatasepowder compacts, together with single crystal
bulk, and new photofunction in anatase–AgI
composites.

957

3. Results
Before describing the results of optical measurements on different anatase specimens, we point out
general optical properties obtained for anatasepowder compacts. When the anatase powder is putin vacuum, the color changes gradually from white
to pale yellow even at room temperature underroom light (usual ﬂuorescent light). When PL
measurement is performed on the anatase specimen in an evacuated specimen chamber, the samechange may occur at the thin surface layer of thespecimen.
Figs. 1(a) and (b) show the PL spectral change
obtained in pristine anatase powder compact
under 325 nm laser light by replacing specimen
atmosphere. Experiments are carried out inalphabetical order: (a)-(b). Irradiation is carried

2. Experimentalin oxygen gas

Crystalline pristine anatase powder was obtained by heating amorphous titanium dioxide ﬁneparticles of 99.9% purity (with an average
diameter of 50 nm) at 673 K in air for 24 h. Powder
compact specimens were prepared by compressing
the powder under a pressure of 0.2 GPa at room
temperature. They were then sintered at 673 K in
ﬂowing oxygen gas for 3 h. Anatase single crystal
was grown by the CVTR method with NH4Cl
agent. Anatase–AgI composites, (x)anatase(1  x)AgI, were prepared by fully mixing AgI
powder and anatase powder in air, heating the
mixture in the lidded crucible at 673 K for 12 h in
air and then annealing at 373 K for 12 h to remove
the stress introduced at the phase transition pointof AgI, 420 K. The specimens were characterized
at 300 K by X-ray diffraction analysis with Cu Ka1
radiation.
Photoinduced spectral change experiments werecarried out with a continuous wave of He–Cd laserline (wavelength l ¼ 325 nm) with a power
density of 5.4 kW/m2. Emitted light was dispersedand detected by using a grating spectrographequipped with a multichannel photodetection
system. All the experiments were carried out atroom temperature.

tir = 0 min

3(c)

tir = 30 min

PL intensity (arb. scale)4in oxygen gas(b)tir = 0 min

34

tir = 30 min

in vacuum(a)tir = 30 min

2
tir = 0 min

400

450

500

1
550

600

650

700

Wavelength (nm)Fig. 1. Reversible photoinduced PL spectral change in anatasepowder compact by replacing specimen atmosphere at room
temperature: (a) in vacuum; (b) in oxygen gas; and (c)comparison of curve 3 with curve 4.

ARTICLE IN PRESS
S. Mochizuki et al. / Physica B 340–342 (2003) 956–959

out at room temperature. Irradiation time tir undera given atmosphere and the kind of atmosphere
are indicated by each curve. The PL intensity of
the anatase powder compact is considerablyincreased by irradiating in vacuum of
1.33  104 Pa for 30 min, as shown in Fig. 1(a):curve 1-curve 2. The PL enhancement factor for
30 min, the integrated PL intensity ratio of thecurve 2 to the curve 1, is about 3.5. Oxygen gas of
0.2 MPa is then introduced into the specimen
chamber under 325 nm laser light irradiation. With
increasing tir ; PL intensity decreases, as seen in the
spectrum shown in Fig. 1(b): curve 3-curve 4. To
see more clearly the spectral change, the curves 3
and 4 are compared by rescaling the intensity of
the curve 4, as shown in Fig. 1(c). The rescaling
factor is 4.65. It is noted that the 325 nm laser lightirradiation in oxygen gas makes the PL component
at longer wavelengths than 520 nm. In the present
experiments, the 325 nm laser light irradiation in
vacuum and in oxygen gas may correspond tophotoreduction and photooxidation, respectively.
Oxygen gas is again evacuated. The change fromweak PL emission to intense one can be seen with
the naked eye. Through many successive experiments, it is found that such the spectral changes
are almost reversible, apart from PL intensity. The
enhancement factor tends to decrease with further
repeating atmosphere change.
It has been also found that single crystal by the
CVTR method shows also similar spectral change.
The results are given in Figs. 2(a) and (b). To see
more clearly the photooxidation effect, the curve 30
and curve 40 are compared by rescaling theintensity of the curve 40 in Fig. 2(c). The rescaling
factor is 1.77. However, the spectral change is not
so prominent as the powder compact specimen.
This may be due to the decreased speciﬁc surfaceof bulk specimen.
Sekiya et al. [8] annealed the CVTR anatasesingle crystals at different temperatures in oxygengas and studied their PL properties. They foundthat PL spectrum consists of three components, F,
E and D, centred at about 517, 577 and 636 nm,
respectively. The components F, E and D are
assigned to excitons bound to partially reduced
titanium ions, self-trapped excitons and oxygen
vacancies, respectively. If the defect structure ofin oxygen gas(c)tir = 0 min

3'

tir = 30 min

4'

PL intensity (arb. scale)958

in oxygen gas(b)tir = 0 min

3'
tir = 30 min

4'

in vacuum(a)tir = 30 min

2'
tir = 0 min

400

450

500

1'
550

600

650

700

Wavelength (nm)Fig. 2. Reversible photoinduced PL spectral change in anatasesingle crystal by replacing specimen atmosphere at room
temperature: (a) in vacuum; (b) in oxygen gas; and (c)comparison of curve 30 with curve 40 .

the CVTR anatase single crystal is identical to thatof the photoirradiated powder compacts, it may beconcluded that the broad PL band reported bymany workers arises not from intrinsic selftrapped excitons but from oxygen defects or
impurities.
Fig. 3 shows the reversible photoinduced PL
spectral change in the (0.5)anatase–(0.5)AgI composite at room temperature. After fully irradiatingwith 325 nm laser light in an oxygen-gas-ﬁlled
specimen chamber, the specimen displays a sharp
PL band due to the radiative decay of excitons in
AgI at about 426 nm [9] and a weak broad PL
band related to anatase, as indicated by curve 100 .
Oxygen gas is then evacuated from the specimen
chamber. With increasing tir ; the sharp AgI-relatedband becomes annihilated, while the anataserelated broad band becomes pronounced, as
indicated by curve 200 . The specimen chamber is

ARTICLE IN PRESS
S. Mochizuki et al. / Physica B 340–342 (2003) 956–959

5in oxygen gas

PL intensity (arb. units)surfaces [5]. These changes may clarify the originof the PL observed in pristine anatase and may
well yield materials for optical sensor devices, forexample, optical oxygen-partial-pressure sensor.

(b)

4

3''

3
tir= 0 min

2

tir= 30 min

1

4''

Acknowledgements

0
5

in vacuum

(a)

2''

4
3

tir= 30 min

2

1''
1
0
400

959

tir= 0 min
450

500

550

600

650

700

Wavelength (nm)Fig. 3. Reversible photoinduced PL spectral change in(0.5)anatase–(0.5)AgI composite by replacing specimen atmosphere at room temperature: (a) in vacuum; and (b) in oxygengas.

again ﬁlled with oxygen gas. The sharp AgI-relatedband reappears as shown in Fig. 3(b): curve 300 curve 400 , and it prevails against the anatase-related
broad band. Through many successive experiments, it is found that such the spectral changes
appear repeatedly. The observed spectral changes
may be explained by excitation energy transfer
between free excitons in AgI domain and photoinduced oxygen defects at the surfaces of anataseparticle.
Finally, the spectral changes observed in thepresent experiments arise from photoinduced
dissociative adsorption and photoinduced associative detachment of O2 molecule near the specimen

This work is partially supported by a Grant-inAid for Scientiﬁc Research from the Ministry of
Education, Science, Sports, Culture and Technology, Japan. This work is supported by Interdisciplinary General Joint Research Grant for Nihon
University. This work is also partially supportedby Project Research Grant from The Institute ofInformation Sciences of College of Humanities
and Sciences (Nihon University) and by Cooperative Research Grant from The Institute of Natural
Sciences (Nihon University).

References[1] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem.
Photobiol. C 1 (2000) 1.
[2] H. Bergar, H. Tang, F. Levy, J. Crystal Growth 130 (1993)
108.
[3] H. Tang, H. Berger, P.E. Schmid, F. Levy, Solid State
Commun. 92 (1994) 267.
[4] S. Mochizuki, Y. Suzuki, T. Nakanishi, K. Ishi, Physica B
308–310 (2001) 1046.
[5] S. Mochizuki, T. Nakanishi, Y. Suzuki, K. Ishi, Appl. Phys.
70 (2001) 3785.
[6] S. Mochizuki, Physica B (in press 28 July 2003).
[7] S. Mochizuki, H. Araki, Physica B (in press 28 July 2003).
[8] T. Sekiya, S. Kamei, S. Kurita, J. Lumin. 87–89 (2000)
1140.
[9] S. Mochizuki, Y. Ohta, J. Lumin. 87–89 (2000) 299.

ARTICLE IN PRESS

Physica B 340–342 (2003) 969–973

UV-laser-light-produced defects and reversible blue–whitephotoluminescence change in silica
Shosuke Mochizuki*, Hiroyuki Araki
Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan

Abstract
The photoluminescence (PL) color of silica glass changes from weak blue to intense white under irradiation with325 nm laser light at room temperature in vacuum. Irradiation at room temperature with the same laser lightunder oxygen gas atmosphere erases the white PL state. Through many successive experiments, it is found thatthe spectral change is a reversible phenomenon. Such intense white PL state is stored for a long time at room
temperature in air under room light after removal of the laser light, in spite of any changes of atmosphere. The observed
phenomenon may relate to the photo-induced defect formation at the silica glass surface. Such observed reversible
phenomenon may well yield materials for erasable optical information storage, oxygen sensors and white-lightemitting devices.
r 2003 Elsevier B.V. All rights reserved.
PACS: 73.20.H; 82.50.F; 42.79.V; 78.55
Keywords: Defects in silica; White-light emission; Photomemory

1. Introduction
Silica glass has been widely used as opticalelement and is of great interest for applications in
optoelectronics. Most of commercially available
silica glasses, however, display unwanted photoluminescence (PL) bands in the ultraviolet (UV)and visible light regions, and the glasses aredamaged by intense UV-laser-light to give additional PL bands [1,2]. On the other hand, it has
been well known that UV light induces a Bragg
grating in Ge-doped silica glass ﬁber [3], which
gives useful optoelectronics devices. The photo*Corresponding author. Tel./fax: +81-3-5317-9771.
E-mail address: motizuki@physics.chs.nihon-u.ac.jp(S. Mochizuki).

induced structural and electronic defects, together
with pre-existing PL centers, may play an important role for the formation of both the UVlight-induced PL centers and the Bragg grating.
Therefore, the defects in silica glass should be fully
characterized. Although different types of studies
have been carried out in silica glass, the defectstructure has been not fully elucidated at present.
Very recently, we have observed for the ﬁrst time
the reversible UV (325 nm)-laser-light-induced
spectral change and intense white PL state indifferent silica glasses, as observed in Eu2O3 [4,5]and Sm2O3. The spectral change may relate closely
to photo-induced defects. In this paper, we report
the experimental results in detail and discuss
UV-laser-light-induced defects in silica glass.

0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2003.09.271

ARTICLE IN PRESS
970

S. Mochizuki, H. Araki / Physica B 340–342 (2003) 969–973

2. Experiments
In the present study, three different commercially available silica glasses of 1 mm thickness
were used as specimens, since laser-irradiation
effects are thought to be sensitive to adsorbed OH.
One is a T-1030 silica glass (Toshiba Ceramics
Co.) which contains a large amount of OH(200 ppm in weight ratio) and it displays considerable optical absorption (optical density OD=0.36)
due to OH at 3650 cm1. This glass contains also asmall amount of aluminum (8 ppm in weight ratio)
as main metal impurity. The others are GE-214
and GE-124 silica glasses (General Electric Co.).
They contain a small amount of OH (less than
5 ppm in weight ratio). The GE-214 glass neverdisplays the optical absorption due to OH, while
the GE-124 glass shows little optical absorption(OD=0.09) at 3650 cm1. Both the GE-214 and124 glasses contain also larger amounts ofaluminum (14 ppm in weight ratio) than theT-1030 glass as main metal impurity. Moredetailed chemical data are given on the Toshiba
website (www. tocera.co.jp) and the GE website(www.gequartz.com).
Irradiations were carried out with a continuous
wave (CW) He–Cd laser line (wavelengthl ¼ 325 nm) with a power density of about
5.4 kW/m2. The same He–Cd laser line excites
luminescence. Emitted light is dispersed anddetected by using a grating spectrograph equippedwith a multichannel photodetection system. Vacuum of about 104 Pa and oxygen gas of about

0.1 MPa are used for control of the atmospherearound the specimen.

3. Results and discussion
Under 325 nm laser-light irradiation in oxygen
atmosphere at room temperature, the T-1030, GE124, and GE-214 glasses display weak blue PL
color which can be seen with the naked eye. As atypical example, this PL color of the T-1030 glass
is shown in Fig. 1(a). The specimen chamber isthen evacuated under 325 nm laser-light irradiation. With increasing irradiation time tir ; the silica
glasses change to show white PL with increasing
intensity. About 90 min after the evacuation, abright white PL can be easily seen with the naked
eye. As a typical example, this PL color of theT-1030 glass is shown in Fig. 1(b). Through manysuccessive experiments, it is found that the PLcolor changes appear repeatedly. After removal of
the laser light, the white PL state is stored for along time at room temperature in air under room
light. Figs. 2(a)–(d) show the reversible PL spectralchange of the T-1030 glass. Figs. 3(a)–(d) show thereversible PL spectral change of the GE-214 glass.
Irradiation was carried out with 325 nm laser line
at room temperature. Experiments were carriedout in alphabetical order: (a)-(b)-(c)-(d). The
tir under given atmosphere and kind of atmosphere
are indicated by each curve. Almost the same
spectral change has also observed for the GE-124
glass. Besides the above-described results, throughFig. 1. Photographs of the T-1030 silica glass fully irradiated with 325 nm laser light in each atmosphere: (a) oxygen gas of about
0.1 MPa; and (b) vacuum of about 104 Pa.

ARTICLE IN PRESS
S. Mochizuki, H. Araki / Physica B 340–342 (2003) 969–973

971

Fig. 2. Reversible PL spectral change of the T-1030 silica glassat room temperature: (a,c) in vacuum; and (b,d) in oxygen.

Fig. 3. Reversible PL spectral change of the GE-214 silica glassat room temperature: (a,c) in vacuum; and (b,d) in oxygen.

present experiments, the following conclusions
were also found for different silica glasses:

molecules is considerably decreased at the silica
glass surface by the interactions, between O2
molecules and the silica glass.
In order to extract the white PL component,
we have calculated the difference spectrum (curve
2–curve 1) for the T-1030 glass. The result isshown in Fig. 4(a). In Fig. 4(b), the difference
spectrum (curve 20 –curve 10 ) for the GE-214 glass
is shown. The T-1030 glass fully 325 nm-laserlight-irradiated in oxygen gas shows a PL band
peaked at 3.15 eV and a broad PL shoulder
ranging from about 3.0 eV to about 2.0 eV. The
T-1030 glass fully 325 nm-laser-light-irradiated in
vacuum shows the 3.15 eV PL band and an intense
broad PL band centered at 2.20 eV. With repeating
atmosphere change, the intense broad PL band
redshifts to 2.10 eV. In the GE-214 glass fully
325 nm-laser-light-irradiated in oxygen gas, the PL
intensity of the 3.13 eV band is enhanced by twicethat of the T-1030 silica glass and extends to about
2.0 eV without showing a clear shoulder. With

(1) Time is required for the spectral change andthe speed of the spectral change increases withlaser ﬂuence. They indicate that some nucleation and growth processes dominate thespectral change.
(2) The change to the white PL state occurs evenat 7 K under CW 325 nm laser light. This
indicates that the spectral change does notarise from a phonon-assisted process and it isa purely electronic mechanism.
The spectral change observed in silica glass
arises from photo-induced-associative detachment
under vacuum and dissociative adsorption of O2
molecules under oxygen atmosphere near the
specimen surface. Since the 325 nm laser lightcannot dissociate directly free O2 molecules asatmospheric gas, the photo-induced oxidation
indicates that the photodissociation energy of O2

ARTICLE IN PRESS
972

S. Mochizuki, H. Araki / Physica B 340–342 (2003) 969–973

Fig. 4. Difference spectra: (a) (curve 2–curve 1) for the T-1030
glass; and (b) (curve 20 –curve 10 ) for the GE-214 glass.

repeating atmosphere change, the intense broad
PL band redshifts to 2.15 eV. After fully 325 nm
laser-light irradiation in vacuum, the 2.20 eV PL
band appears and the PL intensity is nearly thesame as that of the GE-124 and T-1030 glasses.
The study on the 3.15 eV PL band has beenpreviously reported and the band is tentatively
ascribed to the O3  Si2O2O2H group, thetwofold coordinated silicon lone-pair center.
ðO2Si2OÞ
and a germanium impurity [6]. From
the weaker 3.15 eV-PL-band observed in the T1030 glass containing higher OH and lower
aluminum than the GE-214 glass may exclude
the O3  Si2O2O2H group, but we cannot
exclude some aluminum-impurity-induced defects
as a possible origin of the 3.15 eV PL band.
The white PL band photo-induced undervacuum has a peak energy (Ep ) of 2.20 eV and afull-width at half-maximum (FWHM) of about
0.8 eV. Apart from the FWHM, this PL band
resembles the PL band observed in g-ray-irradiated silica glass under 325 nm excitation [7] andsilica glasses damaged by high-power laser light [8]in Ep ; and spectral shape. The g-ray induced 2.2 eV
PL band has been attributed to the radiation froma paramagnetic defect in which one electron isdelocalized over a Si5 cluster. Such a paramagnetic
defect is known as the Ed0 center [7]. Anotherexplanation for the 2.2 eV PL band (g band)
appeared in glass damaged by high-power laser
light under 250 nm excitation and is based on theradiative decay of trapped excitons on thevacancy–interstitial pair consisting of an oxygen
vacancy and two oxygen interstitials ðVO ; ðO2 Þi Þ [8].
Since the PL yield of the laser-light damaged silica
glass at 325 nm, however, is negligibly small andthe FWHM (0.5 eV) of the PL band is relatively
small, the defect structure may be different fromthat induced under 325 nm laser-light irradiation in
vacuum. As described above, the 2.2 eV PL band
observed at the present study is related tophotoreduction at the silica glass surface. In sucha case, we assume that Ed0 -like defects andðVO ; ðO2 Þi Þ-like states may be produced at thesurface. The appearance of defect levels at the silica
glass surface decreases considerably the band gap
at the surface, as observed in gAl2O3 ﬁlm [9] andthen high PL yield at 325 nm can be anticipated.
In conclusion, a new PL color change fromweak blue to intense white was observed indifferent silica glasses under 325 nm laser-lightirradiation in vacuum at room temperature. The
color change was reversed by the same 325 nm
laser-light irradiation in oxygen gas atmosphere atroom temperature. The intense white PL state isstored for a long time at room temperature in air
under room light after removal of the laser light.
The phenomenon is related to the photo-induced
oxygen defect formation at the silica glass surface.
Acknowledgements
This work is partially supported by a Grant-inAid for Scientiﬁc Research from the Ministry of
Education, Science, Sports, Culture and Technology, Japan. This work is supported by Interdisciplinary General Joint Research Grant for Nihon
University. This work is also partially supportedby Project Research Grant from The Institute ofInformation Sciences of College of Humanities
and Sciences (Nihon University) and by Cooperative Research Grant from The Institute of Natural
Sciences (Nihon University).
References[1] S.G. Demos, M. Staggs, K. Minoshima, J. Fujimoto, Opt
Express 10 (2002) 1444.

ARTICLE IN PRESS
S. Mochizuki, H. Araki / Physica B 340–342 (2003) 969–973[2] L. Skuja, J. Non-Cryst. Solids 239 (1998) 16.
[3] K.O. Hill, Y. Fujii, D.C. Johnson, B.S. Kawasaki, Appl.
Phys. Lett. 32 (1978) 647.
[4] S. Mochizuki, T. Nakanishi, Y. Suzuki, K. Ishi, Appl. Phys.
Lett. 79 (2001) 3785.
[5] S. Mochizuki, Y. Suzuki, T. Nakanishi, K. Ishi, Physica B
308–310 (2001) 1046.

973[6] Y. Sakurai, J. Non-Cryst. Solids 271 (2000) 218.
[7] H. Nishikawa, E. Watanabe, D. Ito, Y. Sakurai,
K. Nagasawa, Y. Ohki, J. Appl. Phys. 80 (1996) 3513.
[8] M. Watanabe, S. Juodkazis, H. Sun, S. Matuo, H. Misawa,Phys. Rev. B 60 (1999) 9959.
[9] B. Ealet, M.H. Elyakhlouﬁ, E. Gillet, M. Ricci, Thin Solid
Films 250 (1994) 92.

INSTITUTE OF PHYSICS PUBLISHING

JOURNAL OF PHYSICS: CONDENSED MATTER

J. Phys.: Condens. Matter 15 (2003) 5057–5072

PII: S0953-8984(03)60976-8

Structural, electrical and optical studies on
AgI–anatase composites
Shosuke Mochizuki1 and Fumito Fujishiro
Department of Physics, College of Humanities and Sciences, Nihon University,
3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan
E-mail: motizuki@physics.chs.nihon-u.ac.jp

Received 18 March 2003, in ﬁnal form 3 June 2003
Published 11 July 2003
Online at stacks.iop.org/JPhysCM/15/5057
Abstract
AgI–anatase composites, (x)AgI–(1 − x)anatase, were fabricated over a wide
composition range of 0–100% AgI for the ﬁrst time. The electrical conductivity
at 300 K increases with increasing AgI content and reaches a maximum (about
3×10−2 S m−1 ) at about 40% AgI. The conductivity is enhanced by about threeorders of magnitude in comparison with that of pristine AgI. These composite
specimens have been characterized by both scanning electron microscopy andx-ray diffractometry. The observations with a scanning electron microscope
show that anatase ﬁne particles are densely packed in an AgI particle of several
micrometres in size and such composite particles are three-dimensionallyconnected to each other to form a composite specimen. The photoluminescence(PL), PL excitation and time-resolved PL spectra of these composites have also
been measured at different temperatures between 8 and 278 K. The exciton
spectra obtained related to the anatase particles and the AgI domains in anatasepacked AgI particles are discussed in the light of the morphological, structuraland conductivity data.

1. Introduction
There has been considerable experimental research on ionic conduction in composite solid
electrolytes consisting of ionic conductors and insulator ﬁne particles (Liang 1973, West1984, Shahi and Wagner 1981, Zhao et al 1983, Lee et al 2000a). The electrical andother properties of the composites can be easily controlled by varying the content of theionic conductor component. Among various systems of composites, AgI-based composites
have been extensively investigated by many workers, because they usually show high ionic
conductivity (σ  10−2 S m−1 ) and pristine AgI is well known as a good superionic conductor
at temperatures higher than 420 K (Burley 1963). AgI has three phases designated as α, β and
1 Author to whom any correspondence should be addressed.

0953-8984/03/295057+16$30.00© 2003 IOP Publishing Ltd

Printed in the UK

5057

5058

S Mochizuki and F Fujishiroγ at normal pressures in the order of decreasing temperature with the following properties. At
the superionic transition temperature Tc (420 K), the superionic α phase transforms into thesemiconductor β phase (wurtzite lattice). The α phase has a body-centred cubic arrangementof I− ions with highly mobile Ag+ ions randomly distributed through the equivalent interstices,which has been known as the averaged structure. At room temperature, the semiconductor γphase (zincblende lattice) appears as a metastable state. It is said that the stability of the γ phase
is affected by slight nonstoichiometries and other defects. To date, the conductivity of AgIoxide ﬁne particle composites has been reported in various composites: AgI–Al2 O3 , AgI–SiO2 ,AgI–MgO, AgI–Fe2 O3 (Shahi and Wagner 1981, Zhao et al 1983), AgI–ZrO2 , AgI–CeO2 ,AgI–Sm2 O3 , AgI–MoO3 , AgI–WO3 (Shastry and Rao 1992) and AgI–SnO2 (Gupta et al1996). In all these cases, their electrical conductivities were enhanced by two or three ordersof magnitude higher than that of pristine AgI and a maximum conductivity occurred at proper
AgI content. For example, the maximum occurs in (x)AgI–(1 − x)γ Al2 O3 composites withvery ﬁne alumina particles at x = 0.5–0.6, while composites with larger alumina particles
showed the maximum at smaller AgI content. Such conductivity variation is initially explainedby taking into account a space charge layer around a nucleophilic surface (Jow and Wagner1979). However, several basic problems were raised about AgI-based composites. Why dooxides enhance the conductivity? How does AgI attach to oxide ﬁne particles? What are
the atomic and electronic structures of the AgI/insulator ﬁne particle interface? In orderto solve these problems, it is thought that optical study is effective. Constituent material
AgI shows clear optical spectra due to excitons in the bulk (Mochizuki and Fujishiro 2002),
ﬁlms (Cardona 1963, Mochizuki and Ohta 2000) and microcrystals (Mochizuki and Umezawa
1997). Besides AgI, most oxides also show exciton spectra. The exciton behaviour is very
sensitive to the environment in which excitons are moving, and therefore the optical spectra
due to excitons may provide useful information about the AgI/oxide ﬁne particle interface(Mochizuki and Fujishiro 2003). Very recently, we have fabricated AgI–anatase composites
and observed considerable conductivity enhancement. Since AgI is very photosensitive andanatase (TiO2 ) is known as a typical oxide photocatalyst for decomposing water and organic
matter (Fujishima et al 2000), we can anticipate not only a new solid electrolyte nature but
also a new photofunction for such an AgI–anatase ﬁne particle composite.
In this paper, we report the morphological, structural, electrical and optical studies on
AgI–anatase composites and we discuss the conductivity enhancement of the composites bycomparing the scanning electron microscope images with the data obtained.
2. Experimental details
AgI–anatase composites, (x)AgI–(1 − x) anatase, were prepared by fully mixing AgI powder
and amorphous titanium dioxide ﬁne particles (with an average diameter of 50 nm) in air,
heating the mixture in the lidded crucible at 673 K for 12 h in air and then annealing at 373 K
for 12 h to remove the stress introduced at the phase transition point of AgI, 420 K. The
composition range was from 0 to 100% AgI. At 300 K, the composites were characterized by
x-ray diffraction (XRD) analysis with Cu Kα1 radiation. The composite powder morphology
was studied by a scanning electron microscope (SEM) and an optical microscope with acharge-coupled device camera system. Distribution maps of Ag, I, Ti and O atoms in thecomposites were obtained by using a SEM with an energy dispersive x-ray (EDX) ﬂuorescence
spectrophotometer (Shimadzu SSX-550).
Electrical conductivity measurements were performed in a helium gas stream using theimpedance method. The applied signal frequency ranged from 42 Hz to 1 MHz. The composite
powders were uniaxially compressed under 0.2 GPa for 30 min at room temperature to fabricate

Structural, electrical and optical studies on AgI–anatase composites

5059

pellets. It has been reported that the γ AgI/βAgI ratio is strongly affected by compressing,
grinding and the thermal history of specimens. Such an effect, a so-called memory effect, iserased by heat treatment above 445 K (Bloch and Möller 1931, Burley 1967). Therefore, thepellets were heat-treated at 453 K for 12 h in a lidded crucible and then annealled at 373 K for
12 h. Electrical contact with the pellets was obtained by evaporating silver onto both sides ofthe pellet.
The photoluminescence (PL) spectra were measured by an optical multichannel analyser
consisting of a grating monochromator (focal length = 32 cm) and an intensiﬁed diode-array
detector (gate time: tg  5 ns). A Nd 3+ :YAG laser (wavelength λ = 355 nm, pulse
width < 5 ns) and a monochromatic light source consisting of a 150 W xenon lamp anda grating monochromator (focal length = 20 cm) were used as the excitation source. The
photoluminescence excitation (PLE) spectra were recorded by varying the excitation-lightwavelength λex with an apparatus consisting of the same monochromatic light source describedabove and detecting the emission light intensity at the desired wavelength as a function of
λex with a grating monochromator (focal length = 20 cm) and a synchronous light detection
system. The time-resolved PL (TRPL) spectra were taken using the same optical multichannel
analyser to which two delay pulse generators were attached. The delay time td and gate time
were set with these delay pulse generators which were controlled by a personal computer. A
closed-cycle He refrigerator equipped with a temperature controller was used to change thespecimen temperature between 8 and 278 K.
3. Results and discussion
3.1. Characterization of AgI–anatase composite specimens
The apparent density of the (x)AgI–(1 − x)anatase composites at about 300 K is plotted,
together with the theoretical density, as a function of AgI content x in ﬁgure 1. The
full line and full circles represent the theoretical and experimental (observed) densities,
respectively. Theoretical densities were calculated by using the reported densities of pristine
AgI (5960 kg m−3 ) and pristine anatase (3880 kg m−3 ). The porosity variation with x isshown in the inset. A remarkable difference between the theoretical and experimental density
is observed in the smaller x region. Such a result is known behaviour for the compaction of
mixtures of particles with extremely different hardnesses. The porosity variation with x may
affect the apparent conductivity of the composites.
We studied the surface morphology of (x)AgI–(1 − x)anatase specimens by SEM
observations both on the raw surfaces and the fractured ones. Figures 2(a) and (b) show,respectively, the low- and high-magniﬁcation SEM images for the surface of the (0.4)AgI–(0.6)anatase composite. The EDX ﬂuorescence measurements indicate that large particles andsmall particles are AgI and anatase, respectively. As seen in these ﬁgures, anatase ﬁne particles
are densely packed in AgI particles of several micrometres in size. Henceforth, we call thisparticle the ‘anatase-packed AgI particle’. As shown in these ﬁgures, small anatase particles
are not so different in size from the initial average particle size of 50 nm, while other anataseparticles coagulate to form irregularly shaped particles of several hundred nanometres in size.
The EDX ﬂuorescence measurements indicate that iodine-rich parts tend to appear inside porewalls and that the surroundings of anatase particles are silver-rich. The latter suggests that
Ag+ ions are adsorbed onto the anatase particle surfaces.
Figure 3 shows the XRD patterns of (x)AgI–(1 − x)anatase composites at 300 K. All
XRD patterns in this ﬁgure are normalized at the peak intensity. Amorphous titanium dioxide
powder, as one of the starting materials, was fully transformed into crystalline anatase by5060

S Mochizuki and F Fujishiro(x)AgI-(1-x)anataseT~ 300 K

5000

3

Density (kg/m )theoretical

4000
70
60

Porosity (%)

experimental
3000

50
40
30
20
10
0

2000

0.0 0.2 0.4 0.6 0.8 1.0

AgI content, x
0.0

0.2

0.4

0.6

0.8

1.0

AgI content, x
Figure 1. Density of different (x)AgI–(1− x)anatase composites at about 300 K. Porosity variation
with x is shown in the inset.

heating at 673 K in air for 12 h. With increasing x, some diffraction lines related to AgI
become pronounced. All diffraction lines can be almost assigned to γ AgI, βAgI and anatase.
It seems to be a mere mixture of AgI and anatase. This is very different from that observed in
AgI–γ Al2 O3 composites (Lee et al 2000a, Mochizuki and Fujishiro 2003). In the AgI–γ Al2 O3composites, some broadening of the peaks and weakening of the (h0l) and (100) peaks of βAgI
become pronounced with increasing x, and some new diffraction lines appear. The new lines
can be explained by taking account of the seven-hexagonal-layer polytype AgI (7H-AgI) withthe stacking sequence ABCBCAC (Lee et al 2000a, 2000b). In the present AgI–anatase
composites, however, such polytype-structure lines as observed in AgI–γ Al2 O3 composites
never appear, as shown in the inset. In the inset, the full curve represents the XRD pattern ofthe (0.7)AgI–(0.3)anatase composite, while the broken curve represents that of the (0.73)AgI–(0.27) γ Al2O3 composite which displays clear (1010) and (1011) lines of the polytype structure.
Some broadening is also observed on the βAgI (100) and (002) lines for the small x region.
Such line broadening becomes diminished with increasing x. The broadening may arise mainly
from reduced crystallite size. The intensity ratio of the βAgI (002) line, which coincides withthe γ (111) peak to the βAgI (100) line, increases from 1.919 to 3.384 with increasing x
from 0.2 to 0.8. Such an increase may be explained by either a preferred orientation of βAgI
crystallites or an increased γ AgI component in the anatase-packed AgI particles, accompaniedby stacking disorder in hexagonal βAgI. However, it is noted that the βAgI (101), βAgI (102)and βAgI (103) lines almost disappear, while the γ AgI (331) line becomes sharpened with

Structural, electrical and optical studies on AgI–anatase composites

5061

Figure 2. SEM image of the (0.4)AgI–(0.6)anatase: (a) at low magniﬁcation (×5000), (b) at highmagniﬁcation (×30 000).

increasing x, as shown in ﬁgure 3. Therefore, to explain the XRD patterns obtained, thecontribution from the increased γ AgI component should be more taken into account than thepreferred orientation. In other words, the AgI domain in the anatase-packed AgI particles
consists of many γ AgI crystallites and a small number of βAgI crystallites. Referring to the
known result that a silver-deﬁcient AgI solution crystallizes in the β phase and silver-rich AgI
solutions do so in the γ phase, this is interpreted as follows. Preferential Ag+ ion adsorption onanatase surfaces, as shown in the SEM and EDX ﬂuorescence data, induces some concentration
gradient of Ag+ ions in the AgI domain. The gradient produces then γ AgI crystallites andβAgI ones in the AgI domain. The degree of the gradient is dependent on the anatase content
and, therefore, the diffraction intensity ratio of the γ AgI line and the βAgI line varies with x,
as shown in ﬁgure 3. On the other hand, the linewidth of the broad anatase (101) line is foundto be almost independent of x. It may indicate that the average size of the anatase particle is
not considerably changed by heating. This is consistent with the SEM observation shown inﬁgures 2(a) and (b).

44

β(112), γ(311)48γ(331)

40(1011)β(110), γ(220)36

β(103)

x= 0.8β(112), γ(311)β(110), γ(220)β(102)

x= 1.0β(100)

Intensity (arb. scale)β(002), γ(111)(x)AgI-(1-x)anatase
T= 300 K(1010)S Mochizuki and F Fujishiroβ(103)

5062

x5

x5

x= 0.6x5x= 0.4
x5

20

30

40

50

(204)

(105)
(211)

x= 0.0

(200)

(103)
(004)
(112)

(101)

x= 0.260

2θ (degree)
Figure 3. XRD patterns for different (x)AgI–(1 − x)anatase composites at 300 K. The XRD
pattern for (0.7)AgI–(0.3)anatase (full curve) is compared with that for (0.73)AgI–(0.27)γ Al2 O3(broken curve) in the inset.

3.2. Electrical conductivity
The conductivity σ of (x)AgI–(1−x)anatase composites at 300 K is plotted against x in ﬁgure 4.
The applied frequency was 1 kHz. No frequency dependence of σ was observed between 42 Hz
and 100 kHz. Because of experimental difﬁculties, the transference numbers for ionic andelectronic charge carriers have not been directly determined. However, tentative measurementson the transient current of the asymmetric cell of type (Ag/(x)AgI–(1 − x)anatase/Au) showa small steady-state current. Therefore, we cannot perfectly exclude the contribution fromelectronic conduction from the present discussion. As seen in this ﬁgure, σ increases abruptly
with increasing x. On further increasing x, σ reaches a maximum (about 3 × 10−2 S m−1 ) at
x = 0.4. Since the σ for a pristine AgI specimen prepared in the same way is 1.7×10−5 S m−1 ,the conductivity is enhanced by about three orders of magnitude. In the x region larger
than 0.4, σ falls gradually toward 1.7 × 10−5 S m−1 for pristine AgI. The maximum σ isclose to that observed in the AgI–γ Al2 O3 composite (Uvarov et al 2000). Through thepresent measurements of σ at different temperatures, the following are also found about(x)AgI–(1 − x)anatase composites:(1) The σ varies exponentially with temperature below 373 K, while the activation energy
for the pristine AgI specimen rises continuously from 0.35 to 0.99 eV with increasing
temperature from 300 to 400 K. The activation energy E σ below 400 K is plotted against
x in the inset. The broken curve is the least-squares ﬁt for the plotted E σ .

Structural, electrical and optical studies on AgI–anatase composites

5063

-1

10

(x)AgI-(1-x)anatase
T= 300 K, f = 1 kHz

-2

0.40
-3

10

0.35

Eσ (eV)

Conductivity σ (S/m)

10

0.30

0.25

-4

10

0.20
0.0

0.2

0.4

0.6

0.8

1.0

AgI content, x

-5

10

0.0

0.2

0.4

0.6

0.8

1.0

AgI content, x
Figure 4. Conductivity of different (x)AgI–(1 − x)anatase composites at 300 K. The appliedfrequency was 1 kHz. The activation energy E σ below 400 K is plotted against x in the inset. The
broken curve is the least-squares ﬁt for the plotted E σ .

(2) The σ at room temperature is not considerably changed for a heating–cooling cycle below
Tc .
(3) The composite with larger x (0.4) exhibits more clearly an abrupt σ change near the Tc(420 K) of pristine AgI with some accompanying transition hysteresis, as observed in the
AgI–γ Al2 O3 composite (Lee et al 2000a).
In the light of the porosity data shown in ﬁgure 1, the σ variation in the small x
region (x  0.4) may be mainly caused by the insulating pore effect. On the other
hand, the conductivity enhancement observed in the large x region (0.4) may arise fromsome composite effects, since the starting materials, pristine AgI and anatase, have low
conductivities. The composite effects arise from highly ionic-conductive regions at the
AgI/anatase interfaces and γ AgI/βAgI interfaces in anatase-packed AgI particles. It is known
that the adsorption of Ag+ ions at insulating oxide surfaces leads to the formation of vacancies
on Ag+ sites and the space charge layer. As referred to already in section 3.1, it is also known
that a silver-rich AgI solution crystallizes in the γ phase, while a silver-deﬁcient solution
crystallizes in the β phase. The γ AgI and βAgI crystallites may act as Ag+ ion donor andacceptor, respectively. Thus, we can anticipate high Ag+ ion conductivity at the γ AgI/βAgI
interfaces in anatase-packed AgI particles. With decreasing x from 1 to 0.4, dispersed anataseparticles approach each other and then the space charge effect spreads over the AgI domain to
display a maximum in the σ –x curve. The activation energy E σ for σ shows a complicated x

5064

S Mochizuki and F Fujishiro

dependence, as shown in the inset of ﬁgure 4. The least-squares ﬁtting curve (broken curve)
for E σ exhibits a minimum near x = 0.7. It is noted that the porosity variation with x exhibits
also a minimum near the same x, as shown in the inset of ﬁgure 1. The E σ of 0.35 eV for
the present pristine AgI is smaller than 0.38 eV for an ordered βAgI crystal. This may be
explained by the extensive disorder of Ag+ ions in the stacking faulted structure and such adisordered arrangement of Ag+ ions may provide a higher energy state than in the ordered
βAgI structure with a decrease of E σ (Lee et al 2000b). By adding anatase particles to thepristine AgI, the E σ decreases. This indicates a generation of new ionic transport pathways.
This is due to Ag+ ion adsorption onto anatase ﬁne particles. On further adding anatase, E σ
becomes affected by both the electronic conduction in anatase particles and the existence of
insulating pores. Following more detailed discussion, the transference numbers for ionic andelectronic charge carriers should be determined.
The effects of crystalline defects introduced by some strain coming from stress withdifferent thermal expansion coefﬁcients between AgI and anatase should also be taken into
consideration (Jow and Wagner 1979).
3.3. Optical spectra
Before describing the results of optical measurements on (x)AgI–(1 − x)anatase composites,
we point out general photoinduced properties obtained for AgI-based glasses (AgI–AgPO3
glasses, AgI–Ag2 MO4 and AgI–Ag2 WO4 ) and AgI-based composites (AgI–γ Al2 O3 and AgI–ZnO (Fujishiro and Mochizuki 2003)). These AgI-based ionic conductors are easily blackened
with intense ultraviolet laser light. Such blackening becomes prominent with increasing
temperature and increasing AgI content. The blackening is due to precipitation of silver
clusters or silver ﬁne particles, which is an irreversible reaction. The blackening gives riseto PL intensity reductions, which is the so-called photodarkening. Therefore, we checked
the blackening by examining the reproducibility of measured PL spectra at different laser
ﬂuences and then we found a suitable excitation light intensity for the PL measurement foreach specimen.
In ﬁgure 5, the PL spectra obtained for (x)AgI–(1 − x)anatase composites with different
compositions at 8 K are compared. The PL was excited by the 355 nm laser line of theNd 3+ :YAG laser. All spectra in this ﬁgure are normalized at the peak intensity. Pristine
anatase shows a broad PL band centred at about 536 nm (2.31 eV) with a large Stokes shift of
0.89 eV. The present PLE spectrum measurement of this broad PL band at 8 K shows a sharp
PLE edge, corresponding to the optical absorption edge, at about 3.2 eV which is close to thebandgap energy of pristine anatase. This PL band has been tentatively assigned to the radiative
recombination of self-trapped excitons in pristine anatase by many authors (Tang et al 1994,
Hosaka et al 1997, Suisalu et al 1998). The addition of a small amount of AgI, x = 0.2,
leads to a considerable decrease of this anatase PL and gives a sharp PL band at 428 nm withaccompanying shoulders at about 423 nm and about 438 nm. The spectrum is affected by noiseat wavelengths shorter than 415 nm. The decreased PL intensity of the 536 nm band is due tothe absorption of excitation laser light by the AgI domain in an anatase-packed AgI particle.
In pristine AgI, the bands at about 424 nm and about 429 nm are assigned to the radiative
recombination of Z 1,2 excitons and shallowly trapped Z 1,2 ones, respectively (Mochizuki andFujishiro 2002, Cardona 1963, Mochizuki and Ohta 2000, Mochizuki 2001, Mochizuki and
Umezawa 1997). No measurable spectral change with x is observed in the 536 nm band.
Further addition of AgI, x = 0.4, increases the longer wavelength sideband to generate abroad band centred at 442 nm and reduces the 536 nm band, with an accompanying slightbandwidth narrowing at the longer wavelength side (λ > 536 nm). On further increasing x

Structural, electrical and optical studies on AgI–anatase composites

5065(x)AgI-(1-x)anatase
T= 8 K, λex= 355 nm
x= 1.0x= 1.00

PL intensity (arb. scale)R''R
x= 0.4x= 0.80QR'P

x= 0.2x= 0.6

420

0

424

428

432

x= 0.40x= 0.20x= 0.0
0
400

450

500

550

600

650

700

Wavelength (nm)Figure 5. PL spectra of different (x)AgI–(1 − x)anatase composites at 8 K. Free exciton PL bandsfor (0.2)AgI–(0.8)anatase, (0.4)AgI–(0.6)anatase and pristine AgI are shown in the inset. The PL
bands labelled R, R and R are due to radiative decays of shallowly trapped excitons.

from 0.4 to 0.8, this broad band becomes narrowed and blue-shifted to 441 nm, which is close
to the intensity maximum (437 nm) of pristine AgI. Therefore, it should be interpreted that thebroad band consists of the 438 and 442 nm bands. We interpret the observed spectral variation
with x, as follows. In the specimens with smaller x, since free excitons are conﬁned in smaller
AgI domains, excitons reach AgI domain surfaces and AgI/anatase interfaces and then decay
radiatively. Since the energy bandgap (about 3.1 eV) of AgI is close to that (3.2 eV) of anatase,the potential for trapping free AgI excitons is thought to be shallow. In such a case, unlike the
AgI–γ Al2 O3 composite (Mochizuki and Fujishiro 2003), any PL band due to radiative decayof excitons deeply trapped at the AgI/anatase interfaces may not be apparently observed, as
shown in ﬁgure 5. However, as described below, a small number of excitons deeply trappedat the AgI/anatase interfaces are found through measurements of the excitation-light intensity
dependence of the PL spectrum. It should also be pointed out that the ﬁne structure at the higher
energy side of the 429 nm band becomes prominent at x = 0.4. In the inset, the higher energy
side spectral structure for the (0.2)AgI–(0.8)anatase and (0.4)AgI–(0.6)anatase composites iscompared with that for the pristine AgI specimen which was prepared in the same way. The
shoulders at about 423 nm and about 425 nm, which are respectively labelled P and Q, areseen for the (0.4)AgI–(0.6)anatase composite. As shown in the inset of ﬁgure 6, both the
P and Q shoulders remain clearly even at 129 K (=11.1 meV) in spite of the small energy
separation (13.8 meV) between P and Q. With increasing x, the P and Q shoulders tend to movetoward about 424 nm. The Z 1,2 exciton originates from the doubly degenerate valence band

5066

S Mochizuki and F Fujishiro(0.4)AgI-(0.6)anatase
λex= 355 nm
Q

R1

R2

PL intensity (arb. scale)

P

T (K)
8
18
27
37
57
78
129
188
278

400

450

500

R
129 K
Q
P
8K

420

550

424

600

428

650

432

700

Wavelength (nm)Figure 6. Temperature dependence of the PL spectrum of the (0.4)AgI–(0.6)anatase composite.
The temperature increases in the curves from top to bottom. The PL spectra at 8 and 129 K arecompared in the inset by rescaling their intensities.

8 with light and heavy hole masses. The 8 band tends to split by some crystal-ﬁeld change
caused by mixing with the wurtzite structure or some internal stress caused by different thermal
expansion coefﬁcients between the AgI and anatase. Therefore, such 8 splitting should betaken into account for rigorous assignment of the 423 and 425 nm shoulder bands. Such an
exciton spectrum is similar to that of the microcrystal specimen (Mochizuki and Umezawa
1997). The spectral structure of the 429 nm peak (labelled R) assigned to shallowly trapped
Z 1,2 excitons changes with x as shown in the inset. The changed bands are labelled R and R .
This indicates that there are several shallowly trapped exciton states. In such cases, the lower
energy trapped excitons tend to populate higher energy trapped exciton states with increasing
temperature.
The PL spectra of AgI–anatase composites were studied at different temperatures between
8 and 278 K. The result obtained for the (0.4)AgI–(0.6)anatase composite with maximum σ
is shown in ﬁgure 6 as a typical example. In comparison with the emission intensities of theP-, Q- and R-emission shoulders, the emission of the 442 nm band decreases prominently
with increasing temperature. The intensity peak at about 442 nm was assigned above toradiative decay of excitons trapped by lattice defects and impurities in the AgI domain. On
further increasing temperature above 78 K, the 442 nm band almost disappears and absolutely
disappears at 278 K. Above 188 K, the intrinsic and extrinsic exciton bands are slightly
red-shifted, which is ascribed to the decrease of the bandgap energy of AgI with increasing
temperature. Such a redshift was also observed at pristine AgI (Mochizuki and Ohta 2000).

Structural, electrical and optical studies on AgI–anatase composites

5067(0.4)AgI-(0.6)anatase
T= 8 K, λex= 355 nm

PL intensity (arb. scale)

I0
0.81I00.67I00.44I00.10I00.006I0

0.006I0

I0

400

400

450

500

550

500
600
Wavelength (nm)

600

650

700

700

Wavelength (nm)Figure 7. Excitation-light intensity dependence of the (0.4)AgI–(0.6)anatase composite at 8 K.
The laser ﬂuence increases in the curves from top to bottom. The PL spectra for the weakest andmost intense excitations are compared by rescaling their intensities in the inset.

The disappearance of the 442 nm band may be due to a thermally activated reverse processfrom trapped states to free exciton ones. The 536 nm band becomes weaker and shifted toshorter wavelength with increasing temperature. Such a blueshift was also observed in thepristine anatase specimen. This indicates the decreased exciton-lattice relaxation energy. As
described later, the broad 536 nm band consists of two bands. If these bands have different
temperature dependences, such a blueshift of the 536 nm band is possible. The PL intensity
decrease of the 536 nm band with increasing temperature from 8 to 98 K is about two times
larger than that observed in pristine anatase. This may be attributed to the exciton trapping
and nonradiative exciton decay at the AgI/anatase interface.
The excitation-light intensity Iex dependence of the PL spectrum is also studied at 8 K on
different (x)AgI–(1−x)anatase composites. The result obtained for the (0.4)AgI–(0.6)anatase
composite with maximum σ is shown in ﬁgure 7 as a typical example. With decreasing laser
ﬂuence, the free exciton P- and Q- shoulder emissions and the shallowly trapped exciton R
emission decrease linearly with Iex . On the other hand, the 442 and 536 nm emissions deceasenonlinearly with decreasing Iex , but their decreases are smaller than those of the P, Q and R
emissions. Upon further decreasing Iex , the 536 nm emission intensity becomes comparable
to that of the 442 nm emission with accompanying intensity peak wavelength shift from 536
to 552 nm. This indicates that the 442 and 536 nm emissions have higher yields than thefree exciton emissions and the shallowly trapped exciton emission. But the 442 and 536 nm
emissions become saturated under intense excitation. These saturations arise from a ﬁnite

5068

S Mochizuki and F Fujishiro(0.4)AgI-(0.6)anatase

PL intensity (arb. scale)

T= 9 K, λex= 355 nm, tg= 5 ms

A: td= 0 ns

A-B

400

500
600
Wavelength (nm)700

B: td= 20 ns

400

450

500

550

600

650

700

Wavelength (nm)Figure 8. TRPL spectrum of the (0.4)AgI–(0.6)anatase composite at 9 K. Curves A and B
correspond to the time-resolved spectra integrated from 0 ns to 5000 μs and integrated from 20 ns
to 5000.02 μs after incidence of the laser pulse, respectively. The PL component (curve A − curveB) for short delay time is shown in the inset.

number of shallow- and deep-level PL centres. In the inset, the PL spectra observed underintense (I0 = 33 mJ cm−2 ) and weak (0.006I0 ) excitations at 8 K are compared. We haveassigned tentatively the 536 nm band to radiative decay of self-trapped excitons in pristine
anatase on the basis of both the present PLE spectrum edge (about 3.2 eV) and the assignment
made previously in several papers (Tang et al 1994, Hosaka et al 1997). However, the PL
intensity of the composite saturates at a laser ﬂuence larger than 33 mJ cm−2 , while we cannot
detect any saturation in pristine anatase particles. Therefore, the results may indicate that theexcitons in anatase are affected by adding AgI. Deeply trapped excitons in anatase may beformed by exciton–phonon interactions or by interactions between free electrons and trappedholes by defects at the AgI/anatase interfaces or both. Since the number of defects is limited,the emission from such deeply trapped excitons at the interfaces becomes saturated underintense excitation. The observed redshift from 536 to 552 nm with decreasing Iex may indicatethat anatase-related emission consists of at least two emission bands, labelled as X and Y
bands.
The TRPL spectra of AgI–anatase composites were measured at 9 K. Since the TRPL
measurements need longer data-acquisition times and the composites easily became blackened
by excitation laser light during the measurements, as stated above, we considerably reduced thelaser ﬂuence. As a result, the spectra are considerably affected by noise. In ﬁgure 8, the TRPL
spectra at 9 K shown for the (0.4)AgI–(0.6)anatase composite with maximum σ are typicalStructural, electrical and optical studies on AgI–anatase composites5069

λex= 355 nm

(c)
9K
98 K
178 K

PL intensity (arb. scale)

297 K

(b)
C-D

T= 9 K
λex= 355 nm
tg= 100 ns(a)
C: td= 0 ns

D: td= 50 ns

400

450

500

550

600

650

700

Wavelength (nm)Figure 9. PL spectra of pristine anatase ﬁne particles at 9 K: (a) TRPL spectra, (b) short-delay-timeemission, (c) temperature dependence of the PL spectrum.

examples. Curves A and B correspond to the time-resolved spectra integrated from 0 ns to5000 μs and integrated from 20 ns to 5000.02 μs after incidence of a laser pulse, respectively.
At a ﬁrst glance, the PL spectrum consists of a broad band due to anatase shown as the bottom
curve and another broad band due to AgI. In order to extract the short-delay component,
we have calculated the difference spectrum (curve A − curve B). The result (curve A–B) is
shown in the inset. The difference spectrum gives an image of the ‘pure’ short-time emission
spectrum. Comparing this difference spectrum with those of pristine AgI and pristine anataseshown in ﬁgure 5, one can identify the short-time emission spectrum made up essentially ofseveral kinds of AgI-related emission (400 nm < λ < 500 nm) and weak anatase-related
emission (500 nm < λ < 700 nm). On the other hand, the long-time emission spectrum (fullcurve) may be anatase-related emission, which consists of two components centred at about
465 nm and about 536 nm.
The TRPL spectra of pristine anatase particles are also measured at 9 K. The results are
given in ﬁgure 9(a). Curves C and D correspond to the time-resolved spectra integrated from 0 to
100 ns and integrated from 50 to 150 ns after incidence of the laser pulse, respectively. In order
to extract the short-delay component, we have calculated the difference spectrum (curve C −curve D). The result is shown in ﬁgure 9(b). The short time emission (curve C–D) is blue-shiftedin comparison with the long-time emission (curve D). The blue-shifted emission indicates thatthe broad band emission of pristine anatase particles consists of different radiative decaysof self-trapped excitons. The PL spectrum of the same pristine anatase specimen was also

5070

S Mochizuki and F Fujishiro

measured at different temperatures between 9 and 297 K. The results are shown in ﬁgure 9(c).
As seen in this ﬁgure, the broad PL band is blue-shifted with increasing temperature. This may
be related to the above-mentioned multicomponent of the PL band. We should note the spectral
structure difference between curve B in ﬁgure 8 and curve D in ﬁgure 9(a). We compared the
PL and TRPL spectra of (0.4)AgI–(0.6)anatase with those of pristine anatase particles andsingle crystals (Tang et al 1994, Watanabe et al 2000). In the result, we have reached a viewthat anatase-related excitons are partially trapped at the AgI/anatase interface. Since an exciton
trap is closely related to a defect, the view may yield a conclusion that there are defects at
the AgI/anatase interfaces. Assuming that the effect is explained as a space charge effect, theprobable reason is the adsorption of Ag+ ions at the thereby positively charged oxide surfaces
and the creation of numerous vacancies at the AgI/anatase interfaces. When the interface has
a comparatively large defect density, the interface region may have high ionic conductivity.
The detailed report on the PL and TRPL studies on various pristine anatase specimens will bepresented in a separate paper (Mochizuki et al 2003).
We have already discussed the space charge effects at the AgI/anatase interfaces on ionic
conduction. We need to discuss such space charge effects on excitons. The space charges
generate a potential gradient, namely an electric ﬁeld, which drives the self-trapped exciton
formation, exciton trapping and exciton dissociation.
4. Conclusion and remarks
The results obtained for (x)AgI–(1 − x)anatase composites are brieﬂy summarized as follows.
(1) Conductivity enhancement higher than three orders of magnitude is observed.
(2) The SEM and EDX ﬂuorescence measurements indicate that anatase ﬁne particles arepacked in large AgI particles of several micrometres in size and such anatase-packed AgI
particles are three dimensionally connected to form a composite specimen. We named
these composites particles anatase-packed AgI particles. Silver-rich parts appear aroundanatase particles. This indicates Ag+ ion adsorption on the surfaces of the anatase particles.
The measurements indicate also that some of the anatase particles are nearly isolated andothers aggregate with each other to form larger particles.
(3) The XRD pattern of AgI domains in anatase-packed AgI particles is similar to that of type
II AgI crystals (Lee et al 2000b) and the (311) line of γ AgI is observed. The AgI domain
in the anatase-packed AgI particles consists of γ AgI and βAgI crystallites.
(4) The splitting of the Z 1,2 exciton spectra of AgI is observed as two shoulder PL bands. These
may arise from some crystal-ﬁeld change caused by the mixing with the wurtzite structure
or some internal stress caused by different thermal expansion coefﬁcients between the AgI
and anatase.
(5) The anatase-related exciton spectra in the anatase-packed AgI particles are considerably
affected in comparison with pristine anatase. Excitons in anatase ﬁne particles are partiallytrapped at the AgI/anatase interfaces which may contain numerous crystalline defects.
Results (2) and (3) explain well the observed conductivity data for (x)AgI–(1 − x)anatase
composites. It may be concluded that adding anatase ﬁne particles to AgI produces vacancies
on Ag+ ion sites, a space charge layer and a heterostructure for γ AgI and βAgI crystallites.
In such anatase-packed AgI particles, both the AgI/anatase interface regions of neighbouring
anatase particles and the γ AgI/βAgI crystallite interfaces would provide pathways for highconductivity.
The main limitation of our technique is that there are no direct mesoscopically spaceresolved electrical and optical measurements on the composite specimens. Bearing thisStructural, electrical and optical studies on AgI–anatase composites

5071(0.4)AgI-(0.6)anatase
T= 300 K, λir, λex= 325 nm

PL intensity (arb. scale)in vacuum

A

in O2

B

400

450

500

550

600

650

700

Wavelength (nm)Figure 10. Reversible photoinduced spectral change in (0.4)AgI–(0.6)anatase composites at 300 K.
Both the irradiation laser light wavelength λir and the PLE laser light wavelength λex are the same:
325 nm.

limitation in mind, we have presented some reasonable connections between the measured
data and composite effects in the AgI–anatase system. With a near-ﬁeld optical microscopemeasurement system and a SEM, the space-resolved optical and structural measurements on
different AgI–oxide systems are now in progress in our laboratory.
Very recently, we have measured the Raman scattering spectra of (x)AgI–(1 − x) anatase
composites at room temperature. The present pristine AgI shows a broad band centred at
88 cm−1 . This band is assigned to extra modes coming from defects. When x is decreased
to less than 0.2, the band shifts to 78 cm −1 and becomes resolved into three bands centred atabout 68, 78 and 94 cm−1 . Such a spectral change may give more directly information about
the defect structure at AgI/anatase interfaces (Mochizuki et al 2003).
Finally, we have very recently found a reversible photoinduced spectral change in thepresent (x)AgI–(1 − x)anatase composite specimens at room temperature. The spectralchange obtained for (0.4)AgI–(0.6)anatase is shown in ﬁgure 10 as a typical result. After
fully irradiating at 300 K with 325 nm laser light in a vacuum specimen chamber, the specimen
shows a broad PL spectrum (curve A) due to trapped excitons in anatase. Oxygen gas is thenintroduced into the chamber. With increasing irradiation time, the broad PL band becomes
weak, while a sharp PL band due to the excitons in AgI grows at 426 nm and the broad
PL band prevails, as indicated by curve B. The chamber is again evacuated. The broad
PL spectrum reappears. It is found through many successive experiments that the spectralchanges appear repeatedly. The enhancement of the broad PL is related to photogenerated
oxygen vacancies and is observed for many oxides, for example Eu2 O3 , Sm2 O3 (Mochizuki
et al 2001a, 2001b) and SiO2 (Mochizuki and Araki 2003). The observed spectral changes

5072

S Mochizuki and F Fujishiro

may originate from the photoinduced dissociative adsorption and photoinduced associative
detachment of O2 molecules near the composite surfaces and may well yield materials for
optical sensor devices, for example, optical oxygen-partial-pressure sensors. The detailed
results will be described in a separate paper.
Acknowledgments
This work was partially supported by a Grant-in-Aid for Scientiﬁc Research from the Ministryof Education, Science, Sports, Culture and Technology, Japan. This work is also partially
supported by a Project Research Grant from The Institute of Information Sciences of Collegeof Humanities and Sciences (Nihon University) and by a Cooperative Research Grant fromThe Institute of Natural Sciences (Nihon University).
References
Bloch R and Möller H 1931 Z. Phys. Chem. A 152 245
Burley G 1963 Am. Mineral. 48 1266
Burley G 1967 Acta Crystallogr. 23 1
Cardona M 1963 Phys. Rev. 129 69
Fujishima A, Rao T N and Tryk D A 2000 J. Photochem. Photobiol. C 1 1
Fujishiro F and Mochizuki S 2003 ICDS-22: The 22nd Int. Conf. on Defects in Semiconductors;; Physica B submitted
Gupta R K, Agraval R C and Pandey R K 1996 Solid State Ionics ed B V R Chowdary et al (Singapore: World
Scientiﬁc) p 499
Hosaka N, Sekiya T and Kurita S 1997 J. Lumin. 72–74 874
Jow T and Wagner J B Jr 1979 J. Electrochem. Soc. 126 1963
Lee J-S, Adams S and Maier J 2000a Solid State Ion. 136/137 1261
Lee J-S, Adams S and Maier J 2000b J. Phys. Chem. Solids 61 1607
Liang C C 1973 J. Electrochem. Soc. 120 1289
Mochizuki S 2001 Physica B 308–310 1042
Mochizuki S and Araki H 2003 Physica B submitted
Mochizuki S and Fujishiro F 2002 Proc 6th Forum on Superionic Conductor Physics (Kyoto: The Japanese Societyof Ion Transport) pp 41–6
Mochizuki S and Fujishiro F 2003 Phys. Status Solidi c 0 763
Mochizuki S, Fujishiro F and Shimizu T 2003 (in preparation)
Mochizuki S, Nakanishi T, Suzuki Y and Ishi K 2001a Appl. Phys. Lett. 79 3785
Mochizuki S and Ohta Y 2000 J. Lumin. 87–89 299
Mochizuki S, Suzuki Y, Nakanishi T and Ishi K 2001b Physica B 308–310 1046
Mochizuki S and Umezawa K 1997 Phys. Lett. A 228 111
Shahi K and Wagner J B Jr 1981 Solid State Ion. 3/4 295
Shastry M C R and Rao K J 1992 Solid State Ion. 51 311
Suisalu A, Aarik J, Mandar H and Sildos I 1998 Thin Solid Films 336 295
Tang H, Berger H, Schmid P E and Levy F 1994 Solid State Commun. 92 267
Uvarov N F, Vanek P, Savinov M, Zelezny V, Studnicka V and Petzelt J 2000 Solid State Ion. 127 253
Watanabe M, Sasaki S and Hayashi T 2000 J. Lumin. 87–89 1234
West A R 1984 Solid State Chemistry and Its Applications (New York: Wiley)
Zhao Z-Y, Wang C-Y, Dai S-Y and Chen L-Q 1983 Solid State Ion. 9/10 1175

INSTITUTE OF PHYSICS PUBLISHING

JOURNAL OF PHYSICS: CONDENSED MATTER

J. Phys.: Condens. Matter 16 (2004) 3239–3256

PII: S0953-8984(04)75141-3

In situ real time optical study of ﬁlms grown on low
temperature substrates by vacuum evaporation ofiodine and silver iodide: spectral transitions during
deposition and annealing processes
Shosuke Mochizuki1 and Fumito Fujishiro
Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40
Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan
E-mail: motizuki@physics.chs.nihon-u.ac.jp

Received 20 January 2004
Published 23 April 2004
Online at stacks.iop.org/JPhysCM/16/3239
DOI: 10.1088/0953-8984/16/18/024

Abstract
In order to clarify the growth process and hard to understand reported spectra ofiodine ﬁlm and silver iodide (AgI) ﬁlm, various spectral transitions have beenmeasured in situ: (a) optical density spectra of ﬁlms grown on low temperature
substrates by vacuum evaporation of an iodine lump, during deposition andannealing; (b) optical extinction spectra of vapour zones produced by gas
evaporation of AgI powder, during evaporation; and (c) optical density spectraof ﬁlms grown on low temperature and room temperature substrates by vacuum
evaporation of AgI powder, during deposition and annealing. After thesein situ real time measurements, the ﬁlm specimens were allowed to recover
and were examined by means of comparative measurements of the opticaldensity and photoluminescence spectra at about 12 K. The spectra obtained wereanalysed and compared with those reported previously by other workers, andthe following questions have been optically clariﬁed: (1) How does annealing
improve the crystal quality of iodine ﬁlm? (The annealing at about 200 K
improves the crystal quality considerably.) (2) What is the ﬁlm that is quench
deposited on low temperature sapphire surfaces by thermal evaporation of AgI
powder? (It is Agm In (m, n  1) dispersed iodine ﬁlm.) (3) How does AgI ﬁlm
grow in the deposited ﬁlm? (Above about 220 K, iodine evaporates abruptly
to grow uniform AgI ﬁlm on the substrate.) (4) How does excess iodine affectthe optical spectrum of AgI? (The excess iodine creates both several exciton
absorptions due to polytype structures at wavelengths between 370 and 410 nm,
and a tail at wavelengths longer than 430 nm.) (5) What is the best evaporation
condition for the fabrication of AgI and iodine ﬁlms? (The best quality ofiodine and βAgI ﬁlms can be obtained at a substrate temperature of about
1 Author to whom any correspondence should be addressed.

0953-8984/04/183239+18$30.00© 2004 IOP Publishing Ltd

Printed in the UK

3239

3240

S Mochizuki and F Fujishiro

200 K in the quench deposition method.) Besides this, the present opticalexperiments have answered the following question: is there truly a strongly
enhanced optical absorption in quench deposited AgI ﬁlms, as reported by
Kondo et al (1998 Phys. Rev. B 57 13235)? The present studies have elucidated
that the enhanced absorption is not due to amorphization of AgI but due merely
to intense optical absorption of iodine in the ﬁlms.

1. Introduction
Because of the practical importance of silver iodide, AgI, in electrolytes, photography andcloud condensation nuclei, the physical and chemical properties have been extensively studied
in the last few years. AgI has three phases designated as α, β and γ , in order of decreasing
temperature, at normal pressure with the following properties (Burley 1963). At the superionictransition temperature Tc (420 K), the superionic α phase transforms into semiconductor βphase (wurtzite lattice). The α phase has a body-centred cubic arrangement of I− ions withhighly mobile Ag+ ions randomly distributed through the equivalent interstices; this is known as
the averaged structure. At room temperature, the semiconductor γ phase (zinc-blende lattice)appears as a metastable state. It is said that the stability of the γ phase is affected by slightnonstoichiometries and other defects. To date, the optical spectra of pristine AgI (Cardona
1963, Mochizuki 2001, Mochizuki and Ohta 2000) and AgI nanoparticles (Mochizuki and
Umezawa 1997, Dulin 1993), AgI-based composites (Mochizuki and Fujishiro 2003a, 2003b)
and AgI-based glasses (Fujishiro and Mochizuki 2002, Mochizuki and Fujishiro 2003c) havebeen reported, by many workers. However, it is well known that the AgI specimens tend
to contain excess iodine. The excess iodine species in real AgI specimens are thought tobe dependent on the specimen preparation method and thermal history. Iodine clusters In(n > 2) and ﬁlms exhibit several intense broad optical absorption bands in the visible andultraviolet light regions. Although such excess iodine effects have not been reported in detail,we have frequently experienced these absorption bands due to excess iodine disappearing
or weakening considerably upon annealing at moderate temperature in vacuum, and foundthat βAgI ﬁlm with high optical quality is fabricated by both depositing and subsequently
annealing below room temperature. For example, although excess iodine in ﬁlm specimens isstable at low temperatures (T < 90 K) even in vacuum, it rapidly evaporates above about 220 K
under heating. Kondo et al (1998) prepared ﬁlms on a low temperature sapphire substrate byevaporating AgI in vacuum and observed extremely enhanced absorption compared to that of
crystalline AgI ﬁlm; the enhanced absorption appearing in thick and thin ﬁlms disappeared
suddenly at temperatures higher than 220 and 180 K, respectively. They explained the enhanced
absorption by the p–d hybridization in amorphous AgI and the disappearance of the intense
absorption by an amorphous–crystalline transition. However, we have found very recently thatsuch intense absorptions, spectral shapes and sudden spectral disappearances are very similar
to those observed for pristine iodine ﬁlms. Unfortunately, these excess iodine effects havebeen accorded too little respect in the optical study of AgI and AgI-based materials for a long
period of time and they have led many researchers astray.
In the present study, we have performed in situ real time measurements of the optical
spectra of AgI ﬁlms and pristine iodine ﬁlms deposited on the same low temperature sapphiresurfaces, during both deposition and heating (annealing) processes. Besides this, we havemeasured optical extinction spectra of AgI vapour zones and have clariﬁed the vapour species.
After these measurements, the AgI ﬁlm specimens grown were examined by means ofcomparative measurements of the optical density (OD) and photoluminescence (PL) spectraReal time optical study of ﬁlms

3241

at about 10 K. On the basis of these experimental results, we have studied the AgI ﬁlm growthmechanism below room temperature and elucidated several AgI-related phenomena whichwere previously reported by other authors but were hard to understand.
2. Experimental procedure
Vacuum evaporation experiments were carried out under a base pressure below 10−4 Pa
by using a stainless-steel evaporation chamber with two pairs of opposite optical windows.
The experimental apparatus was composed of a main evaporation chamber and a spectral
measurement system. The apparatus was used previously for cluster and microcrystal studies(Mochizuki 1996, Mochizuki and Ruppin 1993). Optically ﬂat plates of sapphire (R-cut)were used as the substrate. The substrate temperature (Ts ) was monitored with a copper–
constantan thermocouple. For evaporation of AgI, a platinum crucible containing nominallypure AgI (Alpha, 99.999%) powder was gradually heated indirectly in an alumina crucible onwhich a tungsten wire heater was wound. The iodine vapour was obtained by evacuating a
glass tube which contained nominally pure iodine lumps (Wako, 99.8%) and was connectedto the evaporation chamber. In the present experiments, the thickness of ﬁlm on the substrateincreased with time. During the deposition, the OD spectrum of the selected position in theﬁlm was recorded as a function of time (td ) elapsed after the beginning of the measurement with
an optical multichannel analyser system. Each measurement was made at the appropriate time
window and time interval. A time (td ) resolved spectrum consists of 64 spectra. In the present
paper, some of them are selected and presented. Continuum light from a 150 W xenon lamp
was appropriately attenuated to avoid photoinduced blackening and was directed at the wholesubstrate in a transmission conﬁguration without using a lens. After passing the substrate, only
the light through the selected position was collected by a lens and then spectrally analysed andrecorded by a multichannel analyser system (Atago, MAX 3000). The transmission Tr at agiven position is the intensity ratio at the same position before the beginning of the evaporation.
The results for ﬁlms are expressed with respect to the OD spectra, − log Tr .
Generally, the vapour produced by the vacuum evaporation method is too thin for usual
optical absorption measurements and the vapour needs to be conﬁned. In order to conﬁne thevapour, the gas evaporation experiments on AgI were carried out at different gas pressuresand the optical extinction spectra were measured at different stages of evaporation, using theapparatus described above to determine optically the vapour species in the AgI vapour, taking
into account some aggregation of vapour species due to atmospheric gas. Optical extinction
spectra of selected positions in the vapour zones were recorded in the transmission conﬁguration
as a function of td elapsed from the beginning of the measurement. Since the transmissivity
spectra obtained for the vapour zones contain both scattering and absorption effects, the results
obtained are expressed as optical extinction spectra, − log Tr .
After the above-described in situ real time measurements, the recovered AgI ﬁlm
specimens were put in a helium closed-cycle cryostat, whose temperature could be varied fromabout 10 to 300 K. PL was excited by different laser lines (360–440 nm) from a Ti3+ :sapphirelaser excited by a Nd3+ :YAG laser. The PL spectra were examined at different laser lines withdifferent excitation intensities. The emitted light was dispersed using a grating spectrograph(Jobin-Yvon HR-320), and an image-intensiﬁed multichannel photodetection system (Lambda
Vision, LVID) was employed. Both the OD spectra and the PL spectra were measured at about
10 K. The PL spectra were also measured at different temperatures between 7 K and room
temperature. The spectral response of the optical system, including the focusing lens andthe optical ﬁbre, was carefully measured using a calibrated quartz–halogen lamp and used tocorrect the raw data. Through the above optical measurements, the lights from the xenon lamp
and laser were suppressed to avoid photoblackening.

3242

S Mochizuki and F Fujishiro

Ts= 96 K

Optical density

0.6

6

6

5

0.4
1 (x 10+ 0.1)

4

300

400

500

600

700

800

Wavelength (nm)30.2
td (ms)
1: 3360
2: 4760
3: 5600
4: 7000
5: 8400
6: 17640

2

1

0.0
300

400

500

600

700

800

Wavelength (nm)Figure 1. The time evolution of the optical density spectrum of iodine ﬁlm on a sapphire substrate(Ts = 96 K) during deposition. The spectra at the initial and ﬁnal stages of the deposition arecompared in the inset.

3. Results and discussion
3.1. Time evolution of the optical density spectrum of iodine ﬁlm deposited on a sapphire
substrate during vacuum evaporation of pristine iodine
We evaporated iodine at different evaporation speeds and at different substrate temperatures.
Figure 1 shows the time evolution of the OD spectrum of the iodine ﬁlm deposited on asapphire substrate whose temperature was 96 K, as a typical example. Each measurement wasmade for 140 ms at intervals of 140 ms. The time td increases for the curves from bottom
to top. The time evolution corresponds to the ﬁlm thickness increase. With increasing td ,the absorption increases with the spectral broadening. Although the spectra at the transparent
region (>600 nm) are affected by some interference effect, curve 1 can be compared withcurve 6 by rescaling: ×10 + 0.1. The spectra of iodine ﬁlm obtained are very different fromthose of iodine molecules (Mathieson and Rees 1956). This indicates that the strong molecularinteraction in solid iodine affects its electronic structure. The absorption of curve 6 starts to risearound about 760 nm, which is comparable to the reported bulk values deduced from opticaland photoconduction measurements: 733 nm at 78 K and 779 nm at 200 K (Braner and Chen
1963), 775 nm at 123 K (Yamamoto et al 1987), 827 nm at 300 K (Many et al 1961), 992 nm atroom temperature (Riggleman and Drickamer 1963) and 939 nm at room temperature (Moss
1952). The absorption rises gradually up to about 580 nm, and reaches an inﬂection point at
about 580 nm. After reaching a kink at about 505 nm, the absorption rises steeply to anotherkink at about 380 nm. Passing through a maximum at about 320 nm, the absorption decreases

Real time optical study of ﬁlms

3243

200 K

1.0
210 K

0.8

Optical density

160 K

0.6
96 K

0.4

220 K

0.2

230 K

0.0
300

400

500

600

700

800

Wavelength (nm)Figure 2. The temperature dependence of the optical density spectrum (curve 6 in ﬁgure 1) of
iodine ﬁlm on a sapphire substrate during heating.

steeply. We elevated the specimen temperature to room temperature and measured the spectra
at different temperatures. The results are shown in ﬁgure 2. With increasing temperature, theabsorption continues to increase and reaches a maximum at 200 K. On further increasing thetemperature, the absorption decreases monotonically. When the temperature exceeded 220 K,
there was a faint exhaust sound from the vacuum pump attached to the specimen chamber andthe OD spectrum disappeared suddenly. Both the exhaust sound and the disappearance of the
OD spectrum indicate perfect evaporation of the iodine ﬁlm. Through different experiments, it
was found that the spectral disappearance temperature tends to vary with heating rate and ﬁlm
thickness. The temperature dependence of the spectral structure is summarized as follows.
The peak wavelength at about 320 nm is not changed with temperature increase. However,
the absorption components corresponding to the kinks at about 505 and about 380 nm andto the inﬂection at about 580 nm shift toward longer wavelengths. The spectrum at 200 K
seems to consist of at least three absorption bands centred at about 320, about 410, about
510 nm and a tail at wavelengths longer than 600 nm. They correspond to the 340.0, 411.0,
495.0, 537.0 and 602.5 nm absorption bands observed for iodine single-crystal ﬁlms (Schnepp
et al 1965). These ﬁve absorptions are assigned to optical transitions from valence bands
originating from the πg , πu and σg orbits in I2 molecules and to conduction bands originatingfrom σu orbits in I2 molecules (Yamamoto et al 1987, Schnepp et al 1965). It is known that theiodine ﬁlms deposited at 123 K were polycrystals of orthorhombic structure (Grimminger andRichter 1956). It has been experimentally known that, when a substance of anisotropic crystal
structure is evaporated onto a low temperature substrate, the ﬁlm produced tends to containa considerable number of defects. The present ﬁlm was produced at temperature lower than3244

S Mochizuki and F Fujishiro

Ts= 200 K

8

0.6

8

7

1 (x 10)

Optical density

6

0.4

1
5
300

0.2

400

500

600

700

800

Wavelength (nm)4

td (ms)
1: 7020
2: 7540
3: 8060
4: 8580
5: 9100
6: 9620
7: 10140
8: 16380

3

2
1

0.0
300

400

500

600

700

800

Wavelength (nm)Figure 3. The time evolution of the optical density spectrum of iodine ﬁlm on a sapphire substrate(Ts = 200 K) during deposition. The spectra at the initial and ﬁnal stages of the deposition arecompared in the inset.

123 K and therefore it may contain some crystal defects. The absorption intensity increase of
these bands with increasing temperature from 96 to 200 K may arise from some improvementin crystal quality of the ﬁlm upon thermal annealing. Therefore, we deposited iodine on ahigher temperature substrate (Ts > 96 K) and measured the OD spectrum during deposition.
Figure 3 shows the time evolution of the OD spectrum of the iodine ﬁlm deposited on asapphire substrate whose temperature was 200 K, as a typical example. Each measurement
was made for 130 ms at intervals of 130 ms. In the inset, curve 1 obtained at the initial stage iscompared with that obtained at the ﬁnal stage by rescaling curve 1. A shoulder appears at about
439 nm in an absorption tail (>415 nm) of some higher energy band which seems to consist of
several bands. As the deposition progresses, two absorption bands appear at 331 and 435 nm,
and at least two shoulder bands appear and become prominent at about 510 nm and below
600 nm. As the deposition progresses further, the absorption intensities of these bands and theshoulder bands increase. The spectral structure is similar to that of the iodine single-crystal
ﬁlm reported by Schnepp et al (1965). At the ﬁnal stage of the deposition, the 331 and 435 nm
bands become sharper. The 435 nm band is accompanied by some bands between 370 and
410 nm. After the deposition, the iodine ﬁlm was cooled down to 96 K and then it was heated toroom temperature. During the cooling and heating processes, the OD spectrum was measured
as a function of temperature. The result is shown in ﬁgure 4. With temperature decreasing to96 K, the 439 nm band becomes intense and sharp with an accompanying slight red-shift, whilethe 331 nm one is almost unchanged. The 510 nm shoulder becomes a clear absorption band at
about 510 nm. The absorption slope at wavelengths longer than 600 nm grows into a shoulder.

Real time optical study of ﬁlms

3245
96 K

0.6

200 K

Optical density

210 K

0.4
220 K

0.2

230 K

0.0
300

400

500

600

700

800

Wavelength (nm)Figure 4. The temperature dependence of the optical density spectrum (curve 8 in ﬁgure 3) of
iodine ﬁlm on a sapphire substrate during heating.

With increasing temperature from 96 K, the absorption decreases monotonically. As for theﬁlm deposited at 96 K, when the temperature exceeded 220 K, we heard the faint exhaustsound and the OD spectrum disappeared suddenly. The spectral evolution shown in ﬁgures 1
and 3 may be explained by the spectral transition from island-like ultrathin ﬁlm to uniform
thick ﬁlm. In the spectra (curves 1 in the ﬁgures) measured at the initial stage of the deposition,
small numbers of I2 molecules are thought to condense to form each island. In such cases,
the spectrum tends to resemble that of free I2 molecules and the apparent optical absorption
edge shifts to shorter wavelengths, as shown in ﬁgures 1 and 3. The spectral evolution frommolecule to solid, together with such an apparent red-shift of the absorption edge, has been
explained by calculating electronic energy structures for I2 and In (n > 2) (Satoko 2003).
Calculation of stable structures of In in our laboratory is planned.
3.2. Time evolution of the optical extinction spectrum of vapour produced by thermal
evaporation of pristine AgI in helium gas
Figure 5 shows a typical view of the gas evaporation of AgI in helium gas conﬁned at 110 Torr.
This photograph was taken at the middle stage of gas evaporation with illumination from axenon lamp. A dark zone (DZ), ranging from the upper edge of the crucible to a height ofabout 5.2 mm, and a long conical brown smoky zone (BSZ) can be clearly distinguished. The
smoky nature arises from light scattering by particles with sizes comparable to the incident
light wavelength. This suggests that the species of the BSZ are large microcrystals. As the gas
evaporation proceeds, the BSZ grows to produce a multilayer cone structure which consists ofa brown inner zone and a blue outer zone. With decreasing helium gas pressure, the vapour

3246

S Mochizuki and F Fujishiro

BSZh (mm)
122

DZ

crucible

Figure 5. Vapour zones produced by thermal evaporation of AgI powder in He gas conﬁned at
110 Torr. BSZ and DZ are the brown smoky zone and the dark zone, respectively.

zone structure becomes spread and indistinct. As a result, it becomes impossible to measure
the extinction spectrum.
Figure 6 shows the time evolution of the optical extinction spectrum of the BSZ at a heightof 12 mm from the crucible edge at 110 Torr. Each time td is given in the ﬁgure. At the initialstage of the gas evaporation, the observed spectrum (curve 2) consists of an absorption band P
centred at about 510 nm and another absorption band Q increasing toward about 320 nm. With
progressive evaporation, these bands grow as shown by curves 3, 4 and 5. The P band is assigned
to the X–B electronic–vibrational transitions in iodine diatomic molecules I2 (Yamamoto et al1987), while the absorptions due to the X–B transition in isotopes of AgI molecules, 109 AgI
and 107 AgI (Stueber et al 1998), and the interband transition in AgI microcrystals (Mochizuki
and Umezawa 1997, Dulin 1993) appear in the Q-band wavelength region. In the inset, theoptical extinction spectrum of the DZ at a height of 2 mm from the crucible edge is shown.
This spectrum was obtained at the steady state of the gas evaporation. The absorption band
centred at 515 nm and the absorption which increases toward 270 nm are assigned to the X–
B and X–C electronic–vibrational transitions in I2 molecules, respectively (Yamamoto et al1987). Differing from the case for the gas evaporation of pristine silver (Mochizuki and Ruppin
1993), the absorption lines due to atomic silver Ag1 , the absorption bands due to silver diatomic
molecules Ag2 and silver particles Agn (n > 2), and the absorption due to the X–A transitionin AgI molecules (Metropolis 1939) are not resolved clearly in the broad featureless spectra.
However, the existence of silver vapour species is ascertained at the initial stage of the ﬁlm
formation on a substrate whose temperature is 200 K, as shown in the next subsection. Stueber
et al (1998) already found isotopes of AgI molecules (109 AgI and 107 AgI) and Ag atoms as
species in the molecular beam generated by expansion cooling of vapour which was producedby heating AgI. After the gas evaporation experiments, we found some residual substance inthe crucible and thin yellow particles deposited on the upper lid of the evaporation chamber.
We examined them by x-ray diffractometry and photoluminescence spectroscopy. The x-ray
diffractogram showed that the residual substance in the crucible was silver metal and the thin

Real time optical study of ﬁlms
0.14

3247

Vapour zones

I2(X-B)

h=12 mmh= 2 mm0.12
5I2(X-B)

Optical extinction

0.10

0.08
300 400 500 600 700 800

4Wavelength (nm)0.06
3td (ms)
1: 33320
2: 37240
3: 43120
4: 47040
5: 86240

0.04

0.02
2
1

0.00
300

400

500

600

700

800

Wavelength (nm)Figure 6. The time evolution of the optical extinction spectrum of the brown vapour zone at aheight of 12 mm from the crucible edge during evaporation. The optical extinction spectrum of
the dark vapour zone at a height of 2 mm from the crucible edge for the ﬁnal stage of the gas
evaporation is shown in the inset.

yellow deposits were mixtures of βAgI and γ AgI microcrystals. Taking account of the vapour
species aggregation promoted by atmospheric He gas, it is suggested that Agm In (m, n  1)
particles, I2 molecules and Ag atoms may all be vapour species in vacuum evaporation. The
chemical formula Agm In is the general term for AgI diatomic molecules, AgI clusters ((AgI)n )and nonstoichiometric silver iodide clusters ((AgI)n Agδ (δ > 1) and (AgI)n Iε (ε > 1)). The(AgI)n Agδ and (AgI)n Iε are known as fragment clusters. These vapour species from the
crucible are deposited on the high temperature substrate to produce a ﬁlm. When the substratetemperature is lower than 220 K, the vapour species adsorbed on the substrate may condense
to form Agn –Agm In –I2 composite ﬁlm. Since the rate of evaporation of I2 is higher than thosefor other species, Agn and Agm In are thought to be embedded in an iodine ﬁlm matrix. This
speculation may be ascertained spectroscopically, as reported in later subsections.
3.3. Time evolution of the optical density spectrum of ﬁlm deposited on a low temperature
substrate during vacuum evaporation of AgI
Figure 7 shows the time evolution of the OD spectrum of the ﬁlm deposited on a sapphire
substrate at 109 K during vacuum evaporation. Each measurement was made for 1600 ms
at intervals of 4000 ms. In the inset, curve 1 obtained at the initial stage is compared withthat obtained at the ﬁnal stage by rescaling curve 1. The colour of the ﬁlm was light brown.
Although the spectra are broader than those of iodine ﬁlms, faint inﬂection points (a, b, cand d) are seen at about 417, about 475, about 544 and about 597 nm at the initial stage of3248

S Mochizuki and F Fujishiro
Ts= 109 K

11
11
a

0.6

1 (x 27.8)

10

d
b
c

Optical density

9

8

300

0.4

400

500

600

700

800

Wavelength (nm)7

td (ms)
1: 78400 7: 201600
2: 140000 8: 207200
3: 156800 9: 212800
4: 168000 10: 218400
5: 179200 11: 224000
6: 190400

6

0.2

5
4
3
2
1

0.0
300

400

500

600

700

800

Wavelength (nm)Figure 7. The time evolution of the optical density spectrum of the ﬁlm deposited on a sapphire
substrate (Ts = 109 K) during vacuum evaporation of AgI powder. The spectra at the initial andﬁnal stages of the deposition are compared in the inset.

the evaporation. As compared with the absorption spectra of pristine AgI (Cardona 1963,
Mochizuki and Ohta 2000, Mochizuki 2001) and pristine iodine ﬁlms described above, thefollowing are found. The inﬂection at 417 nm is due to the Z1,2 exciton absorption (422 nm).
The inﬂections at 475, 544 and 597 nm arise from the absorption bands centred at 340,411, 495,
537 and 603 nm in iodine ﬁlms. That is, the ﬁlm on the low temperature substrate at the initialstage of the deposition is a composite ﬁlm made of, at least, AgI and iodine. It is hard to deduce
the absorptions due to other species from the observed spectra. With progressing deposition,
the absorption increases and the absorption edge extends to wavelengths longer than 800 nm.
One possible cause of such a spread in absorption is additional optical scattering at the I–AgIinterfaces in the I–AgI composite specimen. We elevated the specimen temperature to room
temperature and measured the spectra at different temperatures during heating. The results areshown in ﬁgures 8(a) and (b). With increasing temperature, the absorption continues to increase
and then reaches a maximum at 210 K. The colour of the ﬁlm becomes dark brown. On further
increasing temperature, the absorption decreases monotonically. Similarly to in the abovedescribed experiments for iodine ﬁlms, when the temperature exceeded 230 K, there was thesame exhaust sound. Simultaneously, the intense broad spectrum disappeared suddenly, a weak
spectrum peculiar to AgI emerging. Both the exhaust sound and the spectral change indicate
the evaporation of excess iodine from the ﬁlm and indicate also that vapour species related to
AgI evaporation are dispersed in an iodine ﬁlm matrix. Through experiments under different
conditions, it is found that the spectral change temperature tends to vary with heating speed andﬁlm thickness. With temperature increasing from 240 to 290 K, the tail at wavelengths longer
than 430 nm becomes considerably weakened and the absorptions between 370 and 410 nm

Real time optical study of ﬁlms
2.5

3249

(a)
210 K

Optical density

2.0

1.5

190 K

1.0
107 K

0.5

0.0
300
2.5

400

500

700

800

(b)
210 K

2.0

Optical density

600

11 K

1.5

240 K
290 K

230 K

1.0
320

360

400

440

Wavelength (nm)
0.5

290 K

240 K

0.0
300

400

500

600

Wavelength (nm)

700

800

Figure 8. The temperature dependence of the optical density spectrum (curve 11 in ﬁgure 7) of
iodine ﬁlm on a sapphire substrate during heating: (a) T  210 K, (b) T  210 K. The exciton
spectra at 240 and 290 K during heating are compared, together with the spectrum at 11 K, in theinset.

decrease. The tail may be due to excess iodine included tightly in AgI ﬁlm. The absorptions
between 370 and 410 nm correspond to the H1 and H2 exciton ones in polytype AgI (Mochizuki
2001), which are induced by some stacking disorder in the AgI lattice. This ﬁlm was cooledagain, down to 11 K, and the OD spectrum was measured. As indicated in the inset, a clear
Z1,2 absorption peak was observed at 422 nm, which indicates that the ﬁlm is γ AgI, and this isdiscussed later in detail. Similarly to that for the iodine ﬁlm evaporated at 96 K, the absorption
increases with increasing temperature from 107 to 210 K. The band centred at about 470 nm,
which appears as a shoulder at 107 K, becomes prominent. The spectral change may arise
from the improvement in crystallinity of iodine domains upon heating (annealing). However,
we cannot exclude effects of some particles (for example, Agm In ) grown at the AgI/iodineinterfaces. Therefore, we deposited iodine on a higher temperature substrate (Ts > 96 K) andmeasured the spectrum during deposition, as follows.
Figure 9 shows the time evolution of the OD spectrum of the ﬁlm deposited on a sapphire
substrate at 200 K during vacuum evaporation. Each measurement was made for 1400 ms
at intervals of 3990 ms. The colour of the ﬁlm was light brown. At the initial stage of thedeposition, the spectrum consists of a broad band centred at 370 nm and a short wavelength
absorption which increases toward some wavelength shorter than 330 nm. In the inset,

3250

S Mochizuki and F Fujishiro
1.2

Ts= 200 K

IBT

8

SPR

1.0
SPR

7

IBT

Optical density

0.8

2
1

6
300

0.6

350

400

450

500

Wavelength (nm)td (ms)
1: 64680
2: 156310
3: 177870
4: 199430
5: 242550
6: 258720
7: 269500
8: 339570

5

0.4
4

0.2

3

2
1

0.0
300

400

500

600

700

800

Wavelength (nm)Figure 9. The time evolution of the optical density spectrum of the ﬁlm on a sapphire substrate(Ts = 200 K) during deposition. Curves 1 and 2 are compared in the inset. SPR and IBT denotethe surface plasmon absorption band and the interband transition band of silver particles deposited,
Agn , respectively.

curves 1 and 2 are expanded and shown. These curves are very similar to those observed forsilver clusters and microcrystals (Mochizuki and Ruppin 1993). The 370 nm band and shortwavelength absorption are assigned to the collective mode resonance of valence electrons (socalled surface plasmon resonance (SPR)) and interband transitions (IBT) of silver particles(Agn : n > 2), respectively. Such absorptions indicate that silver atoms are also one of thevapour species for vacuum evaporation of AgI powder. At later times of deposition, the SPR
band has a long wavelength tail, as shown in the inset. The tail is due to interaction between
adjacent Agn , which was observed frequently in the spectra of gas evaporated Agn ﬁlms
(Hayashi et al 1990) and free Agn (Mochizuki and Ruppin 1993). Thus, it has been deduced
spectroscopically that Ag atoms coming from the evaporation source condense to form Agn
island ﬁlm on the 200 K substrate at the initial stage of the deposition. In curve 6, at the
middle stage of the deposition, a kink appears at about 415 nm. As discussed later, the kink isconnected to the W1 and W2 exciton bands of AgI. The appearance of such an AgI-related kinkmeans that vapour species diffuse easily and grow to become AgI particles in the iodine ﬁlm
matrix at 200 K. At the ﬁnal stage of the deposition, the absorptions at wavelengths longer than
650 nm become prominent, forming the absorption edge structure of solid iodine. We cooled
the specimen to 99 K and then elevated the specimen temperature to room temperature. We
measured the spectra at different temperatures during the cooling and heating processes. The
results are shown in ﬁgure 10. With cooling from 213 K, the absorption continues to increase
and the maximum at about 420 nm shifts to about 430 nm at 99 K. With temperature increasingfrom 99 K, the absorption decreases monotonically. When the temperature exceeded 220 K,

Real time optical study of ﬁlms

3251

1.4
99 K
130 K

1.2

13 K

213 K
307 K
220 K

1.0

Optical density

100 K
300 K

0.8

250 K

0.6

280

320

360

400

440

Wavelength (nm)
230 K

0.4

0.2

240 K
250 K
300 K

0.0
300

400

500

600

700

800

Wavelength (nm)Figure 10. The temperature dependence of the optical density spectrum (curve 8 in ﬁgure 9) of
the ﬁlm on a sapphire substrate during cooling and heating processes. The exciton spectra during
the cooling and heating processes are compared, together with the spectrum at 13 K, in the inset.

there was a faint exhaust sound from the vacuum pump attached to the specimen chamber, as
heard for pristine iodine ﬁlms. Simultaneously, the intense broad spectrum became weakened
and it disappeared suddenly at 240 K, a weak spectrum peculiar to AgI emerging. Throughdifferent experiments, it was found that the spectral change temperature tends to vary withheating rate and ﬁlm thickness. With temperature increasing from 240 to 300 K, the tail at
wavelengths longer than 430 nm disappeared and the absorptions between 370 and 410 nm
decreased. The tail may be due to some excess iodine included tightly in AgI ﬁlms. The
absorptions between 370 and 410 nm correspond to the H1 and H2 exciton ones in polytype
AgI, which are induced by some stacking disorder in the AgI lattice. This ﬁlm was cooledagain, down to 13 K, and the OD spectrum was measured. The W1 , W2 and W3 exciton
absorption bands are observed at 420, 414 and 320 nm, respectively. This indicates that theﬁlm is βAgI. In the inset, the spectra measured at 250 and 300 K for the ﬁrst heating process,
100 K for the second cooling process and 307 K for the second heating process and 13 K forthe third cooling process are compared. A remarkable change occurs at wavelengths shorter
than 320 nm. A possible explanation for such spectral change is desorption of iodine from theﬁlm or reaction of imperfect AgI ﬁlm with iodine during the heating and cooling processes.
3.4. Time evolution of the optical density spectrum of ﬁlm deposited on a room temperature
substrate during vacuum evaporation of AgI
We measured the time evolution of the OD spectrum of the ﬁlm deposited on a room temperature
sapphire substrate during vacuum evaporation. The evaporation condition is the same as in theexperiments with the low temperature substrate. In order to study the optical spectrum for a

3252

S Mochizuki and F Fujishiro
Ts= 300 K

W1

W3

W2

0.3

E

F
W 2 , W1

8

13 K

Optical density

2
1

F

0.2
8
7

E
410

415

420

425

430

435

Wavelength (nm)6

td (ms)
1: 215600
2: 258720
3: 291060
4: 323400
5: 463540
6: 501270
7: 549780
8: 684530

5

0.1
4
3
2
1

0.0
300

400

500

600

700

800

Wavelength (nm)Figure 11. The time evolution of the optical density spectrum of the ﬁlm on a sapphire substrate(Ts = 300 K) during deposition. The spectra at the initial and ﬁnal stages of the deposition arecompared, together with the spectrum at 13 K, in the inset.

wide thickness range, we performed successively the same depositions on the substrate. The
results of the two successive deposition experiments are shown in ﬁgure 11. Each measurement
was made for 1400 ms at intervals of 3990 ms. In the inset, the exciton spectra (curves 1, 2 and8) at different stages of the deposition are compared after appropriate rescaling, together withthe spectrum measured at 13 K. At the initial stage of deposition, two bands, E and F, appear at
420 and 414 nm, respectively. With progressing deposition, the absorption intensity of the Eband becomes superior to that of the F band, and the J band at 320 nm becomes prominent. At
the ﬁnal stage of the deposition, the F band is merged with the E band, while the bands become
well resolved at 13 K. At the initial stage of deposition, vapour species may condense on
the substrate to form AgI microcrystal ﬁlm. In the previous paper (Mochizuki and Umezawa
1997), we reported for AgI microcrystals that the Z1,2 exciton band split into two bands centred
at 420 nm and 415 nm due to the quantum size effects on the doubly degenerate valence band 8
or some crystal ﬁeld change at the crystallite surfaces due to surface reconstruction or surface
defects or the appearance of the wurtzite modiﬁcation. Since the intensity peak wavelengthsof these bands are almost unchanged with progressive deposition, the E, F and J bands are
assigned to the W1 , W2 and W3 exciton bands of βAgI (Cardona 1963, Mochizuki 2001,
Mochizuki and Ohta 2000). Thus, the ﬁlm deposited on room temperature sapphire substrateis optically identiﬁed with wurtzite phase AgI. The relative absorption increase of the W1 (E)band with increasing deposition time (ﬁlm thickness) may arise from the decreased exciton
conﬁnement and decreased surface (surface reconstruction and defects) effects. After theevaporation experiments, we measured the OD spectrum as a function of ageing time at 301 K.

Real time optical study of ﬁlms

3253

T s = 109 K

T s = 200 K

T s = 300 K

W1

W1

Z1,W1
H'1
H''1

S

PL intensity, OD

W2

W2

H''2

H1

H'2
H2

Z2,W2

415

425

OD
(T= 11 K)

G1

OD
W1

OD

(T= 13 K)(T= 13 K)
G1

G2

PL

PL

(T= 11 K)(T= 13 K)

PLW1(T= 13 K)
G2

380 400 420 440 460 480 380 400 420 440 460 480 380 400 420 440 460 480

Wavelength (nm)Figure 12. The optical density and photoluminescence spectra of different AgI ﬁlms near 12 K.

It was found that the absorption at wavelengths shorter than 300 nm grows with increasing
ageing time and, after 60 min, the spectrum comes to resemble that of pristine AgI (Cardona
1963, Mochizuki 2001, Mochizuki and Ohta 2000). This result is very similar to the result for
the low temperature (200 K) substrate.
3.5. Photoluminescence of AgI ﬁlms deposited on different temperatures of substrates
Figure 12 shows the OD and PL spectra of the AgI ﬁlms deposited on the sapphire substrateat three different temperatures (109, 200 and 300 K). First, we discuss the spectra of the
ﬁlm deposited on the 200 K substrate. This ﬁlm displays sharp well resolved absorption
bands at 422 and 416 nm due to W1 and W2 excitons, respectively. The PL spectrum shows
at 423 nm a sharp emission which has a full width at half-maximum (FWHM) of 79 cm−1
(approximately 10 meV) and with a very small Stokes shift (<7 meV) from the W1 exciton
absorption peak. Since the 7 meV shift is within the maximum systematic error between OD
and PL measurements, the emission can be assigned to the radiative decay of free W1 excitons.
The ﬁlm deposited on the 109 K substrate shows an intense absorption band at 422 nm
which coincides with the Z1,2 and W1 exciton wavelengths. However, this band is accompanied
by a faint shoulder band (S) on the short wavelength side, as indicated by an arrow in the inset.
The shoulder wavelength is close to that of the W2 exciton. The 422 nm band and the shoulder
may be assigned to the Z1 and Z2 exciton bands which are split by some crystal ﬁeld change due
to appearance of the wurtzite modiﬁcation, or to defected βAgI. Incidentally, most of the Z1,2
exciton absorption bands previously reported by different authors are also accompanied by afaint shoulder band at about 415 nm. It is also noted that a structured broad absorption band isseen between 380 and 415 nm, which had not been observed for other ﬁlms. Similar absorption
bands was observed previously at 405 and 398 nm for the ﬁlm specimens (Mochizuki 2001),

3254

S Mochizuki and F Fujishiro

which were assigned to the H1 and H2 excitons trapped at some stacking-disorder-induced
polytype structures of βAgI. The 404 and 392 nm shoulders which are labelled respectively
as H1 and H2 , and the 400 and 389 nm peaks which are designated respectively as H1 and H2are the exciton absorptions due to other types of polytype structure of βAgI. The PL spectrumof this ﬁlm displays at least two broader bands. The intensity peaks at 434 and 427 nm which
are designated respectively as G2 and G1 are observed, while the emission due to the radiative
decay of free W1 excitons is not clearly resolved. By comparing the PL spectral structure with
the OD one, the G1 and G2 emissions are assigned to the radiative decays of excitons trappedat crystal defects.
The ﬁlm deposited on the 300 K substrate shows also the W1 and W2 absorption bands
which have larger FWHM than those of the ﬁlm deposited on the 200 K substrate. This
indicates the degradation of the crystallinity of βAgI. The G1 and G2 emissions are also
observed. An extremely faint shoulder is observed at 422 nm. As for the ﬁlm deposited on the
109 K substrate, the G1 and G2 emissions are due to the radiative decays of excitons trappedat crystal defects.
The above optical studies suggest that an optimum substrate temperature for fabricating
βAgI ﬁlm is close to 200 K.
3.6. Film formation by thermal evaporation of AgI powder in vacuum
The present studies have indicated optically that the species in the vapour zones produced bythermal evaporation of AgI powder are Ag atoms (Ag1 ), silver iodide clusters (Agm In ) anddiatomic iodine molecules (I2 ). As deﬁned above, Agm In is the general term for AgI diatomic
molecules, AgI clusters ((AgI)n ) and nonstoichiometric silver iodide clusters ((AgI)n Agδ
(δ > 1) and (AgI)n Iε (ε > 1)). By the optical and time-of-ﬂight mass spectroscopic methods,
Bernstein (2003) has been studying Agm Xn (X: Cl, Br, I) particles (clusters) in an effort tounderstand the structure, dynamics and photochemistry of small clusters, and to explore theproperties of larger Agm Xn structures as they grow from nanoclusters to the bulk material. We
discuss the AgI ﬁlm formation on the substrate whose temperature is below room temperature.
We ﬁrst note that iodine solid has the highest vapour pressure and I2 molecules sublimate
from the deposited ﬁlm on a sapphire substrate prominently above 220 K under evacuation, as
described above. The vapour species coming from the evaporation source arrive at the substratesurface and, when Ts is low enough to suppress re-evaporation, they are well adsorbed to form
a composite ﬁlm of Ag1 , Agm In and I2 . Since the rate of evaporation of I2 is very high, Ag1
and Agm In are thought be dispersed homogeneously in an iodine ﬁlm matrix. These Ag1 andsmall Agm In structures act as condensation nuclei for growing Agm In in the iodine ﬁlm at latertimes. Incidentally, it is known that the cores of photographic sensitization AgI nuclei contain
excessive iodine, producing the Agm In clusters. The composite ﬁlm may display therefore aslightly different OD spectrum from that of pristine iodine ﬁlm, as seen already in ﬁgures 1
and 7. When Ts is moderate, perhaps approximately 200 K, some of the adsorbed excess
I2 desorbs from the substrate, while Ag1 and Agm In grow, generating larger Agn structuresand larger Agm In structure, respectively, through some coalescence, absorption and reaction
processes. Since the composite ﬁlm, however, exhibited only the SPR absorption of Agn at theinitial stage of the deposition, as shown in ﬁgure 9, the number of Agm In structures may be small
in the ﬁlm or the Agm In structures may be too small to display measurable optical absorption.
With progressing deposition, the Agn acts as condensation nuclei, absorbing I2 and small Agm In
structures coming later from the evaporation source, generating larger nearly stoichiometric
AgI particles. As a result, the kink at the exciton wavelength may appear in the OD spectrumof the composite ﬁlm, as shown by curves 6, 7 and 8 in ﬁgure 9. Since I2 neighbouring
Agm In is thought to be bound more tightly in the composite ﬁlm than in pristine iodine ﬁlm,

Real time optical study of ﬁlms

3255

different temperature dependences may be observed in the OD spectra of the pristine iodine
and composite ﬁlms, as shown in ﬁgures 4 and 10. Incidentally, the spectral structure is also
affected, as shown in ﬁgures 3 and 9. When Ts is room temperature, re-evaporation of I2
from the substrate, reactions and coalescence between adsorbed vapour species are enhanced,
generating nearly stoichiometric AgI ﬁlm, as shown in ﬁgure 11. The reactions, coalescences
and annealing effects become promoted with Ts , while the re-evaporation and desorption of
I2 with increasing Ts are also promoted, increasing the deviation from the stoichiometriccomposition of AgI. Therefore, an optimum Ts for fabricating the highest quality AgI ﬁlm
below room temperature may exist. Since the ﬁlm deposited on the 200 K sapphire substrate
showed the sharpest OD and PL spectra, 200 K is thought to be the optimum Ts .
In the present discussion, we have ignored the effects of the kinds of substrate materials andthe crystal orientation of the substrate surface. Up to the present, we have found the following
substrate effects. When AgI powder is ﬂash evaporated and deposited on different kinds ofroom temperature substrates (silica glass, sapphire single crystal and GaAs single crystal),βAgI tends to crystallize on the sapphire single-crystal and GaAs single-crystal substrates,
while γ AgI tends to crystallize on the silica glass. Thus, there is a need to study the substrateeffects. More detailed study with different kinds of substrate materials and with different
orientations of substrate surfaces is in progress, in an attempt to fully explain the mechanismof AgI ﬁlm formation.
4. Conclusion
We have evaporated AgI powder in vacuum by setting different evaporation conditions for theevaporation and deposition. The OD spectrum of AgI ﬁlm on a substrate was measured as afunction of the time elapsed from the beginning of the deposition for the ﬁrst time. The same
experiment was carried out for pristine iodine. Besides these vacuum evaporation experiments,
we have produced two vertically well separated vapour zones of AgI above the evaporation
source in a conﬁned He gas atmosphere and obtained the optical extinction spectra as a functionof the time elapsed after the beginning of evaporation. On the basis of these results, wehave studied the process of AgI ﬁlm formation on the low temperature (<room temperature)
substrate and the following have been found.
(1) The vapour species produced by thermal evaporation of AgI powder in vacuum are Ag1 ,Agm In and I2 .
(2) The ﬁlm deposited at low temperatures (200 K) is a composite ﬁlm. That is, Ag1 , Agn
and Agm In are dispersed in an iodine ﬁlm matrix to form a composite ﬁlm.
(3) Intense iodine optical absorption, which is slightly affected by Ag1 and Agm In , has led
many researchers to the wrong conclusion that amorphous AgI ﬁlms prepared by quench
deposition display strongly enhanced optical absorption in comparison to crystalline ﬁlm.
(4) AgI ﬁlm is grown through desorption of excess I2 from the composite ﬁlm and a coalescent
reaction among ﬁlm elements (Ag1 , Agn , Agm In and I2 ) with increasing temperature.
(5) The composite ﬁlm grown on the substrate at 200 K becomes transformed to thespectroscopically best βAgI ﬁlm by heating up to room temperature in vacuum.
The main limit of the present experimental technique is that there is no in situ
characterization of the ﬁlm thickness and crystal structure. Bearing these limits in mind,
we have presented a reasonable connection between measured optical data and the AgI ﬁlm
formation process, with a convincing coherence with the results on pristine iodine, the vapour
zone produced by thermal evaporation of AgI powder in a helium gas atmosphere conﬁned at
different pressures and the results obtained by the mass resolved excitation spectroscopy of an
expansion cooled AgI beam.

3256

S Mochizuki and F Fujishiro

Finally, a comparative study of the OD and PL spectra has also been performed at low
temperature for the ﬁlms deposited at different substrate temperatures. The exciton absorption
band at 422 nm, which is usually assigned to Z1,2 exciton absorption, has a faint shortwavelength shoulder. The absorption intensity of the shoulder is dependent on the specimen
preparation and the thermal history. The appearance of the shoulder has been explained by
splitting of the doubly degenerate 8 valence band. Such splitting may arise from some stress,
with different thermal expansion coefﬁcients for AgI and substrate materials (Mochizuki 2001),
or some crystal ﬁeld change due to mixing of the wurtzite (β) modiﬁcation and stacking disorderdue to nonstoichiometry. This speculation is in agreement with the well known result that γ AgI
appears at room temperature as a metastable state and the stability of the γ phase is affectedby slight nonstoichiometries and defects. Both the AgI ﬁlms displaying the less resolved
W2 exciton absorption band and the Z1,2 exciton absorption band with the short wavelength
shoulder show a broader PL band and the long wavelength PL band. On the other hand, theAgI ﬁlm displaying well resolved W1 and W2 absorption bands shows the sharpest single PL
band. Therefore, the PL spectrum measurement could be a useful tool for characterizing the
local structure in AgI ﬁlm. On the basis of this notion, we are now developing nanometre-scale
space resolved PL measurement with near-ﬁeld optical microscopy.
Acknowledgments
This work was partially supported by a Grant-in-Aid for Scientiﬁc Research from the Ministryof Education, Science, Sports, Culture and Technology, Japan. This work was also supported
in part by an Interdisciplinary General Joint Research Grant from Nihon University. Also, thiswork was partially supported by a Project Research Grant from The Institute of Information
Sciences of the College of Humanities and Sciences (Nihon University) and by a Cooperative
Research Grant from The Institute of Natural Sciences (Nihon University).
References
Bernstein E R 2003 http://franklin.chm.colostate.edu/erb/home metal.htm
Braner A A and Chen R 1963 J. Phys. Chem. Solids 24 135
Burley G 1963 Am. Mineral. 48 1266
Cardona M 1963 Phys. Rev. 129 69
Dulin M N 1993 Z. Phys. D 26 172
Fujishiro F and Mochizuki S 2002 Nonlinear Opt. 29 443
Grimminger V H and Richter H 1956 Z. Naturf. a 11 942
Hayashi S, Koga R, Ohtuji M, Yamamoto K and Fujii M 1990 Solid State Commun. 76 1067
Kondo S, Itoh T and Saito T 1998 Phys. Rev. B 57 13235
Many A, Simhony M, Weisz S Z and Levinson J 1961 J. Phys. Chem. Solids 22 285
Mathieson L and Rees A L G 1956 J. Chem. Phys. 25 753
Metropolis N 1939 Phys. Rev. 55 636
Mochizuki S 1996 J. Lumin. 70 60
Mochizuki S 2001 Physica B 308–310 1042
Mochizuki S and Fujishiro F 2003a J. Phys.: Condens. Matter 15 5057
Mochizuki S and Fujishiro F 2003b Phys. Status Solidi c 0 763
Mochizuki S and Fujishiro F 2003c Phys. Status Solidi c 0 767
Mochizuki S and Ohta Y 2000 J. Lumin. 87–89 299
Mochizuki S and Ruppin R 1993 J. Phys.: Condens. Matter 5 135
Mochizuki S and Umezawa K 1997 Phys. Lett. A 228 111
Moss T S 1952 Photoconductivity of the Elements (London: Butterworths)
Satoko T 2003 private communication
Schnepp O, Rosenberg J L and Gouterman M 1965 J. Chem. Phys. 43 2767
Stueber G J, Foltin M and Bernstein E R 1998 J. Chem. Phys. 109 9831
Riggleman B M and Drickamer H G 1963 J. Chem. Phys. 38 2721
Yamamoto H, Seki K, Mori T and Inokuchi H 1987 J. Chem. Phys. 86 1775

ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 272–276 (2004) 428–429

Pressure-induced phase transition in a-MnS
Y. Ishidaa, Y. Mitaa,*, M. Kobayashia, S. Endob, S. Mochizukica

Materials Physics, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japanb
Research Center for Materials Science at Extreme Conditions, Osaka University, Osaka 560-8531, Japanc
College of Humanities and Sciences, Nihon University, Tokyo 156-8550, Japan

Abstract
The IR reﬂection measurements of a-MnS have been performed at room temperature under various pressures. It isobserved that the reﬂectivity increases drastically at the pressure range of 23–29 GPa and becomes almost constant at
the higher pressure. The carrier concentrations obtained from the reﬂectivity spectra at the pressures higher than
29 GPa are the order of 1022 cm3. Therefore it is concluded that pressure-induced semiconductor–metal transition
occurs at the pressure range of 23–29 GPa. This result is quite consistent with the newest results of X-ray analysis.
r 2003 Elsevier B.V. All rights reserved.
PACS: 71.30.+hKeywords: MnS; Pressure; Reﬂectivity; Phase transition

The discovery of colossal magnetoresistance in
FexMn1xS solid solutions [1] revived interest in studying the physical properties of a-MnS, which is the basic
component of these substances. An a-MnS possesses an
NaCl-type face-centered cubic (B1) lattice which undergoes a rhombohedral distortion along the cube diagonalin the (1 1 1) plane with decreasing temperature. The
antiferro-paramagnetic phase transition occurs at
TN ¼ 150 K. In the paramagnetic phase, the a-MnS is
a p-type semiconductor with an activation energy
E ¼ 0:3 eV. In high-pressure region, the luminescenceand excitation spectra were studied up to 3.9 GPa andthe pressure dependences of crystal ﬁeld strength Dq
and Racah parameter B were obtained [2]. On the other
hand, the pressure-induced phase transitions have been
explored by many researchers, but their results wereinconsistent: Clendenen and Drickamer observed astructure distortion at approximately 10 GPa [3] andKraft and Greuling detected a structural phase transition from the B1 to the B16 (GeS type) at 7.2 GPa [4].
McCammon performed powder X-ray diffraction at
298 K from 0 to 21 GPa. However the data show no*Corresponding author. Tel.: +81-6-6850-6417; fax: +81-66845-4632.
E-mail address: mita@mp.es.osaka-u.ac.jp (Y. Mita).

evidence of either transition, and are well ﬁt by a single
equation of state at whole the pressure range [5]. In
1993, Sweeney and Heinz observed a phase transition of
a-MnS at 26 GPa from B1 phase to a high pressure one
whose symmetry is lower than hexagonal but notconcluded and it is stable to at least 46 GPa [6]. In thatcase, change of some optical properties must be
observed around the pressure region.
We performed the IR reﬂection measurements of
a-MnS under high pressure up to 37 GPa, which havebeen obtained by using a diamond anvil cell (DAC) at
room temperature. The reﬂectance spectrum for theinterface between the sample and diamond in mid-IR
range was measured utilizing a Fourier transform
infrared spectrometer FT/IR-610 (JASCO) combinedwith an infrared microscope installed with two mirror
objectives (10  ) and an MCT detector. In order toobtain the absolute reﬂectivity, powdered sample wascompressed directly by diamond anvil without using any
pressure mediums.
The reﬂection spectra obtained under various pressures and their pressure dependence at the point of
0.4 eV are shown in Figs. 1 and 2, respectively.
With increasing pressure, the reﬂectivity shows drastic
increase at the pressure range of 23–29 GPa. Note that
this pressure coincides with that of the structural change

0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2003.11.141

ARTICLE IN PRESS
Y. Ishida et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 428–429

429

Here eb ; G; N and m are background dielectric
constant, damping constant, carrier concentration andeffective mass of the carrier, respectively. The normal
reﬂectance RðoÞ is calculated using the Fresnel relation:

Fig. 1. Reﬂection spectra of a-MnS obtained under variouspressures.

RðoÞ ¼ jðns  nD Þ=ðns þ nD Þj2 ;ð3Þ

n2s ¼ eðoÞ;ð4Þ

where ns and nD are the reﬂective index of sample andanvil diamond (=2.4), respectively. Our reﬂectivity
measurements were performed using infrared microscope, which means that the incident condition is notjust normal. However deviation angle of the incident
light on sample from the normal is smaller than 8
because of high reﬂective index of anvil diamond.
Therefore we regarded the condition as nearly normal.
Parameter ﬁtting of the reﬂection spectra was performed using these relations and the obtained carrier
concentrations at high-pressure region (P > 29 GPa)
assuming m ¼ m0 are the order of 1022 cm3 which is
10–100 times higher than that of typical semimetal. It isalso observed that the reﬂectivity is independent of theapplied pressure at the pressure higher than 29 GPa.
These results indicate the band reconstruction to ametallic phase occurs at 23–29 GPa instead of theincreased band overlap. Therefore it is concluded thata phase transition occurs at 23–29 GPa in a-MnS andthe high-pressure phase is metallic.
The authors thank to Mr. T. Musha of Osaka
University for experimental supports.

Fig. 2. Pressure dependence of the reﬂectivity at the point of
0.4 eV. Abrupt increase of reﬂectivity is seen clearly around
26 GPa.

reported by Sweeney and Heinz (26 GPa) [6] and, inpresent study, no other optical changes were detected at
7.2 and 10 GPa. According to the Drude model, thedielectric function eðoÞ is written aseðoÞ ¼ eb  o2p =ðo2 þ G2 Þ þ io2p G=ðo3 þ G2 oÞ;

ð1Þ

o2p ¼ 4pNe2 =m:

ð2ÞReferences[1] G.A. Petrakovskii, L.I. Ryabinkina, N.I. Kiselev, D.A.
Velikanov, A.F. Bovina, G.M. Abramova, JETP Lett. 69(1999) 949.
[2] M. Kobayashi, T. Nakai, S. Mochizuki, N. Takayama,
J. Phys. Chem. Solids 56 (1995) 341.
[3] R.L. Clendennen, H.G. Drickamer, J. Chem. Phys. 44(1966) 4223.
[4] A. Kraft, B. Greuling, Cryst. Res. Technol. 23 (1988) 605.
[5] C. McCammon, Phys. Chem. Miner. 17 (1991) 636.
[6] J.S. Sweeney, D.L. Heinz, Phys. Chem. Miner. 22 (1993) 63.

ARTICLE IN PRESS

Journal of Luminescence 112 (2005) 267–270
www.elsevier.com/locate/jlumin

The reversible UV-laser-light-induced spectral change andorigin of the 2.4 eV luminescence band in SrTiO3
Shosuke Mochizuki, Seiko Minami, Fumito Fujishiro
Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan
Available online 24 November 2004

Abstract
In order to clarify the origin of the 2.4 eV luminescence band in SrTiO3, as-grown and annealed SrTiO3 single crystals
are irradiated with a continuous-wave ultraviolet laser light (l ¼ 325 nm) in different atmospheres at room temperature.
Under the laser light irradiation in a vacuum, the 2.4 eV luminescence band grows with increasing irradiation time,
while it returns to the original weak-luminescence state under the same laser light irradiation in oxygen gas. The
excitation-intensity, temperature and time dependences of the photoluminescence spectra are also measured for the
crystals. All the results indicate that the 2.4 eV luminescence band arises not from intrinsic self-trapped excitons but
from extrinsic excitons trapped around oxygen defects. Near-band-edge emissions have been observed, for the ﬁrst time,at 3.2 and 2.9 eV under intense excitation.
r 2004 Elsevier B.V. All rights reserved.
PACS: 71.35.y; 78.55.m; 82.50.m; 61.72.yKeywords: Quantum paraelectric state; Photo-induced defects

A large number of the optical studies have been
devoted to clarify the quantum paraelectric state instrontium titanate (SrTiO3). Grabner [1] measured
the photoluminescence (PL) properties of doped
and undoped SrTiO3 crystals, and observed anear-infrared luminescence band around approximately 1.6 eV and a visible luminescence bandCorresponding

author. Tel.: +81 3 5317 9771;+81 3 5317 9771.
E-mail address: motizuki@physics.chs.nihon-u.ac.jp(S. Mochizuki).

fax:

around 2.4 eV.These PL bands were Stokes-shifted
considerably. He assigned them to intrinsic defects
or intrinsic excitons. Recently, Hasegawa et al. [2]measured the optical absorption and PL properties
of SrTiO3 crystal. They assigned the Stokes-shiftedlong-lasting luminescence around 2.4 eV to theradiative decay of intrinsic self-trapped excitons.
However, many metal oxides display the similar
visible PL, which are related to oxygen defects[3–6]. Unfortunately, such oxygen defect effect has
been accorded too low evaluation in the optical
study of SrTiO3 and it has led many researchers

0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jlumin.2004.09.050

ARTICLE IN PRESS

PL intensity (arb. units)

15x10

as-grown SrTiO3

3

60 min.
50 min.
40 min.

10

5

2 min. 10 min.
0 min. 4 min.
30 min.
20 min.

0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Fig. 1. The CW 325 nm laser light-induced spectral change ofan as-grown SrTiO3 crystal at room temperature in vacuum.
The tir is given in min.

15x10

as-grown SrTiO3

3

tir = 60 min.
10

tir = 0 min.
(rescaled: x 2.5)5

Difference

0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Fig. 2. The CW 325 nm laser light-induced PL component ofan as-grown SrTiO3 crystal at room temperature.

PL intensity (arb. units)

astray. Very recently, we have observed that the
2.4 eV luminescence band of SrTiO3 grows under acontinuous wave (CW) 325 nm-laser-light irradiation in a vacuum, while it disappears under thesame laser light irradiation in oxygen gas [7]. It
was found that the observed phenomenon relates
closely to the creation and annihilation of oxygen
defects under the laser light. Taking account of theoxygen defects in real SrTiO3 crystals [8–10], thePL properties of SrTiO3 should be now restudied
in detail. In the present paper, we report the PL
properties and reversible photo-induced spectralchange of as-grown and annealed SrTiO3 single
crystals.
SrTiO3 single crystals were grown by the Vernuil
method. As-grown crystals are dark blue and they
become transparent by annealing in an appropriate atmosphere. During CW 325 nm laser-lightirradiation, we have measured the PL spectra ofthese as-grown and annealed crystals as a functionof irradiation time tir. Fig. 1 shows the photoinduced PL spectral change of the as-grown
SrTiO3 crystal at room temperature in a vacuum
as a function of tir (in min). As seen in this ﬁgure,the initial PL spectrum (tir ¼ 0) is very broad, andit has an intensity peak at about 2.8 eV and two
faint kinks at about 2.4 and 2.15 eV. With
increasing tir ; the intensity increases, growing the
PL component below 2.8 eV. With tir increasing
further, the rate of the spectral change becomesdecreased. The spectrum at tir ¼ 0 min is rescaled,

PL intensity (arb. units)

S. Mochizuki et al. / Journal of Luminescence 112 (2005) 267–270

268

1400

annealed SrTiO 3

1200

60 min.

1000

50 min.
40 min.

800
600
400

0 min.
10 min.
20 min.
30 min.

200
0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Fig. 3. The CW 325 nm laser light-induced spectral change ofan annealed SrTiO3 crystal at room temperature in vacuum.
The tir is given in min.

and then a difference between the spectrum at tir ¼
60 min and the rescaled spectrum is calculated,which is shown in Fig. 2. This ﬁgure shows clearly
that a broad PL band grew at 2.4 eV under 325 nm
laser light irradiation in a vacuum. Fig. 3 shows
the photo-induced PL spectral change of theannealed SrTiO3 crystal at room temperature ina vacuum. It is noted that the annealing treatment
reduces the PL intensity to about one-tenth. The
PL component below 2.8 eV grows with increasingtir. With tir increasing further, the rate of thespectral change becomes decreased. As done forthe results of the as-grown crystal, the difference
spectrum is also calculated, which is given in
Fig. 4. It indicates clearly that a broad PL band of
the annealed crystal grew at 2.4 eV under 325 nm
laser light irradiation in a vacuum. The same
spectral change in the 2.4 eV band was observed

ARTICLE IN PRESS

1600

tir = 0 min. (rescaled: x 1.34)

annealed SrTiO 3

1200

50x10

269

3

as-grown SrTiO3

tir = 60 min.

12 K

800
400
0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Fig. 4. The CW 325 nm laser light-induced PL component ofan annealed SrTiO3 crystal at room temperature.

also at 12 K in our previous study [7]. As well as
the as-grown crystal, it is found, under the samelaser light irradiation in oxygen gas, the 2.4 eVband disappears, returning to the original weak PL
state. Since the 325 nm laser light never dissociates
directly free O2 molecule, the observed spectralchanges arise from the photo-induced associative
detachment and photo-induced dissociative adsorption of O2 molecules near the SrTiO3 surfaces[6].
We have studied the PL spectra of the as-grown
and annealed SrTiO3 crystals at 13 K as a functionof excitation laser ﬂuence of the third harmonics(l ¼ 355 nm) of a Nd3+-YAG laser. The results
are shown in Figs. 5 and 6. We have conﬁrmed
that, unlike CW 325 nm laser light, the pulsed
355 nm laser light cannot induce any reversible
spectral change even at 14 mJ/cm2. At the lowest
laser ﬂuence, both crystals exhibit only the intense
2.4 eV band. However, the center energy for the asgrown crystal is slightly smaller than that for theannealed crystal. We compare the PL spectra
observed at 4 mJ/cm2, before and after annealing.
The annealed crystal displays more intense 2.4 eV
luminescence than that for the as-grown crystal.
This may be due to decrease in the number of nonradiative trap by the annealing treatment. With
increasing laser ﬂuence, two new PL bands appear
at 3.2 eV, which is near indirect gap of SrTiO3 andat 2.9 eV. On the contrary to the 2.4 eV band, the
3.2 and 2.9 eV bands for the as-grown crystal are
more intense than those of the annealed crystal.
This indicates that the 3.2 eV luminescence and2.9 eV one are enhanced by oxygen defects. As
seen from Fig. 5, with increasing laser ﬂuence, the2.4 eV band for the as-grown crystal grows and

PL intensity (arb. units)

40

Difference

120 K
30

2

14 mJ/cm

297 K
3.2 2.8 2.4 2.0
Photon energy (eV)22 mJ/cm

20
2

12 mJ/cm

2

0.8 mJ/cm

2

10 mJ/cm

108 mJ/cm
4 mJ/cm

0.16 mJ/cm

2

2

2

0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Fig. 5. The excitation laser intensity dependence of the PL
spectra of an as-grown SrTiO3 crystal at 12 K. The temperaturedependence of the PL spectrum is shown in the inset.
3

35x10

annealed SrTiO3
12 K

30

PL intensity (arb. units)

PL intensity (arb. units)

S. Mochizuki et al. / Journal of Luminescence 112 (2005) 267–270

39 K

4.0 mJ/cm

25

2

297 K
20

2

3.2 2.8 2.4 2.0
Photon energy (eV)

10 mJ/cm

152

14 mJ/cm
105-2

8.1x10 mJ/cm

2

0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Fig. 6. The excitation laser intensity dependence of the PL
spectra of an annealed SrTiO3 crystal at 12 K. The temperaturedependence of the PL spectrum is shown in the inset.

then becomes saturated at 8 mJ/cm2, while the 3.2
and 2.9 eV bands continue to grow without any
saturation. As seen in Fig. 6, the PL intensity of
the 2.4 eV band for the annealed crystal increases

ARTICLE IN PRESS
270

S. Mochizuki et al. / Journal of Luminescence 112 (2005) 267–270

also with increasing laser ﬂuence, and then it
reaches a maximum at 4 mJ/cm2, while the 3.2 and2.9 eV bands continue to grow without displaying
any maximum. Besides these measurements, thePL spectra were measured at different temperatures between 12 and 297 K. Of them, the PL
spectra at 12, 120 and 297 K for the as-grown
crystal are given in the inset of Fig. 5, and the PL
spectra at 12, 39 and 297 K for the annealed crystal
are given in the inset of Fig. 6. As seen from these
insets, the 3.2 and 2.9 eV bands grow deteriorating
the 2.4 eV band with increasing temperature. The
2.4 eV band observed for the as-grown andannealed crystals disappeared at 120 and 39 K.
The 3.2 eV band becomes weak and broad withincreasing temperature. The 2.9 eV band remains
even at room temperature. We have also measured
the time-resolved spectra and found that the 2.4 eV
luminescence has long lifetime (Z1 ms), while boththe 3.2 luminescence and the 2.9 eV one have short
lifetime (550 ns), at 12 K [7].
Finally, the experimental results obtained in thepresent study are summarized and discussed asfollows. Since the PL intensity of the 2.4 eV band isaffected considerably by oxygen defects and it
saturates under intense excitation, the 2.4 eV bandis assigned to the radiative decay of extrinsic
excitons trapped around oxygen defects. The
excitation intensity dependences of the PL shownin Figs. 5 and 6 indicate that, with increasing laser
ﬂuence, the 3.2 and 2.9 eV bands grow deteriorating the 2.4 eV band. This intensity relation among
these PL bands at 2.4, 3.2 and 2.9 eV withincreasing excitation intensity may arise fromtwo causes: (1) ﬁnite number of the 2.4 eV
luminescence centers, and (2) electron–hole recombination enhanced with higher laser ﬂuence.
Since the recombination rate increases withincreasing carrier density, the near-band edge
emissions at 3.2 and 2.9 eV increase linearly withincreasing laser ﬂuence, while the rate of electron(including hole) migration to the 2.4 eV luminescence centers decreases with increasing laser
ﬂuence. The excitation-intensity and temperaturedependences of the 3.2 and 2.9 eV bands also
indicate that two PL bands occur from the same
origin. We measured also the PL excitation (PLE)
spectrum for the 2.4 eV band and found a sharp

PLE edge at 3.26 eV.This edge corresponds well tothe indirect gap of SrTiO3. Generally, an indirectgap-type semiconductor crystal does not exhibit
intense PL. However, some impurities and crystal
defects activate the PL of indirect excitons,
exhibiting intense PL band near band edge.
Therefore, the intense PL at 3.2 and 2.9 eV may
be assigned to the radiative decay of indirect
excitons affected by oxygen defects, creating
phonons. The energy difference (271 cm1) between the PLE edge and the peak energy of the
3.2 eV band is close to the transverse optic (TO)phonon frequency [11]. Thus, the 3.2 eV band isassigned to the one TO phonon sideband, while
the broad 2.9 eV band is also assigned to acontinuum consisting of other phonon sidebands.

Acknowledgements
This work is supported by a Grant-in-Aid for
Scientiﬁc Research from the Ministry of Education, Science, Sports, Culture and Technology,
Japan. This work is partially supported byInterdisciplinary General Joint Research Grant
for Nihon University.

References[1] L. Grabner, Phys. Rev. 177 (1969) 1315.
[2] T. Hasegawa, M. Shirai, K. Tanaka, J. Lumin. 87–89(2000) 1217.
[3] S. Mochizuki, T. Shimizu, F. Fujishiro, Physica B 340-342(2003) 956.
[4] Y. Kawabe, A. Yamanaka, E. Hanamura, T. Kimura, Y.
Takiguchi, H. Kan, Y. Tokura, J. Appl. Phys. 88 (2000)
1175.
[5] S. Mochizuki, H. Araki, Physica B 340–342 (2003) 969.
[6] S. Mochizuki, T. Nakanishi, Y. Suzuki, K. Ishi, Appl.
Phys. Lett. 79 (2001) 3785.
[7] S. Mochizuki, S. Minami, F. Fujishiro, J. Phys.: Condens.
Matter., submitted for publication.
[8] S. Kimura, J. Yamauchi, M. Tsukada M, Phys. Rev. B 51
(1995) 11049.
[9] V.E. Henrich, G. Dresselhous, H.J. Zeiger, Phys. Rev. B 17
(1978) 4908.
[10] R. Astala, P.D. Bristowe, Modell. Simul. Mater. Sci. Eng.
9 (2001) 415.
[11] R.A. Cowley, Phys. Rev. 134 (1964) A981.

ARTICLE IN PRESS

Journal of Luminescence 112 (2005) 71–74
www.elsevier.com/locate/jlumin

Photoluminescence studies on AgI–ZrO2 composites
Fumito Fujishiro, Shosuke Mochizuki
Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan

Abstract
AgI–ZrO2 ﬁne particle composites were fabricated over a wide composition range of AgI. We have measured the Xray diffractograms and photoluminescence properties of the composites, together with those of pristine AgI and pristine
ZrO2 ﬁne particles. The results give the information about the atomic structure of AgI/ZrO2 interfaces, which may
provide the pathway for the high ionic conduction.
r 2004 Elsevier B.V. All rights reserved.
PACS: 78.55.m; 61.72.y; 66.10.Ed
Keywords: Exciton; Photoluminescence; Ionic conductor; Interface; Oxygen defect

The ionic conductivity of AgI is considerablyenhanced by adding oxide ﬁne particles [1,2]. It isfrequently assumed that highly conductive layers
are created at AgI/oxide ﬁne particle interfaces.
However, it is difﬁcult to clarify experimentally theatomic structure of the interfaces. In order toinvestigate such interface structure, the photoluminescence (PL) study is effective. Besides AgI [3],most oxides also show structure-sensitive PL. Veryrecently, we have fabricated AgI–ZrO2 composites
and observed considerable conductivity enhancement [4]. In the present paper, we report thestructural and PL studies on AgI–ZrO2 compoCorresponding author. Tel./fax: +81 3 5317 9771.

E-mail address: fumito@phys.chs.nihon-u.ac.jp(F. Fujishiro).

sites, and we discuss the structure of AgI/ZrO2 ﬁneparticle interfaces.
AgI–ZrO2 composites were prepared by mixing
AgI powder (99.999%, Alfa) and ZrO2 powder(498%, Aldrich) with an average diameter of
26 nm, compressing uniaxially the mixture under
0.2 GPa for 30 min into pellets and then heating
the pellets in the lidded crucible at 673 K in air for
12 h. The composite pellets were characterized by
the X-ray diffraction analysis with Cu Ka1 radiation. The third harmonics, l ¼ 355 nm (=3.49 eV), of a pulsed Nd3+–YAG laser light wasused to excite PL.
Fig. 1 shows the X-ray diffractograms ofdifferent (x)AgI–(1–x)ZrO2 composites. All of
the diffraction lines observed for a pristine ZrO2(x=0) specimen are assigned to the monoclinic

0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jlumin.2004.09.086

ARTICLE IN PRESS
F. Fujishiro, S. Mochizuki / Journal of Luminescence 112 (2005) 71–74

72

phase, except for the tetragonal phase line at 30.21.
Since the diffraction line widths are hardly
changed with adding AgI, the average size ofZrO2 particles in the composites is thought to bealmost the same as that of the starting material
powder. On further increasing x, the AgI diffraction lines become pronounced. The diffraction
intensities of the bAgI(100) line at 22.31 andbAgI(200) line at 45.61 are considerably enhanced(x)AgI(1x)ZrO2
x = 1 (AgI)XRD intensity (arb. scale)x = 0.8
x = 0.6

in comparison with other AgI-based composites[1], which may be due to preferred-orientations.
The PL spectra for different (x)AgI–(1x)ZrO2
composites at 12 K are shown in Fig. 2. The
excitation laser ﬂuence Iex was 0.066 mJ/cm2.
Pristine ZrO2 shows a broad PL band centered
at about 2.9 eV. The PL peak energy depends onthe excitation photon energy Eex, as shown in
Figs. 3(a) and (b). Adding small amount(0.1p x p0.2) of AgI, the intensity of the broad
PL band decreases extremely, and a new PL band
appears at about 2.80 eV. At x ¼ 0:4; two PL
bands appear at 2.89 and 2.83 eV. On further
adding AgI (0.6p x p0.8), another new sharp PL
band appears at 2.94 eV, accompanying by someweak PL bands at its lower energy side. The
2.94 eV band is assigned to the radiative decay of
free excitons (FE) in AgI domain [3]. At x=1,

x = 0.4
x = 0.2

x = 0 (ZrO2)
20

30

40
2T (degree)

50

Fig.
1. TheX-ray
diffractograms(x)AgI–(1x)ZrO2 composites at 297 K.

60

for

PL intensity (arb. scale)x = 0.1

2:Eex= 4.59 eV

1:Eex= 4.96 eV

x = 0.6

PL peak energy (eV)(x)AgI(1x)ZrO2

x = 0.4(b)x = 1 (AgI)x = 0.8

4:Eex= 3.44 eV

3:Eex= 4.13 eV

different(a)

PL intensity (arb. scale)

ZrO2
5:Eex= 3.10 eV

3.2

3.0

2.8
2.6
2.4
Photon energy (eV)

2.2

3.0
2.9
2

2.8

4
3

2.7

5

1

2.6
5.0

4.5
4.0
3.5
Excitation photon energy Eex (eV)PLE

x = 0.1

E

x = 0 (ZrO2)
3.2

3.0

2.8

2.6
2.4
Photon energy (eV)

2.2

2.0

Fig. 2. The PL spectra for different (x)AgI–(1x)ZrO2
composites at 12 K.

(c)

6

5

4
Photon energy (eV)

3.0PL
Eex= 4.13 eVD

PL intensity

x = 0.2

2.0

Eex= 3.49 eV

32Fig. 3. (a) The PL spectra of pristine ZrO2 excited with various
Eex at 13 K; (b) the PL peak energy plotted against Eex; (c) the
PL and PLE spectra for pristine ZrO2 at 13 K.

ARTICLE IN PRESS
F. Fujishiro, S. Mochizuki / Journal of Luminescence 112 (2005) 71–74(x)AgI(1x)ZrO2

12 K

288 K

x = 1 (AgI)

12 K
288 K

PL intensity (arb. scale)x = 0.8
12 K
287 K
x = 0.6
13 K
288 K
12 K

99 K

99 K

x = 0.4
x = 0.2
x = 0.1

12 K

3.0

2.9

(a)
PL intensity

namely, pristine AgI, besides the free exciton
emission, more intense PL bands grow at 2.89
and 2.86 eV. These intense PL bands are assigned
to radiative decays of excitons trapped arounddefects and grain boundaries in AgI specimen.
In pristine ZrO2 (x=0), we have found that the
PL intensity peak energy depends on Eex, as shownin Fig. 3(a). The PL intensity peak energy isplotted against Eex in Fig. 3(b). It is noted thateach PL spectrum is considerably Stokes-shifted,
which indicates that there are different localized
electronic states in the energy gap (5.8 eV) of ZrO2.
The PL and PL excitation (PLE) spectra are also
shown in Fig. 3(c). The PLE curve a is monitored
at 2.99 eV, which corresponds to the PL intensity
peak energy for the excitation Eex=3.49 eV, while
the PLE curve b is monitored at 2.53 eV whichcorresponds to the PL intensity peak energy forthe excitation Eex=4.13 eV. The PLE spectra
ascertain the existence of the luminescent localized
electronic states in the energy gap. Since the PL
intensity of ZrO2 was decreased remarkably byadding AgI, as shown in Fig. 2, such luminescencecenters may be at ZrO2 particle surfaces.
The temperature dependence of PL spectra fordifferent (x)AgI–(1x)ZrO2 composites was measured. The results are shown in Figs. 4(a) and (b).
Each spectrum is normalized in intensity at eachmaximum. In small x range (0.1p x p0.2), withincreasing temperature, the 2.80 eV band is considerably weakened and almost disappears above
about 100 K. The broad PL band observed at 99 K
may arise from ZrO2. However, the PL spectral
shape above 2.85 eV is affected by the opticalabsorptions due to AgI [3]. At x ¼ 0:4; the PL
intensity of the 2.83 eV band decreases, while the
PL intensity of the 2.89 eV band increases incomparison with that of the 2.83 eV band, withincreasing temperature. It suggests that phononassisted reverse transition from the 2.83 eV luminescence center to the 2.89 eV luminescence centeroccurs. In large x range (0.6p x p0.8), withincreasing temperature, free exciton emission
decreases in intensity, while another PL band
appears at 2.90 eV. This indicates that the 2.90 eVband arises from radiative decay of excitons
trapped around thermally ionized defects orthermally ionized impurities.

73

2.7

12 K
288 K

3.2

(b)

2.8
Photon energy (eV)

3.0

2.8
2.6
2.4
Photon energy (eV)2.6

x = 0 (ZrO2)

2.2

2.0

Fig. 4. The temperature dependence of the PL spectra: (a) for
different (x)AgI–(1x)ZrO2 composites; (b) for pristine ZrO2.

Figs. 5(a) and (b) show the Iex dependence of the
PL spectra for different (x)AgI–(1x)ZrO2 composites at 12 K. Each spectrum is normalized inintensity at each maximum. Labels, I and W,
indicated by the curves denote intense (X1.0 mJ/cm2) and weak (p0.1 mJ/cm2) excitations, respectively. For small x region (p0.2), the PLcomponent below 2.80 eV saturates at intense
excitation, which indicates that the luminescencecenter giving this PL component is limited in
number, though it has high quantum yield. We
note the x dependence of the spectra for weakexcitation. With adding AgI, the 2.80 eV band
shifts to 2.78 eV at x ¼ 0:2 and, with further
adding AgI, it shifts to 2.81 eV at x ¼ 0:4: At bothx ¼ 0:6 and 0.8, the band remains as a shoulder of
AgI-related PL bands. Since adding AgI to ZrO2
induces the oxygen defects, Zr1–xAgxO2–3x/2(Vö)3x/2 [5], the shift may be connected with thechange in the electronic states at AgI/ZrO2
interfaces. On further adding AgI, the shoulders
at 2.94 and 2.86 eV become prominent.

ARTICLE IN PRESS
F. Fujishiro, S. Mochizuki / Journal of Luminescence 112 (2005) 71–74(x)AgI-(1-x)ZrO2

I (x 1)W (x 72)

PL intensity (arb. scale)I (x 1)

x = 1 (AgI)

W (x 67)
x = 0.8

I (x 1)W (x 1361 )

54(0.1)AgI-(0.9)ZrO2

I (x 1)W (x 120)x = 0.4

I (x 1)

2
1
0
3.2

2.9

PL intensity

(a)

2.8
Photon energy (eV)I (x 1)

3.0

2.8
2.6
2.4
Photon energy (eV)

3.0

2.8

2.6
2.4
Photon energy (eV)

2.2

2.0

Fig. 6. Reversible photo-induced PL spectral change of(0.1)AgI–(0.9)ZrO2 composite at 295 K.

x = 0.2
W (x 48)2.7

2.6

x = 0 (ZrO2)
W (x 65)

3.2

in oxygen

W (x 69)x = 0.1

3.0

in vacuum3

x = 0.6

I (x 1)(b)

PL intensity

74

2.2

2.0

Fig. 5. The Iex dependence of the PL spectra: (a) for different(x)AgI–(1x)ZrO2 composites; (b) for pristine ZrO2 at 12 K.

The spectral transition with adding AgI is
explained as follows. For small x region, the free
excitons generated in AgI domain collides frequently with AgI surfaces, and they are trapped at
AgI/ZrO2 interfaces, creating bound excitons.
With increasing x, the size of AgI domainincreases and free excitons are able to propagate
in a long distance. Therefore, before trapping at
AgI/ZrO2 interfaces, a part of free exciton decays
spontaneously with generating the 2.94 eV luminescence, and other part of free exciton is trappedat crystal defects and impurities within AgI
domain. The radiative decay of these trapped
excitons generates the 2.93 and 2.86 eV luminescence.
Besides the above PL properties, we have foundalso a reversible photo-induced PL spectralchange in the present AgI–ZrO2 composites atroom temperature. One of the results is shown in
Fig. 6, as a typical example. After fully irradiatingwith CW 325 nm laser light in an evacuatedspecimen chamber for 10 min, the specimen showsa broad PL band at 2.57 eV. Oxygen gas is thenintroduced into the specimen chamber and, withincreasing irradiating time, the PL component
below 2.45 eV decreases in intensity. Throughmany successive experiments, it is found that such
spectral changes appear repeatedly. The observed
spectral changes may originate from excitationenergy exchange between the photo-induced oxygen vacancies on ZrO2 particle and AgI. The
spectral transitions yield materials for opticalsensor devices.
In summary, we have studied the PL propertiesof (x)AgI–(1x)ZrO2 (0p x p1) composites. The
PL spectra have shown well the behavior of free
and trapped excitons at AgI/ZrO2 interfaces whereAg+ ions may move with high-ionic conductivity.
The specimen fabrications using different startingmaterial ZrO2 powders and more detailed studies
are now in progress.
This work is supported by a Grant-in-Aid for
Scientiﬁc Research from the Ministry of Education, Science, Sports, Culture and Technology,
Japan. This work is partially supported byInterdisciplinary General Joint Research Grant
for Nihon University.

References[1] S. Mochizuki, F. Fujishiro, J. Phys.: Condens. Matter 15(2003) 5057.
[2] M.C.R. Shastry, K.J. Rao, Solid State Ionics 51 (1992)
311.
[3] S. Mochizuki, F. Fujishiro, J. Phys.: Condens. Matter 16(2004) 3239.
[4] F. Fujishiro, S. Mochizuki, in preparation.
[5] F.A. Kröger, H.J. Vink, Solid State Phys. 3 (1956) 307.

INSTITUTE OF PHYSICS PUBLISHING

JOURNAL OF PHYSICS: CONDENSED MATTER

J. Phys.: Condens. Matter 17 (2005) 923–948

doi:10.1088/0953-8984/17/6/011

Photoluminescence and reversible photo-induced
spectral change of SrTiO3
Shosuke Mochizuki1 , Fumito Fujishiro and Seiko Minami
Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40
Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan
E-mail: motizuki@physics.chs.nihon-u.ac.jp

Received 7 June 2004, in ﬁnal form 4 January 2005
Published 28 January 2005
Online at stacks.iop.org/JPhysCM/17/923
Abstract
When strontium titanate (SrTiO3 ) single crystal is irradiated at room
temperature with a 325 nm laser light in an evacuated specimen chamber,the luminescence intensity increases, creating a broad visible luminescencecentred at about 2.4 eV. Then, introducing oxygen gas into the specimen
chamber, the photoluminescence spectrum returns reversibly to the original
weak luminescence under the same laser light irradiation. After removing thelaser light irradiation, each photoluminescent state is stored for a long time atroom temperature under room light, regardless of any changes of atmosphere.
Such photo-induced spectral change has been observed also at different
temperatures from 13 K to room temperature. The observed phenomenon
is explained by means of the photo-induced oxygen defect formation at thesurfaces of SrTiO3 crystal. For the same SrTiO3 single crystal, we have studied
the photoluminescence properties. Besides the 2.4 eV luminescence band, we
have observed new two luminescence bands centred at about 3.2 eV and about
2.9 eV. The energy, 3.2 eV, is close to both the photoluminescence excitationedge energy and the reported band edge energy of SrTiO3 crystal. Both the
3.2 eV luminescence and the 2.9 eV luminescence decay rapidly after a pulsed
photoexcitation, while the 2.4 eV luminescence lasts for several seconds at
13 K. The excitation light intensity dependence of these luminescence bands
has been also measured at 13 K. The 2.4 eV luminescence increases in intensitywith increasing excitation intensity up to 4 mJ cm−2 , and then it becomesdecreased with further increase in the excitation intensity. On the other hand,both the 3.2 eV luminescence and the 2.9 eV luminescence increase in intensitywith increasing excitation intensity, without any saturation. Although the
2.4 eV luminescence had been assigned to the radiative decay of intrinsic selftrapped excitons in a superparaelectric state by several workers, the present
studies have clariﬁed that the luminescence originates mainly from crystal
defects (oxygen defects and chemical heterogeneity in the surface region).
1 Author to whom any correspondence should be addressed.

0953-8984/05/060923+26$30.00© 2005 IOP Publishing Ltd

Printed in the UK

923

924

S Mochizuki et al

Both the 3.2 eV luminescence and the 2.9 eV luminescence are discussed
qualitatively.

1. Introduction
Considerable attention has been given to clarifying the quantum paraelectric state in strontium
titanate (SrTiO3 ). The lattice of SrTiO3 has the cubic perovskite-type structure at room
temperature. At 105 K, it transforms into a tetragonal structure. With temperature decreasingfrom room temperature, the static dielectric constant increases prominently, and it attains
several ten thousands at 4 K without any paraelectric–ferroelectric transition (Müller and
Burkard 1979). Between 4 and 0.3 K, the dielectric constant was found to be independent of
temperature. Recently, a prominent enhancement of the dielectric constant has been observed
under ultraviolet light irradiation (Katsu et al 2001, Takesada et al 2003, Hasegawa et al 2003),
but it is now under dispute, involving some complicated problems relating to electrode/SrTiO3
interfaces. In order to explain the quantum paraelectric state, one may assume ﬁrst that Ti4+
ions are ﬂuctuating quantum mechanically in the SrTiO3 lattice, as if they are in the critical state
just before the paraelectric–ferroelectric transition in usual ferroelectrics. Müller et al (1991)
proposed that SrTiO3 is in a coherent quantum state associated with a rotonic minimum in thetransverse acoustic mode. For the photo-enhanced dielectric constant, Nasu (2004) assumed
that ultraviolet light broke the crystal inversion symmetry, creating ferroelectric microdomains.
He assumed also that such microdomains ﬂuctuated and itinerated quantum mechanically in the
SrTiO3 crystal. SrTiO3 crystal has an indirect gap and a direct gap in an energy range between
about 3.2 eV and about 3.5 eV (Cohen and Blunt 1968, Blazey 1971, Capizzi and Frova 1970,
Capizzi et al 1972, Cardona 1965, Zollner et al 2000, Hasegawa et al 2000). It is known thatthe luminescence intensity is specimen dependent (Grabner 1969). Hasegawa et al (2000)
measured the optical absorption and photoluminescence spectra at temperatures between 10 K
and room temperature. They found that the crystal exhibited a broad photoluminescence bandaround 2.4 eV. Through time-resolved photoluminescence measurements, they assigned the
2.4 eV luminescence to the radiative decay of intrinsic self-trapped excitons. However, many
metal oxides display very similar broad photoluminescence between 2 and 3 eV: for example,
Sm2 O3 (Mochizuki 2003), Eu2 O3 (Mochizuki et al 2001), anatase TiO2 (Mochizuki et al 2003),
Al2 O3 (Mochizuki and Araki 2003a),vitreous SiO2 (Mochizuki and Araki 2003b) and LaAl2O3(Kawabe et al 2000). Moreover, their luminescence intensities are specimen dependent,
especially as regards the thermal history and crystal preparation method. Unfortunately, thisspecimen-dependent nature of the 2.4 eV luminescence has been accorded too little respect inthe optical study of SrTiO3 and it has led many researchers astray.
Very recently, we have found at room temperature that the 2.4 eV luminescence band of
SrTiO3 grows under a 325 nm laser light irradiation in vacuum, while it disappears under thesame laser light irradiation in oxygen gas. Through many successive experiments involving
replacing the specimen atmosphere, it was found that the spectral change occurs reversibly
under the 325 nm laser light irradiation. The observed phenomenon relates to the photoinduced oxygen defects at the surfaces of SrTiO3 . Now or never is the time to reinvestigate
the photoluminescence properties of SrTiO3 in detail, taking account of such oxygen defects.
In the present paper, we report the photoluminescence spectrum, its excitation intensity
dependence, the decay proﬁle, the photoluminescence excitation spectrum, the time-resolved
photoluminescence spectrum and the reversible photo-induced spectral change of SrTiO3 single
crystal at different temperatures between 12 K and room temperature in detail.

Photoluminescence and reversible photo-induced spectral change of SrTiO3925

2. Experimental detail
The SrTiO3 crystal was well grown by the Verneuil method and the as-grown crystal is dark blue.
The as-grown crystal was then annealed under an appropriate reducing atmosphere. The crystal
became perfectly colourless and transparent under the annealing. Several as-grown (dark blue)
and annealed (colourless transparent) SrTiO3 single crystals, 10 × 10 × (0.5–1.0) mm in size,
were supplied by Sinkosya Corporation. The broad surfaces of the crystals are optically ﬂatand they are oriented in the [100] direction. It should be noted that such transparent SrTiO3
crystals grown by the Verneuil method have been widely used by many workers for studying
electrical and optical properties. As reference specimens, we have used also several SrTiO3
powder compact discs which were sintered at 1273 K and annealed at different atmosphere.
The photoluminescence (PL) spectra were measured by using an optical multichannel
analyser consisting of a grating monochromator (focal length = 32 cm, grating = 1200 or
150 grooves mm−1 ) and an image-intensiﬁed diode-array detector (number of channels =1024, gate time = 5 ns). An Nd3+ :YAG laser (wavelength = 355 nm, pulse width = 4–5 ns,
repetition = 10 Hz), a continuous wave (CW) He–Cd laser (wavelength = 325 nm) and a
monochromatic light source consisting of a 150 W xenon lamp and a grating monochromator(focal length = 20 cm) were used as the excitation source. The excitation light intensity
dependence of the PL spectrum was measured by attenuating the incident laser light intensitywith glass attenuators calibrated in transmissivity. The PL excitation (PLE) spectra wererecorded by varying the excitation light wavelength λex with the same monochromatic lightsource as described above, and by detecting the luminescence light intensity at a desired
wavelength λob as a function of λex with a grating monochromator (focal length = 20 cm) anda synchronous light detection system. The time-resolved PL spectra were taken using the same
optical multichannel analyser to which two delay pulse generators were attached. The delay
time td , which is the measurement start time after a laser pulse incidence, and the gate time tg ,which is the time after td of the spectral measurement, were set with these delay pulse generators
which were controlled with a personal computer. The actual minimum gate time was 5 ns. The
decay curves were measured with an apparatus consisting of a grating monochromator (focal
length = 20 cm) equipped with a photodetection system (time constant = about 1 μs) andthe Nd3+ :YAG laser oscillating at 355 nm. The optical density (OD) spectrum was measured
using another optical multichannel analyser.
An optical cryostat was used for the specimen chamber, in which a closed-cycle
helium refrigerator equipped with a temperature controller was used to change the specimen
temperature between 12 K and room temperature. Another specimen chamber was also used
for the measurement of the reversible spectral change at room temperature. An oil-free vacuum
system was used for evacuating these specimen chambers.
3. Results
3.1. Photoluminescence spectra of SrTiO3 under weak excitation
Figure 1(a) shows the PL spectra of SrTiO3 at different temperatures under a pulsed 355 nm
laser light excitation (pulse width = 4–5 ns, repetition = 10 Hz, ﬂuence = 0.079 mJ cm−2 ).
A broad luminescence band is observed around 2.4 eV at 12 K. The band seems slightly
asymmetrical in spectral shape and it consists of two luminescence bands centred at 2.46 and2.40 eV, which are labelled B and B , respectively. This indicates that this photoluminescencecomes from at least two kinds of luminescent centres. The spectral shape and intensity differ
slightly with the specimens. We measured also the PL spectra of an as-grown crystal as afunction of annealing time at 973 K in a reducing atmosphere. It has been found that the PL

926

S Mochizuki et al

PL intensity (arb. units)

70x10

3

B'

50
40

25 K

30

27 K

20

28 K

NBE

30 K

10

37 K
3.2

2500

(a)

12 K

0

PL intensity (arb. units)

B"
B

60

3.0

2.8

2.6

2.4

2.2

2.0

1.8

NBE
(b)

2000
1500

B
A

1000

37 K

38K

500

58K

0

PL intensity (arb. units)

3.2

3.0

2.8

2.6

2.4

Photon energy (eV)

2.2

2.0

1.8

1

(c)

0.1
0.01

B luminescence

0.001
20

30

40

50

-1

60

70

80

1000/T (K )Figure 1. The photoluminescence spectra of SrTiO3 at different temperatures: (a) T  37 K; (b)T  37 K. (c) The temperature dependence of the integrated intensity. The excitation laser ﬂuence
was 0.079 mJ cm −2 . The spectral integration was performed in a photon energy range between
2.9 and 1.77 eV.

intensity increases with increasing annealing time, while the PL intensity peak energy andspectral width are slightly dependent on the annealing time. With increasing temperature, the2.46 eV luminescence band grows more and then this band becomes together with the 2.40 eV
luminescence band, displaying an asymmetrical band centred at about 2.44 eV above 27 K. We
call this luminescence band the ‘B luminescence band’. On further increasing the temperature,
the B luminescence intensity continues to decrease, while a new luminescence band appears at
2.89 eV and becomes prominent, as seen in ﬁgure 1(b). We tentatively call this luminescenceband the ‘A luminescence band’. The A luminescence band seems to extend above 3.2 eV.
The A luminescence band grows to the detriment of the B luminescence and it is superior
to the B luminescence band above 58 K. The integrated PL intensity of the B luminescenceband is plotted against inverse temperature on a semi-logarithmic scale in ﬁgure 1(c). The

Photoluminescence and reversible photo-induced spectral change of SrTiO3

927

spectral integration was performed in a photon energy region between 2.9 and 1.77 eV. In thisﬁgure, the maximum luminescence intensity is normalized to unity. As seen in this ﬁgure, theluminescence intensity decreases gradually with increasing temperature to 25 K and, on further
increasing the temperature above 28 K, the intensity decreases exponentially. Approaching
43 K, the intensity becomes constant. Although such temperature dependence approaching a
constant has been not reported for SrTiO3 , a slight sign of such constant intensity appearedin the reported curve (ﬁgure 3(b): Hasegawa et al 2000), as some deviation from the curve
ﬁtting to the data. The constant intensity is due to the A luminescence component which
grows gradually at high temperatures. Although the PL band consists of several luminescencecomponents as described above, we subtract the constant intensity from the data and we assume
a phonon-assisted non-radiative decay process for the observed B luminescence. Then, we
perform a curve ﬁtting by using the well-known expression for the integrated PL intensity I (T),I (T ) =1,
1 + C exp(− E/kT )(1)

where C is a constant and E is the height of an average potential barrier separating aluminescent state from the ground state. In addition to such a phonon-assisted non-radiative
process, thermal ionization of luminescence states is also considered to participate in theintensity decrease with increasing temperature. In such a case, E may correspond to theionization energy. In the ﬁtting, the data obtained above 40 K are taken off to avoid the effectof the A luminescence. The best-ﬁtted curve is obtained with a E = 32 meV. The value
is slightly smaller than the value (43 meV) reported by Hasegawa et al (2000). At present,
it is not clear whether the difference arises from the specimen-dependent nature or from theerror in the specimen surface temperature measurements or from the manner of the curve
ﬁtting—performed probably ignoring the effect of the A luminescence.
As seen in ﬁgures 1(a) and (b), a sharp luminescence band is also observed at 3.23 eV
which is close to the observed PLE edge (3.26 eV) and the reported indirect band gap energy
Egid (3.27 eV) of SrTiO3 . We tentatively call this luminescence band appearing near the bandedge the ‘NBE luminescence band’. The steep rise above 3.26 eV is due to the parasitic
luminescence of the optical multichannel analyser used.
3.2. Photoluminescence excitation spectra of SrTiO3
The photoluminescence excitation (PLE) spectra of the same SrTiO3 single crystal have also
been measured at different temperatures. Some of the spectra are selected and shown in ﬁgure 2,
in which the PL spectrum observed at 13 K is also shown. The luminescence intensity wasmonitored at the intensity peak energy of the B luminescence band. The PLE spectrum displays
a sharp edge at 3.26 eV and a kink at 3.42 eV, which are close to the reported indirect gap
energy Egid (3.27 eV) and the direct gap energy Egd (3.46 eV) of SrTiO3 (Capizzi and Frova
1970), respectively. The PLE spectrum indicates that the B luminescence is caused mainlyby the interband excitations. The energy difference of the B luminescence band from the
PLE edge is about 0.80 eV. The shape of the PLE spectrum differs in an energy region higher
than the PLE edge from the absorption spectra reported by Blazey (1971), Capizzi and Frova(1970), Capizzi et al (1972), Cardona (1965) and Zollner et al (2000). Generally, there aremany crystal defects at the specimen surfaces where photoexcited states tend to decay nonradiatively. Such a luminescence quench effect appears more prominently in an energy region
higher than the fundamental absorption edge. The excitation with photons above the band gap
energy produces energetic electrons and holes, which are eventually thermalized by emitting
phonons. Therefore, the PLE above the band gap is governed not necessarily by the direct

928

S Mochizuki et al

T = 13 K

(b)

PLE

13 K

PL

λob = 2.45 eV

λex = 3.49 eV

25 K
5.0

4.5

4.0

B

3.5

3.0

Photon energy (eV)

Optical density

PL intensity (arb. scale)

20 K

(a)

2.0
1.5
1.0

13 K

0.5

NBE

0.0
5.0

4.0

3.0

2.0

Photon energy (eV)
5.0

4.5

4.0

3.5

3.0

2.5

2.0

Photon energy (eV)Figure 2. The photoluminescence (PL), optical density (OD) and photoluminescence excitation(PLE) spectra of SrTiO3 at 13 K. The luminescence is monitored at the peak energy (=2.45 eV) of
the B luminescence band. The OD spectrum at 13 K and the PLE spectra at different temperaturesare shown in inset (a) and inset (b), respectively.

quenching of highly excited electrons at defect sites but by the change in the probability of theinteraction of electron–hole pairs with defects.
As shown in inset (a) of ﬁgure 2, the OD spectrum has a faint tail below 3.2 eV. It isfound that the OD of the tail decreases with annealing in a reducing atmosphere. Although
the B luminescence band is well excited by the interband transition, the PLE spectra indicatethat the luminescence can also be excited even by photons having energies lower than indirect
gap. Under a 442 nm (=2.80 eV) laser light excitation, we have certainly detected the B
luminescence spectrum through a laser light sharp cut ﬁlter (HOYA: Y48). The B luminescence
can also be ascertained clearly by the naked eye through the same ﬁlter. Under the same
442 nm excitation, we have also found more intense luminescence for sintered SrTiO3 powder
compact specimens (Mochizuki and Fujishiro 2005). These results indicate that there aredifferent defect-related luminescent levels in the energy gap.
As seen in inset (b), the PLE spectral shape changes with increasing temperature,
except for the absorption edge proﬁle. The suppression seen at photon energies higher than
3.7 eV becomes prominent with increasing temperature. The suppression may arise from thetemperature change in the probability of the interaction of electron–hole pairs with defects.
In the following sections, the observed energy difference (0.8 eV), together with the photoinduced spectral change, will be discussed, and it will be concluded that the B luminescenceband comes from the extrinsic luminescent centres around which photo-generated carriers (freeelectron or free hole, or both) are trapped.

Photoluminescence and reversible photo-induced spectral change of SrTiO3

929

(a)

PL intensity (arb. units)

2

10
8
6

30 K

14 K

4

2

59 K

1

49 K

40 K

B luminescence

8
6

10

-5

10

-4

10

-3

10

-2

Time (s)(b)

PL intensity (arb. units)

2

10
8
6

14 K

4

2

1

B luminescence

8
6

0

2

4

6

8

10

12

14

Time (ms)Figure 3. The photoluminescence decay proﬁle of the B luminescence band of SrTiO3 : (a)log (PL intensity)–log t plots at different temperatures, (b) log (PL intensity)–t plots at 14 K.
The photoluminescence was monitored at 2.45 eV. The 355 nm excitation laser ﬂuence wasapproximately 0.2 mJ cm −2 .

3.3. The photoluminescence decay proﬁle of SrTiO3
We have measured the luminescence decay curves of the B luminescence of the same SrTiO3
single crystal at different temperatures. The luminescence was monitored at an intensity peakwavelength of the B luminescence band. Figure 3(a) shows the PL decay proﬁles of the B
luminescence at different temperatures. The PL intensity is plotted against time on a log–log
scale. The decay proﬁle at 14 K is also displayed as a semi-log plot in ﬁgure 3(b). It is notedthat the crystal displays luminescence for times longer than several tens of milliseconds at
14 K. The proﬁle at 14 K can be well expressed as a three-component exponential decay curve,(2)
I (t) = I1 exp(−t/τ1 ) + I2 exp(−t/τ2 ) + I3 exp(−t/τ3 ),with the following parameters:
I1 = 15.408,
τ1 = 2.6533 × 10−6 s,
I2 = 11.134,
τ2 = 5.4218 × 10−4 s,
I3 = 16.909,
τ3 = 2.8396 × 10−3 s.
Since the photodetection system used for decay curve measurements has a long time constant(about 1 μs), the shorter lifetime component is not resolved. Therefore, the B luminescence
may arise from at least three kinds of luminescence centres.

930

S Mochizuki et al
35x10

3

PL intensity (arb. units)

T = 13 K

4.0 mJ/cm

2

(a)

30
25
2

10 mJ/cm

20
15

2

14 mJ/cm
10

-2

5

2

8.1x10 mJ/cm

0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)
PL intensity (arb. units)

35x10

3

T = 13 K

B luminescence

30

(b)

25
20
15
10

NBE luminescence

5
0
0

2

4

6

8

2

10

12

14

Laser fluence (mJ/cm )Figure 4. The excitation light intensity dependence of the photoluminescence spectrum of SrTiO3
at 13 K: (a) spectra under intense, medium and weak excitations; (b) the excitation laser ﬂuence
dependence of the peak intensities of the near-band-edge luminescence (NBE) band and the B
luminescence band. The NBE luminescence peak energy and the B luminescence peak energy
are 3.2 and 2.4 eV, respectively.

Alternatively, the same decay data were examined with a power law (t −n ). It is foundthat the curve cannot be uniquely expressed with the t −2 law (Tsang and Street 1979); rather
it has a t −1−2 dependence. Hasegawa et al (2000) performed also decay curve ﬁtting with thet −n law, and they inferred suitability of the t −2 law for SrTiO3 . However, a caution should be
required for result to differ greatly by which data points are selected for the curve ﬁtting withthe power law.
At the present stage, the observed decay curve should be regarded obediently as an
exponential decay curve consisting of different components. The long lifetime component
may arise from some radiative transitions of different deeply trapped states. After stopping thelaser irradiation of the as-grown and annealed SrTiO3 crystals, we frequently observed faintphosphorescence lasting for several seconds, with the naked eye. Unfortunately, the decay timeof such a phosphorescence component (t  10−3 s) cannot be determined from the decay
curve analysis, because the phosphorescence contribution to the decay curve is too small.
3.4. Photoluminescence spectra of SrTiO3 under intense excitation
We have measured the PL spectra of the same SrTiO3 single crystal at different excitation laser
ﬂuences. A typical result is shown in ﬁgure 4(a). In ﬁgure 4(b), the peak intensities of thePhotoluminescence and reversible photo-induced spectral change of SrTiO3

PL intensity (arb. units)

50x10

B

3

40

931

12 K
17 K
19 K

NBE

(a)

21 K
23 K
25 K
27 K
29 K

30

20

A

10

31 K
39 K

0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

PL intensity (arb. units)

Photon energy (eV)
300

NBE

T = 13 K

(b)

200
100
0
3.35

3.30

3.25

3.20

3.15

3.10

Photon energy (eV)Figure 5. The photoluminescence of SrTiO3 under intense photoexcitation: (a) photoluminescence
spectra at different temperatures (T  39 K). The 355 nm excitation laser ﬂuence was
0.93 mJ cm −2 . (b) Near-band-edge (NBE) emission at 13 K. The 355 nm excitation laser ﬂuence
was 12 mJ cm −2 .

NBE and the B luminescence bands are plotted against laser ﬂuence. Solid squares and hollow
circles correspond to the NBE luminescence and the B luminescence, respectively. Under
weak excitation, only a B luminescence band is observed. With increasing laser ﬂuence,
the B luminescence intensity increases monotonically, and the NBE and A luminescencebands appear. On the laser ﬂuence further increasing up to 4 mJ cm−2 , the B luminescenceattains a maximum, while both the NBE luminescence and the A luminescence become
prominent. On laser ﬂuence exceeding 4 mJ cm −2 , the B luminescence intensity begins todecrease monotonically, while both the NBE luminescence band and the A luminescence band
continue to grow, degrading the B luminescence band. Incidentally, we have found that the
B luminescence intensity maximum appeared at 0.4 mJ cm−2 for the sintered SrTiO3 powder
compact. This suggests that most of the B luminescence centres are at the specimen surface.
In order to study the NBE, A and B luminescence bands in detail, we have measured
the PL spectra at different temperatures under an intense excitation of 0.93 mJ cm−2 . This
excitation intensity is enough to show up all of the luminescence bands. The results are shownin ﬁgures 5(a), 6(a) and (b). We subtract the spectrum measured at 297 K from the spectrum
measured at 12 K, in an attempt to extract the B luminescence component. The difference
spectrum is given in ﬁgure 6(c). By comparing this spectrum with the spectrum (ﬁgure 1(a))
observed at the same temperature under a weak photoexcitation, it was found that the spectral

932

S Mochizuki et al

30x10

159 K

3

(a)

139 K

20

PL intensity (arb. units)

10

99 K

39 K

0
30x10

3

198 K
237 K
278 K

159 K

(b)

20
297 K

10
3

60x100
50

(c)

40
30
20
10
0

PL intensity (arb. units)

3.2

3.0

2.8

2.6

2.4

Photon energy (eV)

2.2

2.0

1.8

1

(d)
0.1
0.01

weak excitation
intense excitation

B luminescence

0.001
20

30

40

50

-1

60

70

80

1000/T (K )Figure 6. The photoluminescence of SrTiO3 under intense photoexcitation (0.93 mJ cm −2 ): (a)photoluminescence spectra at different temperatures (39 K  T  159 K); (b) photoluminescence
spectra at different temperatures (159 K  T  297 K); (c) the difference spectrum; (d) thetemperature dependence of the integrated intensity of the B luminescence band for weak andintense excitations which were 0.079 and 0.93 mJ cm −2 , respectively. The spectral integration was
performed in a photon region between 2.9 and 1.77 eV.

shape of the B luminescence band is almost independent of the photoexcitation intensity. With
temperature increasing up to 39 K, the B luminescence intensity decreases and then it disappears
at 39 K, while the NBE and A luminescence intensities are almost constant. With temperature
increasing above 39 K, the A luminescence band continues to grow more without changing
the spectral shape and then it attains a maximum at 159 K, while the NBE luminescence band
becomes weak with the spectral broadening and is merged in the A luminescence band at 198 K.
At present, it is not clear why such a phonon-assisted nature of the A luminescence band arose.
With the temperature further increasing above 159 K, the A luminescence intensity decreases.
This intensity decrease may be due to non-radiative traps. The PL spectrum measured at 297 K
was subtracted from that measured at different temperatures, and then the spectral integration
was performed over a photon energy range between 2.9 and 1.77 eV. The integrated intensities
thus obtained for the B luminescence band are plotted against inverse temperature, as hollow

PL intensity (arb. units)

Photoluminescence and reversible photo-induced spectral change of SrTiO3

1400
1200

933

2

(a)

T=13K: 4.0 mJ/cm

1000
0ns - 1ms

800
600

50ns - 1ms

400

0ns - 2μs

200
0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)
PL intensity (arb. units)

1400
2

(b)

T=13K: 10 mJ/cm

1200
1000

0ns - 1ms

800
600
400

0ns - 2μs

20050ns -1ms

0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Figure 7. The time-resolved photoluminescence spectra of SrTiO3 at 13 K: (a) laser
ﬂuence = 4.0 mJ cm −2 ; (b) laser ﬂuence = 10 mJ cm −2 .

squares, in ﬁgure 6(d). The data obtained for weak excitation, which are already shown inﬁgure 1(c), are also plotted as solid circles in this ﬁgure.
Under a high laser ﬂuence (12 mJ cm−2 ), more detailed measurements were performed
also on the NBE luminescence and the A luminescence, using a high resolution monochromator.
This laser ﬂuence is enough to show up the NBE and A luminescence bands. The result obtained
at 13 K is given in ﬁgure 5(b). As seen in this ﬁgure, the NBE luminescence band is on the riseof the A luminescence band. The NBE luminescence peak is about 77 meV lower in energy
than the onset energy of the A band, and it is about 34 meV lower than the PLE edge energy.
The NBE luminescence band and the A luminescence have similar temperature and excitation
intensity dependences. This suggests that the NBE luminescence and the A luminescence havethe same origin.
Most of luminescent metal oxides show some photo-darkening effects under intense
photoexcitation. It is found that the PL spectrum and PL intensity of the SrTiO3 single crystal
used were almost unchanged, in spite of such intense 355 nm photoexcitation (14 mJ cm−2 ).
3.5. Time-resolved photoluminescence spectra of SrTiO3Time-resolved photoluminescence spectra of the same SrTiO3 single crystal have beenmeasured at 13 K under different excitation intensities. To study the time evolutions of
all of the luminescence bands, the measurements were performed under intense excitation.
The spectra obtained under excitations of 4.0 and 10 mJ cm −2 are shown in ﬁgures 7(a)and (b), respectively. The measurement times, td and tg , are given by each curve. At the934

S Mochizuki et al

initial stage of photoexcitation, the laser light pulse is seen at twice the wavelength, 710 nm(=355 nm × 2 = 1.745 eV), of each spectrum, which is common to every grating
monochromator. Both the NBE luminescence and the A luminescence disappear rapidly
within 50 ns after excitation. Taking account of the parasitic delay, timing jitter times of theapparatus used and the actual laser pulse duration (10 ns), the lifetimes of the NBE and A
luminescences are estimated to be less than 10 ns. The B luminescence band is growing overtime between 50 ns and 1 ms, and the spectral shape is almost unchanged with further increasein time. The B luminescence observed between 50 ns and 1 ms is slightly shifted toward
the lower energy side of the B luminescence observed between 0 ns and 1 ms, as shown inﬁgure 7(a). The shift is due to the time integration effect on the PL intensity. The measured
intensity between td and tg is expressed as follows:  tgη j N j (t = 0)e−t/τ j dt,(3)
I (t) =
jtd

where η j , N j (t = 0) and τ j are the quantum yield, number and lifetime of j th luminescencecentre including the extrinsic and intrinsic luminescence centres, respectively. If theluminescence centres responsible for the B luminescence band differ slightly in energy, asmall shift may be anticipated, as is in fact observed.
Both the PLE spectrum shown in ﬁgure 2 and these time-resolved spectra show clearly thatthe B luminescence follows the NBE and A luminescence. The long lifetime luminescence,
continuing for more than several tens of milliseconds, like the B luminescence, is not peculiar to
SrTiO3 but such long lasting luminescence is frequently observed for different metal oxides, forexample, ZrO2 (Fujishiro and Mochizuki 2005). This indicates that the B luminescence cannot
necessarily be connected with the nature of the quantum paraelectric state. Such long lifetime
luminescence observed for different oxides is thought to arise from accidental recombinationof shallowly trapped electrons with distant holes localized around defects. In such a case, theluminescence has slightly different long lifetimes, as seen in ﬁgures 3(a) and (b), and the B
luminescence spectral shape is almost unchanged with time, as seen in ﬁgures 7(a) and (b).
3.6. Reversible photo-induced spectral change in SrTiO3When the same SrTiO3 single crystal is irradiated with a CW 325 nm laser light at room
temperature in an evacuated chamber, the B luminescence intensity is enhanced to about
twice. Then, introducing oxygen gas into the specimen chamber, the intense PL state returns
to the original weak one under the same laser light irradiation, with increasing irradiation time.
Through many successive experiments, it was found that the spectral change is nearly reversible
under 325 nm laser light irradiation. After removing the 325 nm laser light irradiation, each
PL state is stored for a long time at room temperature under room light, regardless of any
changes of atmosphere. A typical example of the photo-induced spectral change obtained isshown in ﬁgure 8. The experiments were carried out in the following order: (a) → (b) → (c)→ (d). Irradiation times tir under a given atmosphere and the kind of atmosphere are indicatedby each curve. Irradiating under oxygen gas considerably decreases the PL intensity of thespecimens. In ﬁgure 9, the difference spectrum, curve 7 − curve 8, is shown. The PL spectralchange observed for the SrTiO3 single crystal is mainly characterized by an intensity changeof the B luminescence band. A slight degradation of the PL intensity is observed, which may
be caused by some photo-induced non-radiative centres. The same experiments were carriedout by changing the wavelength of the irradiating laser light. No spectral change was observed
except for the CW 325 nm laser light. It was found that the speed of the change accelerates
on increasing the power density of the activating light.

Photoluminescence and reversible photo-induced spectral change of SrTiO3935

in vacuum
1500

2: tir = 60 min

(a)

1000
500

1: tir = 0 min
0in oxygen

PL intensity (arb. units)

1500

3: tir = 0 min

(b)

1000
500

4: tir = 60 min
0in vacuum

(c)

1500

6: tir = 60 min

1000
500

5: tir = 0 min
0in oxygen

(d)

1500

7: tir = 0 min
1000
500

8: tir = 60 min
0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Figure 8. The photo-induced reversible spectral change of SrTiO3 at room temperature: (a) in
vacuum, (b) in oxygen gas, (c) in vacuum, (d) in oxygen gas. The experiments were carried out inthe following order: (a) → (b) → (c) → (d). Each irradiation was carried out for 60 min under agiven atmosphere. Each spectral change is indicated by an arrow. The 325 nm laser ﬂuence was
0.79 W cm −2 .

Similar photo-induced spectral change has been observed at different temperatures from13 K to room temperature. The result obtained at 13 K in vacuum under irradiation with325 nm laser light is shown in ﬁgure 10(a), as a typical result. The tir in vacuum is indicatedby each curve. The peak intensity is plotted against tir in ﬁgure 10(b). With increasing tir , theintensity increases and then tends to saturate at 90 min. The intensity increase is proportional
to tirn (n < 1). It has been found that the magnitude of the spectral change for 60 min is nearly
independent of temperature. After stopping the laser light irradiation, the photoluminescenceintensity decreases slightly with increasing time and then reaches a steady state value, which
is a photomemory phenomenon.

936

S Mochizuki et al800

curve 7 - curve 8

PL intensity (arb. units)

600

400

200

0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Figure 9. The difference spectrum for curve 7 and curve 8. Curves 7 and 8 are shown in ﬁgure 8.

Like for Sm2 O3 (Mochizuki 2003), Eu2 O3 (Mochizuki et al 2001), anatase TiO2
(Mochizuki et al 2003) and vitreous SiO2 (Mochizuki and Araki 2003b), the observed spectral
transition may arise from photo-induced oxidation and photo-induced reduction.
3.7. Photoluminescence properties of as-grown SrTiO3
In order to elucidate the crystal defect effects on the photoluminescence more directly, we
have studied the reversible photo-induced spectral transition for an as-grown Verneuil SrTiO3
crystal at room temperature. The as-grown SrTiO3 crystal is dark blue. The luminescence was
excited also with a 325 nm laser line of the He–Cd laser. The result is shown in ﬁgure 11(a).
Curve 1 is the spectrum of the as-grown crystal. The PL intensity of the as-grown crystal isstronger than several thousands times that of the annealed crystal. The spectrum consists of atleast three luminescence bands, at 2.9, 2.3 and 2.2 eV. Curve 2 is the spectrum measured after
irradiating in oxygen gas for 60 min. Under 325 nm laser light irradiation in oxygen gas, thePL intensity is decreased considerably. The degradations of the 2.3 and 2.2 eV luminescencebands are especially prominent. The specimen chamber is evacuated again. Curve 3 andcurve 4 are the spectra measured at 60 and 480 min after beginning irradiation in vacuum,
respectively. Then, oxygen gas was introduced again into the specimen chamber. Curve 5 isthe spectrum measured at 840 min after beginning irradiation in oxygen gas. The integrated
PL intensity of curve 4 is increased more than 16 times compared with that of curve 5. In
ﬁgure 11(b), curve 4 is compared after rescaling curve 5 so as to ﬁt to curve 4 at photon energies
higher than 2.8 eV. The rescaled curve is shown as curve 5 . Like for the annealed transparent

Photoluminescence and reversible photo-induced spectral change of SrTiO3

35x10

3

T = 13 K

30

PL intensity (arb. units)

937

(a)

tir = 90 min

25
60 min20

20 min
6 min

15

0 min

10
5
0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)

PL intensity (arb. units)

25x10

T = 13 K

3

(b)

20

15

under irradiation
050

without irradiation
100

150

tir (min)Figure 10. The photo-induced spectral change of SrTiO3 at 13 K in vacuum: (a) photoluminescence
spectra for 325 nm laser light irradiation; the irradiation time is indicated by each curve; (b) theirradiation time dependence of the peak intensity of the photoluminescence. The 325 nm laser
ﬂuence was 0.79 W cm −2 .

SrTiO3 crystal, an A-like luminescence appears in the as-grown crystal: compare curve 5 inﬁgure 11(a) with curve 8 in ﬁgure 8(d) and the curve observed at 297 K in ﬁgure 6(b). To deducethe information on the B luminescence observed in the annealed SrTiO3 crystal, the difference
spectrum (curve 4 − curve 5 ) was calculated; it is shown in ﬁgure 11(b). We can ﬁnd a broad
luminescence band centred at 2.3 eV, which is slightly shifted to the lower energy side of the
B luminescence band of the annealed crystal. It is noted that the B luminescence intensity
for the as-grown and the annealed crystals can be varied by 325 nm laser light irradiation in
vacuum and in oxygen gas. Since the light penetration depth is very small at 325 nm for
SrTiO3 (Zollner et al 2000), the observed spectral change of the B luminescence band could
be closely related to the change of surface electronic states.
Using the same as-grown crystal, we have measured the photoluminescence spectra atdifferent temperatures under an intense excitation (14 mJ cm−2 ). As a typical result, thespectrum obtained at 12 K is shown in ﬁgure 12. The spectrum obtained at 13 K under the938

S Mochizuki et al

30x10

3

(a)
4: in vacuum for 480 min

PL intensity (arb. units)

25

20
3: in vacuum for 60 min
15
1: as grown10
2: in oxygen for 60 min
5
5: in oxygen for 840 min

0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)
PL intensity (arb. units)

35x10

3

(b)

30

4

25

5'

20
15
10

curve 4 - curve 5'

5
0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Figure 11. The photo-induced reversible spectral change of as-grown SrTiO3 at room temperature:(a) photo-induced spectral change for replacing specimen atmosphere, (b) comparison of curve 4
with curve 5. The 325 nm laser ﬂuence was 0.79 W cm −2 .

same excitation intensity for the annealed crystal is also shown; this was already shown inﬁgure 4(a). The NBE and A luminescence are considerably enhanced in the as-grown crystal.
The ﬁne structure and PL intensity of the NBE luminescence band depend on the annealing
time. The results suggest that the NBE and A luminescence are assisted by crystal defects.
3.8. Miscellaneous
We have measured the PL spectra of sintered SrTiO3 powder compacts at different temperaturesand different laser ﬂuences. The powder specimens show clearly the NBE, A and B
luminescences. The NBE and A luminescences remain at room temperature, while the B
luminescence disappears above 40 K. Most of the PL properties were very similar to thoseof single-crystal specimens. Like for other luminescent oxides, the powder compacts wereless luminescent and their luminescence decay lifetimes were shorter than those of the single
crystals. The main difference between the cases for the powder compacts and the single
crystals is that the B luminescence of the powder compacts saturates at lower laser ﬂuence(approximately 0.5 mJ cm−2 ), while that of the single crystals saturates at 4 mJ cm−2 .

Photoluminescence and reversible photo-induced spectral change of SrTiO3

50x10

939

3

2

T ~ 13 K, 14 mJ/cm

PL intensity (arb. units)

40

30

as-grown

20

annealed
10

0
3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

Photon energy (eV)Figure 12.
The photoluminescence spectra of the as-grown dark blue and annealedtransparent SrTiO3 crystals at about 13 K under an intense 355 nm photoexcitation (laser
ﬂuence = 14 mJ cm −2 ).

4. Discussion
4.1. The reversible photo-induced spectral change
The present experimental results on the reversible photo-induced spectral change of SrTiO3
single crystal may be summarized as follows.
(1) The photo-induced reversible spectral change between weak PL and intense PL can beobserved at different temperatures, from 13 K to room temperature. The spectral changetakes time. The speed of the spectral change is nearly independent of temperature but it
increases with increasing light intensity.
(2) The spectral change tends to saturate at a long light irradiation time (more than 90 min).
(3) The spectral change is induced by changing the specimen atmosphere between oxygengas and vacuum only under a CW 325 nm laser light. On the other hand, intense pulsed
355 nm laser light (14 mJ cm−2 ) never induced such spectral change.
(4) After removing the 325 nm laser light irradiation, each PL property persists for a long timeeven at room temperature under room light, regardless of any changes of atmosphere.
The result (1) indicates that the observed photo-induced spectral changes are purely
electronic phenomena and they are not phonon-assisted ones. The result (1) indicates also thatthe B luminescence is an extrinsic luminescence due to the luminescence centres introduced

940

S Mochizuki et alby a CW 325 nm laser light. Since the 325 nm (=3.81 eV) laser light never dissociates
directly free O2 molecules, the result (3) indicates that the phenomena arise from the photoinduced associative detachment and photo-induced dissociative adsorption of O2 molecules
near the SrTiO3 surface. In this case, photoexcited SrTiO3 crystal acts as a photocatalyst fordecomposing O2 molecules and also as an oxygen reservoir. The results also summarize how
spectral change arises naturally from photo-activated oxidation and reduction.
Although some chemical heterogeneity on the surface has been pointed out for the surfacesof SrTiO3 crystal reduced and oxidized at high temperatures (Szot et al 1997, Szot and Speier1999), we discuss tentatively the observed photo-induced spectral changes at room temperatureby regarding the measured SrTiO3 crystal as chemically homogeneous, as follows. The
chemical heterogeneity effects will be discussed in the next subsection. The reduction of
the SrTiO3 surface under a CW 325 nm laser light in vacuum may be accompanied by thecreation of electron-captured oxygen vacancies VO , as follows:(O2− in lattice) + (CW 325 nm photon) → VO + (two electrons) + 12 O2 ,(4)

where Kröger–Vink notation is used (Kröger and Vink 1956). Using a local spin density
approximation plane wave pseudopotential method, Astala and Bristowe (2001) have shownthat the doubly positively charged state is the most stable. This is in good agreement withthe experimental result that oxygen deﬁciency in SrTiO3 enhances the electrical conductivity(Jourdan and Adrian 2003) even at low temperatures. This means that most of the electrons
around oxygen vacancies are released and, therefore, such oxygen vacancy sites are relatively
positively charged. Therefore, the oxygen vacancies tend to trap photo-generated electrons.
On the other hand, several authors (Henrich et al 1978, Cord and Courths 1985, Kimura et al1995) proposed another type of defect at the SrTiO3 surface as follows:(O2− in lattice) + (CW 325 nm photon) → (Ti3+ –VO –Ti3+ ) complex + 12 O2 .

(5)

In the complex, the Ti3+ ions act as hole traps, while the vacancy VO tends to trap electrons.
For SrTiO3 crystal, it is known that electrons determine the transport properties, for example,
electrical conductivity and photoconduction, well, while no phenomena related to holes havebeen observed. This indicates that holes are almost trapped around crystal defects. Incidentally,
the high dielectric constant (several tens of thousands) of SrTiO3 crystal suppresses, creating
excitons (ex). Toyozawa (1983) proposed three types of symmetry-breaking instability for
excitons in the phonon ﬁeld and, when the electron and the hole have deformation potentials
of opposite sign, decomposition into a pair of self-trapped particles occurs. In other words,
the instability of an exciton leads to lattice decomposition into an electron centre (an anion
vacancy) Se and a hole centre (an anion interstitial) Sh , if the exciton is formed in the bulk.
On surfaces, the hole centre is emitted from the surface, thus leaving only the electron centre.
Therefore, such exciton instability may result in oxygen desorption at SrTiO3 surfaces invacuum and the oxygen vacancies thus produced give rise to B luminescence. Similar
desorption has already been discussed for photo-induced breaking of the Si–O bond in asilica (Shluger and Stefanovich 1990). By a quantum chemical calculation, Eglitis et al(2004) has demonstrated that the chemical bonding of the perovskite structure contains aconsiderable covalent component, like the case for silica. It is noted that CW 325 nm laser
light is indispensable for spectral change due to the photo-induced oxidation and reductionin SrTiO3 . The CW laser ﬂuence in the present experiments was at most 0.8 W cm−2 , andtherefore any bond-breaking mechanism based on the instability under high density excitation
may be excluded from the present discussion. Unlike the case for alkali halides, a single
interband excitation never induces the desorption of surface atoms in semiconductors, since
the binding energy of atoms is larger than the energy gap. In such photoexcited semiconductor

Photoluminescence and reversible photo-induced spectral change of SrTiO3

941

cases, only atoms around crystal defects at the surfaces can be released (Puchin et al 1993, Itohet al 1995, Itoh 1995, Itoh and Stoneham 2000, Song and Williams 1993). Astala and Bristowe(2001) calculated the oxygen vacancy formation energy of about 7 eV, which is approximately
twice the bulk energy gap and slightly smaller than twice the CW 325 nm He–Cd laser photon
energy, at the TiO2 -terminated (100) surface of SrTiO3 . Both the oxygen defects (for example,
Ti3+ –OV –Ti3+ and OV ) inherent to SrTiO3 surfaces and the inﬁnite continuity of the laser light
may enable the 325 nm laser photons to release O2 molecules from the surface. Incidentally,
pulsed 355 nm laser light (the pulse width is 4–5 ns), although the photon energy is close to thatof the CW 325 nm laser light, never induced spectral change even at 14 mJ cm−2 , as a resultof (3). Besides the surface defects, a small amount of OH− ions introduced inevitably during
crystal growth by the Verneuil method should also be taken into account for the photo-induced
spectral change. Incidentally, Ohmukai et al (1999) have observed a 126 nm laser-induced
bond breaking and some polycrystalline precipitates on a SiO2 glass. We could not detect any
light scattering due to precipitates on the 325 nm laser light irradiated SrTiO3 crystal.
It has been said that the desorption via such an excited state occurs within 10−13 s afterlaser pulse incidence. However, it is found that a relatively long time (more than 90 min) istaken to complete the spectral change in SrTiO3 , as a result of (2). This indicates that thephoto-induced oxygen desorption probability is very low at the SrTiO3 surface. The observed
saturation tendency with further increasing tir suggests that photo-induced oxygen defects
increase the potential energy of the light irradiated surface. In such a case, equilibrium may
be realized in a system composed of matter (SrTiO3 crystal) and a radiation ﬁeld. It can beassumed that the intensity, coherency and continuity of laser light determine the degree of
ﬂuctuation in the system and the dynamics of photo-induced spectral change. Unlike the casefor a pulsed laser light, the possibility of successive electronic excitation is not ignored for
CW laser light. More detailed experiments with varying frequency, pulse width and duty ratioof a periodic laser light pulse are in progress in our laboratory.
In relation to the UV laser light-induced oxygen defects and white luminescence, it
should be noted that the crystals (for example, LaAlO3 ) grown by light heating methods(for example, the ﬂoating zone methods and the arc furnace method) display intense white
luminescence. During the growth, the crystals are exposed to intense ultraviolet radiation
and, therefore, oxygen defects may be produced, exhibiting oxygen defect-related white
luminescence between 3 and 2 eV (Kawabe et al 2000).
4.2. The B luminescence band
The present experimental results on the B luminescence of SrTiO3 single crystal may besummarized as having the following features.
(1) The broad B luminescence band is observed well under band-to-band excitation(>3.27 eV) in SrTiO3 with a large energy shift (0.80 eV). Although the quantum yieldis small, the B luminescence is also observed under a 442 nm (=2.80 eV) laser light
excitation.
(2) The B luminescence intensity is considerably increased by photo-induced reduction with325 nm (=3.81 eV) laser light irradiation in vacuum.
(3) The PL intensity increases with progressive annealing in a reducing atmosphere. The
intensity peak energy and spectral width of the B luminescence depend on annealing
history.
(4) The B luminescence decay curve consists of at least four components.
The
phosphorescence component lasts for at least several seconds. It is hard to ﬁt the decay
curve uniquely to a power law time (t −2 ) dependence.

942

S Mochizuki et al(5) With increasing temperature, the B luminescence intensity decreases, increasing the NBE
and A luminescence bands. The B luminescence of both the single crystal and the powder
compact disappeared above about 40 K.
(6) With increasing excitation intensity, the B luminescence intensity increases. On further
increasing the excitation intensity, the B luminescence intensity attains a maximum atabout 4 mJ cm−2 and then decreases, increasing the NBE and A luminescence bands. The
SrTiO3 power compacts display the B luminescence intensity maximum at approximately
0.5 mJ cm−2 .
Feature (1) indicates that the B luminescence arises from several kinds of PL mechanisms.
Both feature (2) and the feature (3) indicate that oxygen deﬁciency is a main cause of B
luminescence. Feature (4) indicates that the deviation from stoichiometry creates different
types of defect which give different luminescence centres and traps. Most workers have been
studying SrTiO3 crystals produced by the Verneuil method. Unfortunately, the compositional
deviation of the crystals produced by the Verneuil method is known to be between 10−2 and10−4 even for simple transition metal oxides (MnO, CoO, NiO and TiO2 ). The chemical
purities and isotope separation of the constituent elements (especially strontium and titanium)
are not necessarily satisfactory and therefore most of the available SrTiO3 crystals are at most
99.99% pure. These are problems common to many transition metal oxides. Moreover, since
a ﬂame consisting of H2 gas and O2 gas is used for the Verneuil crystal growth process, somecontamination due to OH− ions may be inevitable. Feature (6) indicates that most of the
B luminescence centres are at the specimen surface. Ignoring the above-described chemical
impurity effects and noting only the effects due to the deviation from SrTiO3 stoichiometry,
we will discuss the results of the PL measurements.
We ﬁrst discuss the excited states created in SrTiO3 crystal by the third harmonics (355 nm)of the pulsed Nd 3+ :YAG laser. Unlike CW 325 nm laser light, this 355 nm laser light neverinduced the spectral changes discussed in the previous subsection. The valence and conduction
bands of SrTiO3 mainly consist of the 2p orbits of O2− ions and the 3d orbits of Ti4+ ions,
respectively. Therefore, the fundamental absorption transfers the electron from the O2− ionto the Ti4+ one. After such photoexcitation, some of the photo-produced electrons and holes
recombine directly, displaying luminescence via some processes with short lifetime. However,perfect SrTiO3 crystal is an indirect gap semiconductor and, therefore, the PL intensity is so
weak that it cannot be measured. Another portion of electrons and holes may be trapped byintrinsic crystal defects at the surface and by the defects related to the deviation from thestoichiometry. In such a case, the PL property differs with the type of defect and also with thesites of defects, as follows.
4.2.1. Oxygen defects at the surface and in the bulk. Stashans and Vargas (2001) havecalculated structural and electronic properties of the F centres (two electrons trapped by anoxygen vacancy) in SrTiO3 , as have Astala and Bristowe (2001). The results indicate thatthe wavefunctions of the two extra electrons extend over the titanium atoms closest to thetwo vacancies and over other nearby atoms, which is the same as the Ti3+ –oxygen vacancy
complex model proposed by Henrich et al (1978), Cord and Courths (1985) and Kimura et al(1995). In other words, most of the oxygen vacancies are Ti3+ –oxygen vacancy complexes
in real SrTiO3 crystal. In this complex, Ti3+ ion is one of the candidate hole-trapping centres
and the oxygen vacancy is one of the candidate electron-trapping centres. Since electricalconduction due to holes is not observed, most of the photo-generated holes may be deeply
trapped at Ti3+ sites of the complexes. Unlike such holes, photo-generated electrons may
be itinerant, experiencing repeatedly collisions with phonons and trapping by oxygen defects.

Photoluminescence and reversible photo-induced spectral change of SrTiO3

943

When some portions of photo-generated holes and electrons are trapped at distant complexes, aluminescence arises from accidental recombination of shallowly trapped electrons with distantholes localized around the defects, exhibiting a long lasting luminescence. Taking account of
the electronic conductivity, both the absorption tail in the OD spectrum shown in the inset (a)of ﬁgure 2 and the energy difference of 0.80 eV may guarantee the existence of such deeply
trapped hole states and shallowly trapped electron states. The broadness of the OD tail shownin the inset (a) of ﬁgure 2 suggests that such hole trap levels and electron trap ones arise
from many slightly different defects. Using ultraviolet photoemission spectroscopy (UPS),
Henrich et al (1978) found that the Fermi level of the fractured SrTiO3 lies about 3 eV above
the top of the valence band, which places it within 0.1 eV of the bottom of the conduction
band. Also, they observed surface sensitive band gap emission band at energies between the
top of the valence band and the Fermi level. These UPS results conﬁrm also the existence ofband gap states of the SrTiO3 surface. Thus, the energy difference of 0.8 eV may correspondto the average energy difference between the shallow electron trap levels and the deep hole
trap levels in the band gap. Since the observed luminescence decay curve was expressed asa multi-component exponential curve (at most four components), the overlapping between
electron and hole wavefunctions is thought to be almost time independent.
With increasing laser ﬂuence, the numbers of photo-generated electrons and holes increase
and then the PL intensity may increase. With further increasing laser ﬂuence, photo-generated
electrons and holes become diffused more rapidly inside the crystal (bulk), and therefore the
PL intensity decreases, as shown in ﬁgures 4(a) and (b). This is why the PL intensity maximum
appears at 4 mJ cm −2 . In other words, this means that most of the defect luminescence centres
are at the specimen surface. Since a powder specimen has a large speciﬁc surface area, a
SrTiO3 powder specimen may exhibit a PL intensity maximum at the laser ﬂuence smaller thanthat of the single-crystal specimen. This suggestion was certainly conﬁrmed by the present
experimental result that the PL intensity maximum of the B luminescence for the powder
compacts appeared at laser ﬂuences (approximately 0.5 mJ cm−2 ) lower than that (4 mJ cm−2 )observed for the single crystal. On the other hand, the luminescence arising from the defects inthe bulk is thought to saturate at higher laser ﬂuence. In addition to these defect effects,it should
be taken into consideration that, at higher laser ﬂuence, the recombination of photo-generated
electrons and holes is enhanced and therefore it also reduces the luminescence intensity.
4.2.2. Chemical heterogeneity in the surface region. As stated in the previous sections, acolourless transparent SrTiO3 single crystal is obtained by thermally annealing dark blue asgrown crystal at high temperature under an appropriate reducing atmosphere. Although it iscolourless and transparent, the crystal is thought to be affected to some extent by such annealing
treatment. In particular, it has been reported that the surfaces of reduced and oxidized SrTiO3
contain not only the above-described oxygen defects but also Ruddlesden–Popper phases(Ruddlesden and Popper 1957, 1958). The combination of x-ray diffraction analysis withsurface sensitive techniques has revealed a chemical inhomogeneity in the surface region of
single crystals of SrTiO3 prepared under low and high partial pressures of oxygen at elevatedtemperatures (Szot et al 1997). A solid state reaction leads to the formation of a multilayertype structure. For oxidized crystals, they observed SrO-rich Ruddlesden–Popper phases atthe surface and Magnelli phases of Ti in deeper layers of the surface region. The order ofthe layered structure is reversed for the reduced crystals, with Ti oxides of different oxidationlevels at the surface and Ruddlesden–Popper phases in lower parts of the surface region.
Measurements by atomic force microscopy are also reported for the (100) and (110) surfacesof SrTiO3 single crystals prepared with different oxidizing and reducing conditions at elevatedtemperatures (1073–1273 K) (Szot and Speier 1999). The morphology of the surfaces turns

944

S Mochizuki et alout to be drastically altered for both oxidized and reduced crystals in comparison with theoriginal stoichiometric surfaces. The observed changes on the surface of SrTiO3 due to theapplied extensive thermal treatment cannot be explained by the formation of point defects,
relaxation of the uppermost surface layer, rumpling or reconstruction due to vacancy ordering.
Instead, the results have to be interpreted in terms of segregation processes and solid state
reactions at elevated temperatures, which cause the formation of new chemical phases on thesurface and in the region underneath. On the surface of oxygen-annealed SrTiO3 , this leads to
the growth of steps perpendicular to the surface with step heights larger than the unit cell of
the perovskite structure. Crystals prepared above 1173 K are shown to exhibit a step height of
1.18 nm which is attributed to the formation of a Ruddlesden–Popper phase SrO∗ (SrTiO3 )nwith n = 1 on the surface. In the case of reduced crystals, the topographic changes on thesurface are caused by the formation of Ti-rich phases such as TiO and Ti2 O on the surface
above 1173 K. The complex interplay of the processes at the surface for different temperatures,in particular its dependence on the details of the heat treatment, is discussed. The induced
chemical heterogeneity of the surface and in the near-surface region is interpreted in terms ofa kinetic demixing.
Certainly, as shown in ﬁgures 11(a) and (b), we observed complicated spectral change
under CW 325 nm laser light irradiation in oxygen gas and vacuum for the as-grown SrTiO3
crystal, which cannot be explained entirely by the simple oxygen point defect model. Since
the as-grown crystal has a higher light absorption coefﬁcient than the colourless annealed
crystal, the observed spectral change may be more affected by the surface crystal defects,
including the chemical heterogeneity (for example, Ruddlesden–Popper phases) in the surface
region.
Bearing these chemical heterogeneities in mind, we have measured the PL propertiesof the as-grown crystal at different stages of annealing in a reducing atmosphere and alsomeasured those of a colourless transparent stepped TiO2 -terminated SrTiO3 crystal whose
step height is 0.4 nm. The detailed results will be reported in a separate paper (Mochizuki
and Fujishiro 2005). The results are summarized as follows. The low temperature PL spectral
structures of these crystals are almost the same as those of the colourless transparent (100) faced
crystal, apart from the PL intensity. However, with progressive annealing, the dark blue colour
becomes weak, the intensities of the NBE, A and B luminescence bands decrease and the lower
energy tail of the PLE for the B luminescence becomes weak. Moreover, the excitation laser
ﬂuence giving the B luminescence intensity maximum (we call this ﬂuence Fmax ) increases
with progressive annealing. Like that for the as-grown crystals, the Fmax for the stepped
TiO2 -terminated SrTiO3 crystal decreases considerably in comparison with the Fmax for thecolourless transparent SrTiO3 crystal. The observed change in Fmax may be due to the change inthe probability of electron–hole pairs with surface defects arising from chemical heterogeneity.
We have observed also at least ﬁve broad optical absorption bands at about 2.9 eV, about 2.4 eV,
about 2.1, 1.6 eV and about 0.9 eV for the as-grown crystal at different annealing stages. The
dark blue colour of the as-grown crystal is attributed to this spectral structure in which the blue
colour wavelength range is a spectral window. With progressive annealing, these absorptions
decrease in intensity. These optical absorption bands may be assigned to oxygen defects andsome chemical heterogeneity (Szot et al 2002). The broad OD tail observed for the colourlesstransparent crystal is thought to be a trace from these absorption bands; see the inset (a) ofﬁgure 2 in the present paper and the ﬁgure 4 in the paper reported by Szot et al (2002). In
order to clarify the optical excitation in the Ruddlesden–Popper phases, more detailed surface
sensitive spectroscopic measurements (for example, reﬂectivity and PL measurements under
total reﬂection conditions) and morphological and structural measurements for the as-grown
crystals at different stages of annealing are now in progress in our laboratory.

Photoluminescence and reversible photo-induced spectral change of SrTiO3945

Taking account of these studies on different kinds of defects in the real SrTiO3 crystal
grown by the Verneuil method, we thus assign the B luminescence band to the radiative decayof the crystal defect-related excited electronic states. Alternatively, it is theoretically assumed
that a long lasting luminescence arises also from some forbidden transition, and therefore wecannot ignore the contribution from de-excitation of some triplet excited states. Previously,
many workers have discussed the long lasting B luminescence on the basis of intrinsic selftrapped electronic (exciton) states, ignoring the effects of crystal defects. As discussed above,
it should now be apparent that defect effects are important for such self-trapping, and more
careful consideration of them is necessary for deducing the intrinsic nature of SrTiO3 fromexperimental data.
4.3. The NBE and A luminescence bands
Both the NBE luminescence and the A band luminescence have the following features:(α) Both luminescences become prominent under higher excitation intensity and do notsaturate under intense excitation (14 mJ cm−2 ). They grow to the detriment of the B
luminescence with increasing excitation intensity.
(β) With increasing temperature, both luminescences become prominent to the detriment of
the B luminescence and survive even at high temperatures. The spectral structures of thetwo luminescences at room temperature are hardly changed by changing the excitation
laser wavelength from 325 to 355 nm.
(γ ) Both luminescences are considerably enhanced in the as-grown crystal.
(δ) Both luminescences appear near the band edge energy region and have very short decay
times (50 ns).
Feature (α) indicates that both the NBE luminescence and the A luminescence arise not froma small number of impurities and crystal defects, but from numerous luminescent species.
Feature (β) indicates also that the NBE and A luminescence centres are stable at hightemperatures, without any thermal exhaustion. This feature indicates that both luminescences
are assisted by phonons. Feature (γ ) indicates that the luminescence is emission assisted by
crystal defects. Together with the features (α), (β) and (γ ), feature (δ) suggests that the twoluminescences arise from the same origin and are assignable to some kind of electron–holerecombination emission assisted by crystal defects and phonons.
We discuss these luminescence bands from two different viewpoints as follows. The
frequency shift of the NBE luminescence band from incident 355 nm laser light is about
2207 cm −1 . This is too large for assigning this band to non-resonant Raman scattering (Sirenko
et al 1999). Non-resonant Raman scattering in a cubic centrally symmetric crystal is forbiddenby the odd parity of the optical phonons of F1u symmetry, and therefore cubic SrTiO3 crystal
displays weak broad Raman spectrum due to second-order Raman scattering in which two
phonons are involved in the scattering process. In the tetragonal phase below 105 K, thephonon mode splits into two Raman-inactive modes of A2u and Eu symmetry. When crystal
defects (impurities and vacancies) are introduced (Kleemann et al 1997) or when uniaxial
external ﬁelds (electric ﬁeld and stress) are applied (Akimov et al 2000, Worlock and Fleury1967), these ﬁrst-order Raman-inactive modes can be observed only below 1000 cm −1 . They
cannot explain the observed large frequency shift (about 2207 cm−1 ) of the NBE luminescence.
On the other hand, since the photon energy of the incident 355 nm (=3.49 eV) laser light isclose to the reported direct gap energy (=3.47 eV; Capizzi and Frova 1970) and the observed
PLE edge energy (=3.26 eV), we have to take account of the resonant Raman scattering effects.
In this case, the Raman spectrum may display several resonance lines due to the scattering bydifferent phonons near the direct and indirect gap energies, together with other critical points of946

S Mochizuki et al

the interband transitions (Cardona 1982). As is well known theoretically, the resonant Ramanprocess involves effects due to different intraband and interband scatterings, which display abroad resonance. If we regard the NBE and A luminescences as a result of such resonanceRaman scattering, the phonon energy may be estimated from the difference between the indirect
band edge energy (PLE edge energy = 3.26 eV) and the NBE luminescence peak energy (=3.23 eV): the estimated value is 30 meV (=242 cm−1 ), which is comparable to the transverse
optical phonon frequency. Further discussion on the basis of such resonant Raman scattering
requires measurements at different incident wavelengths and information about the chemical
heterogeneity and defect effects on the Raman scattering.
Next, we discuss the NBE and A luminescence bands as follows. The NBE and the A
luminescences were seen even at high temperatures, and they do not exhibit any PL intensity
saturation under higher excitation intensity. Although an explanation with so-called donor–
acceptor pair (D–A pair) luminescence due to unwanted impurities is frequently given for
the intense broad luminescence band with intensity peaks for semiconductors, such D–Apair luminescence related to inevitable impurities may saturate under intense photoexcitation
because of the limited numbers of donors and acceptors. Moreover, the luminescence intensity
decreases remarkably with increasing temperature. Moreover, such a luminescence spectrum
for D–A pairs related to unwanted impurities is thought to be time resolved, since separations
between donors and acceptors differ. Therefore, we can ignore the contribution from theimpurity-related D–A pair luminescence in the present discussion.
Now, we note that, since ideal SrTiO3 crystal has an indirect gap, the crystal does not exhibit
directly any intense intrinsic edge emission. When crystal defects giving shallow levels are
introduced at the surface, a PL process becomes possible. We note moreover that the NBE
and A luminescence bands are observed even at high temperatures under intense excitationand that these luminescence bands are considerably enhanced for the as-grown crystal, as seen
in ﬁgure 12. Therefore, we regard the observed NBE and A luminescence bands as defectinduced edge luminescence (emission) ones, in which processes different phonons are created.
The NBE luminescence band having a shift of 242 cm−1 may be assigned to one of the opticalphonon lines. A zero-phonon line anticipated at 3.26 eV is thought to be buried in the riseof the A luminescence band. The full width at half-maximum (FWHM) of the NBE bandis about 30 meV. Such a large FWHM is thought be due to some structural inhomogeneityand damping. It is noted that the A luminescence displays an almost smooth and continuousband. This suggests that, besides optical phonons, many longitudinal and transverse acoustic
phonons are created during electron–hole recombination.
5. Conclusion and remarks
In summary, we have investigated the reversible PL spectral change and the basic PL propertiesof as-grown and annealed SrTiO3 single crystals, together with sintered SrTiO3 powder
compacts, at different temperatures from 12 K to room temperature. Three luminescence bands,
NBE, A and B, are observed around 3.2, 2.9 and 2.4 eV, respectively. The B luminescenceintensity changes with CW 325 nm laser light irradiation in oxygen gas and a vacuum. This
result indicates that the B luminescence is not intrinsic luminescence but results from radiative
decays of the oxygen defect-related photoexcited states and the chemical heterogeneity inthe surface region. The NBE and A luminescences remain at high temperatures and they do
not saturate under high density photoexcitation (at least 14 mJ cm−2 ). Their PL properties(temperature dependence, excitation intensity dependence and time dependence) suggest that
the NBE luminescence and the A luminescence arise from the same origin. Taking accountof crystal defects, the NBE and the A luminescence are discussed qualitatively from two

Photoluminescence and reversible photo-induced spectral change of SrTiO3

947

Figure 13. Some of the possible photoluminescence processes in real SrTiO3 crystal. The solid
arrow and dotted arrow indicate the optical absorption and radiative transition, respectively. The
hollow arrow and double arrow indicate the luminescence and phonon creation, respectively. E gid
and eslt denote the indirect gap energy and the shallow electron trap level, respectively. For theaccidental recombination model, the short dotted arrow and the broken arrow indicate the electron
trapping and hole trapping, respectively.

different viewpoints: defect-induced resonant Raman scattering and defect-induced electron–
hole recombination luminescence accompanying creation of phonons. At present, we do not
exclude the possibility of other explanations. Although it constitutes a difﬁcult problem, thesurface defect structure and the defect-induced effects on the resonant Raman scattering andthe edge luminescence (edge emission) should be considered in more detail.
A tentative energy diagram showing some of the possible PL processes in real SrTiO3
crystal containing crystal defects is given in ﬁgure 13. The solid arrow and dotted arrow
indicate the optical absorption and radiative transition, respectively. The hollow arrow anddouble arrow indicate the luminescence and phonon creation, respectively. eslt indicates ashallow electron trap. For the accidental recombination model, the short dotted arrow and thebroken arrow indicate the electron trapping and hole trapping, respectively.
Incidentally, Pontes et al (2002) found intense PL in amorphous SrTiO3 at room
temperature under 488 nm (=2.539 eV) radiation from an Ar + ion laser. They ascribed the PL
excited by low energy photons to non-bridging oxygen defects, for example, TiO5 . Study of
how the non-bridging oxygen defects enhance the PL in amorphous SrTiO3 is also helpful in
discussing the PL properties of real SrTiO3 crystal surfaces.
Finally, more detailed PL measurements, with excitation laser wavelengths varying aroundthe band gap energies, on different annealing stages of SrTiO3 crystals are now progress in ourlaboratory. Experiments with chopped laser light with different duty ratios on the reversible
photo-induced spectral change are also now in progress. The results will be reported in a
separate paper.
Acknowledgments
This work was partially supported by a Grant-in-Aid for Scientiﬁc Research from the Ministryof Education, Science, Sports, Culture and Technology, Japan. This work was supported by
an Interdisciplinary General Joint Research Grant for Nihon University. This work was also
partially supported by a Project Research Grant from The Institute of Information Sciences of
the College of Humanities and Sciences (Nihon University) and by a Cooperative Research
Grant from The Institute of Natural Sciences (Nihon University).

948

S Mochizuki et al

References
Akimov I A, Sirenko A A, Clark A M, Hao J-H and Xi X X 2000 Phys. Rev. Lett. 84 4625
Astala R and Bristowe P D 2001 Modelling Simul. Mater. Sci. Eng. 9 415
Blazey K W 1971 Phys. Rev. Lett. 27 146
Capizzi M and Frova A 1970 Phys. Rev. Lett. 25 1298
Capizzi M, Frova A and Dunn D 1972 Solid State Commun. 10 1165
Cardona M 1965 Phys. Rev. 140 A651
Cardona M 1982 Light Scattering in Solids II (Berlin: Springer)
Cohen M I and Blunt R F 1968 Phys. Rev. 168 929
Cord B and Courths R 1985 Surf. Sci. 162 34
Eglitis R I, Kotomin E A and Borstel G 2004 Comput. Mater. Sci. 30 376
Fujishiro F and Mochizuki S 2005 in preparation
Grabner L 1969 Phys. Rev. 177 1315
Hasegawa T, Mouri S, Yamada Y and Tanaka K 2003 J. Phys. Soc. Japan 72 41
Hasegawa T, Shirai M and Tanaka K 2000 J. Lumin. 87–89 1217
Henrich V E, Dresselhaus G and Zeiger H J 1978 Phys. Rev. B 17 4908
Itoh N 1995 Butsuri 50 704 (in Japanese)
Itoh N, Kansaki J, Okano A and Nakai Y 1995 Annu. Rev. Mater. Sci. 25 97
Itoh N and Stoneham A M 2000 Material Modiﬁcation by Electronic Excitation (Cambridge: Cambridge University
Press)
Jourdan M and Adrian H 2003 Physica C 388/389 509
Katsu H, Tanaka H and Kawai T 2001 J. Appl. Phys. 90 4578
Kawabe Y, Yamanaka A, Hanamura E, Kimura T, Takiguchi Y, Kan H and Tokura Y 2000 J. Appl. Phys. 88 1175
Kimura S, Yamauchi J and Tsukada M 1995 Phys. Rev. B 51 11049
Kleemann W, Albertini A, Kuss M and Linder R 1997 Ferroelectrics 203 57
Kröger F A and Vink H J 1956 Solid State Physics vol 3 (New York: Academic) p 307
Mochizuki S 2003 Physica B 340–342 944
Mochizuki S and Araki H 2003a Physica B 340–342 913
Mochizuki S and Araki H 2003b Physica B 340–342 969
Mochizuki S, Nakanishi T, Suzuki Y and Ishi K 2001 Appl. Phys. Lett. 79 3785
Mochizuki S, Shimizu T and Fujishiro F 2003 Physica B 340–342 956
Mochizuki S and Fujishiro F 2005 in preparationMüller K A, Berlinger W and Tosatti E 1991 Z. Phys. B 84 277
Müller K A and Burkard H 1979 Phys. Rev. B 19 3593
Nasu K 2004 Rep. Prog. Phys. 67 1607
Ohmukai M, Takigawa Y and Kurosawa K 1999 Appl. Surf. Sci. 137 78
Pontes F M, Longo E, Leite E R, Lee E J H, Varela J A, Pizani P S, Campos C E M, Lanciotti F, Mastellaro V and
Pinheiro C D 2002 Mater. Chem. Phys. 77 598
Puchin V E, Shluger A L and Itoh N 1993 Phys. Rev. B 47 10760
Ruddlesden S N and Popper P 1957 Acta Crystallogr. 10 538
Ruddlesden S N and Popper P 1958 Acta Crystallogr. 11 54
Shluger A and Stefanovich E 1990 Phys. Rev. B 42 9664
Sirenko A A, Akimov I A, Fox J R, Clark A M, Li H C, Si W and Xi X X 1999 Phys. Rev. Lett. 82 4500
Song K S and Williams R T 1993 Self-Trapped Excitons (Berlin: Springer)
Stashans A and Vargas F 2001 Mater. Lett. 50 145
Szot K and Speier W 1999 Phys. Rev. B 60 5909
Szot K, Speier W, Carius R, Zastrow U and Beyer W 2002 Phys. Rev. Lett. 88 075508-1
Szot K, Speier W, Herion J and Freiburg C 1997 Appl. Phys. A 64 55
Takesada M, Yagi T, Itoh M and Koshihara S 2003 J. Phys. Soc. Japan 72 37
Toyozawa Y 1983 Physica B 112 & 118 23
Tsang C and Street R A 1979 Phys. Rev. B 19 3027
Worlock J M and Fleury P A 1967 Phys. Rev. Lett. 19 1176
Zollner S, Demkov A A, Liu R, Fejes P L, Gregory R B, Alluri P, Curless J A, Yu Z, Ramdani J, Droopad R, Tiwald T
E, Hilﬁker J N and Woollam J A 2000 J. Vac. Sci. Technol. B 18 2242

Polyhedron 23 (2004) 1605–1611
www.elsevier.com/locate/poly

Salts of tetrachloroauric acid with pyridine N -oxide having
various base/acid ratios of 1/1, 4/3, 3/2 and 2/1:
crystal structures, 35Cl NQR and phase transitions
Tetsuo Asaji a,*, Eigo Akiyama a, Fukue Tajima b, Kazuo Eda b,
Masao Hashimoto b, Yoshihiro Furukawa caDepartment of Chemistry, College of Humanities and Sciences, Nihon University, Sakurajosui, Setagaya-ku, 3-25-40, Tokyo 156-8550, Japanb
Department of Chemistry, Faculty of Science, Kobe University, Nada-ku, Kobe 657-8501, Japanc
Department of Science Education, Graduate School of Education, Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-8524, Japan
Received 27 January 2004; accepted 24 March 2004
Available online 30 April 2004

Abstract
We determined crystal structures of [(PyO)D][AuCl4 ], [(PyO)4 D3 ][AuCl4 ]3 and [(PyO)3 D2 ][AuCl4 ]2 (PyO: pyridine N-oxide) atroom temperature (1/1, 4/3 and 3/2 D-salts, respectively). The powder X-ray diﬀraction patterns of these salts (D-salts) wereidentical to those of the corresponding ones having normal hydrogen (H-salts) unlike the case of [(PyO)2 D/H][AuCl4 ]. In the crystalsof the 4/3 and 3/2 salts, there exist not only the monomer-type protonic adducts of PyO, but also dimer-type adducts in which two
PyO units are bridged by a proton. A few types of aggregation of AuCl4 units (sheet and ribbons) were found in these crystals. The
variety in the aggregation motif of the anion and that in the structure of the cation seem to be related to the presence of the various
salts having several base-to-acid ratios. The isotope eﬀect on the preparation of the salts is discussed. 35 Cl NQR and phase transitions of these compounds are also reported.
Ó 2004 Elsevier Ltd. All rights reserved.
Keywords: Crystal structure; Phase transition; Isotope eﬀect; Hydrogen bond; NQR; Tetrachloroaurate(III)

1. Introduction
It is known that pyridine N -oxide (PyO) forms amonomeric hydrogen adduct [(PyO)H]þ in its salts. Examples can be seen in several compounds [1,2]. In addition, PyO has a tendency to form a monovalent diadducttype cation [(PyO)2 H]þ , in which the two PyO units arebridged by a proton. This type of cation exists, for instance, in [(PyO)2 H][AuCl4 ] [3] and other compounds[4,5]. There is the possibility that both types of cations areincorporated in a salt. In fact, we prepared a compound[(PyO)3 H2 ][AuCl4 ]2 having a PyO/HAuCl4 ratio of 3/2,
which suggests the coexistence of [(PyO)H]þ and[(PyO)2 H]þ in the crystal [4]. Furthermore, we could
separate a compound [(PyO)4 H3 ][AuCl4 ]3 with another*

Corresponding author. Tel./fax: +81-3-5317-9739.
E-mail address: asaji@chs.nihon-u.ac.jp (T. Asaji).

0277-5387/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2004.03.022

PyO/HAuCl4 ratio of 4/3 by controlling the PyO/
HAuCl4 ratio in the solution. These ﬁndings led us todetermine the crystal structures of these compounds to
conﬁrm their composition and the structure of the cation.
As a structural change induced by substitution of D
for H atom, we recently reported that the structure of
the cation [(PyO)2 D]þ in [(PyO)2 D][AuCl4 ] and its
whole crystal structure itself were dramatically diﬀerent
from those of [(PyO)2 H][AuCl4 ] [4]. Since we have beeninterested in this phenomenon induced by substitutionof a hydrogen isotope, a part of this work was devoted
to examine the eﬀect of deuteration on the structures ofthe various salts mentioned above. We also investigated
the stability of [(PyO)2 D][AuCl4 ] and [(PyO)2 H][AuCl4 ]to show that both of them are in stable phases.
The results of DTA/DSC as well as preliminary
35
Cl NQR measurements of [(PyO)3 (H/D)2 ][AuCl4 ]2 ,[(PyO)4 H3 ][AuCl4 ]3 and [(PyO)D][AuCl4 ] are reported.

1606

T. Asaji et al. / Polyhedron 23 (2004) 1605–1611

We brieﬂy discuss the phase transitions in [(PyO)3 (H/D)2 ][AuCl4 ]2 and [(PyO)D][AuCl4 ].

2. Experimental
2.1. Synthesis
The various complex salts [(PyO)m (H/D)n ][AuCl4 ]n(m=n ¼ 1=1, 4/3, 3/2, 2/1) between PyO and (H/D)AuCl4
were prepared by slow evaporation of aqueous solutions(H2 O or D2 O) containing PyO and (H/D)AuCl4 in themolar ratio shown in Table 1. In the text, following
abbreviations will be used; [(PyO)m (H/D)n ][AuCl4 ]n :m=n (H/D)-salt. The 4/3 H-salt was not prepared.
DAuCl4 was obtained by dissolving Au2 O3 in a DClsolution in D2 O. Elemental analyses were carried out at
the Center for Organic Elemental Microanalysis, Kyoto
University. The results of the analyses have already beenreported in [4] for [(PyO)2 D][AuCl4 ], [(PyO)3 H2 ][AuCl4 ]2 , [(PyO)H][AuCl4 ] and [(PyO)D][AuCl4 ]. In thefollowing, the results of [(PyO)3 H2 ][AuCl4 ]2 , [(PyO)3 D2 ][AuCl4 ]2 and [(PyO)4 D3 ][AuCl4 ]3 are reported. Anal.
Calc. for [(PyO)3 H2 ][AuCl4 ]2 : C, 18.7; H, 1.8; Cl, 29.4;
N, 4.4; O, 5.0. Found: C, 18.4; H, 1.8; Cl, 29.2; N, 4.2;
O, 4.8%. Anal. Calc. for [(PyO)3 D2 ][AuCl4 ]2 : C, 18.6; H/
D, 2.0; Cl, 29.3; N, 4.4; O, 5.0. Found: C, 18.5; H/D, 1.9;
Cl, 29.4; N, 4.3; O, 4.8%. Anal. Calc. for [(PyO)4 D3 ][AuCl4 ]3 : C, 17.1; H/D, 1.9; Cl, 30.3; N, 4.0; O, 4.6.
Found: C, 17.2; H/D, 2.0; Cl, 30.4; N, 4.0; O, 4.3%. The
extent of the deuteration has been estimated by use of2
H NMR measurements of the aqueous solution to be
H/D ¼ approximately 15/85 for [(PyO)3 D2 ][AuCl4 ]2 ,[(PyO)4 D3 ][AuCl4 ]3 and [(PyO)D][AuCl4 ].
The 3/2 (H/D)-salts and the 4/3 D-salt showed an
extraordinarily wide temperature range of melting (seeTable 1). This is probably due to gradual decompositions of the salts.
2.2. Physical measurements
X-ray powder patterns were recorded by the use of a
Rigaku RINT 2100S diﬀractometer with Cu Ka radia-

tion. Silicon powder was used as an internal standard of
the diﬀraction angle. DTA and DSC measurements
were performed using a home-made apparatus and
DSC3100S (Bruker AXS), respectively. For the2
H NMR measurements of the solution, a JEOL JNMEX270 NMR spectrometer was employed. The densityof the crystal at room temperature was determined by aﬂoating method, using a mixture of o-xylene andbromoform or benzene and methylene iodide. A pulsed
spectrometer based on the Matec gated ampliﬁer 515A
was used for the 35 Cl nuclear quadrupole resonance(NQR) measurements. The temperature of the NQR
sample was controlled in the range 4.2–300 K using anelectronic controller (LakeShore 331) and measured
with an accuracy of 0.5 K by use of a gold +0.07% ironversus chromel thermocouple. Solid-state 1 H NMR experiments were carried out by use of a standard pulsed
NMR spectrometer at the Larmor frequency of 42.5
MHz.
2.3. Crystallography
A SMART 1000/CCD diﬀractometer (Bruker) was
employed for the diﬀraction measurements usinggraphite monochromated Mo Ka radiation. Data collections were carried out at room temperature. Crystal
data and the details of the experiments and reﬁnements
are given for the 3/2 D-salt, 4/3 D-salt and 1/1 D-salt inTable 2.
The structures were solved by direct methods andreﬁned by full-matrix least-squares calculations basedon Fo2 with absorption correction (S A D A B S ) using
S H E L X L -97 [6]. Non-hydrogen atoms of each salt were
included in the least-squares calculations with anisotropic displacement parameters.
It was found that the monomeric adduct [(PyO)D]þ
in the 1/1 D-salt had a mirror plane, being perpendicular
to the ring plane and containing the N–O bond, and a
C2 -axis, being normal to the mirror plane and passing
through the center of the ring (C2h symmetry) indicating
an orientational disorder, in which each of the O, N andrelevant C (C1) atoms was located on two sites with anequal population. The positions of the N and C1 atoms

Table 1
The complex salts [(PyO)m (H/D)n ][AuCl4 ]n (m=n ¼ 1=1, 4/3, 3/2, 2/1) between PyO and (H/D)AuCl4 obtained from aqueous solutions (H2 O or D2 O)
Chemical formula

Melting point (K)Molar ratio in crystal
PyO:(H/D)AuCl4

Molar ratio in solution
PyO:(H/D)AuCl4[(PyO)2 H][AuCl4 ][(PyO)3 H2 ][AuCl4 ]2[(PyO)H][AuCl4 ][(PyO)2 D][AuCl4 ][(PyO)3 D2 ][AuCl4 ]2[(PyO)4 D3 ][AuCl4 ]3[(PyO)D][AuCl4 ]368–369
428–491
521–528
367–368
438–484
478–497
517–5202:1
3:2 (1.5:1)
1:1
2:1
3:2 (1.5:1)
4:3 (1.3:1)
1:1

2:1
1:1
1:2 (0.5:1)
5:1
2:1a
2:1a
2:1a ; 1:1aFrom the 2:1 solution, three kinds of crystals were obtained case by case.

T. Asaji et al. / Polyhedron 23 (2004) 1605–1611

1607

Table 2Crystal data and details of the experiments and reﬁnements
Base/acid

1/1

3/2

4/3

FormulaFormula weight
X-ray radiationWavelength (A)Crystal system
Space group

a (A)

b (A)

c (A)

C5 H5 DONAuCl4
435.87
Mo Ka
0.710730
orthorhombic
Pbam
7.673(2)8.526(2)7.944(2)

C15 H15 D2 O3 N3 Au2 Cl8
966.86
Mo Ka
0.710730
monoclinic
C2=c16.46(2)9.038(9)18.65(2)

a (°)
b (°)
c (°)
3 )
Cell volume (AZ
Dc (g cm3 )Dm (g cm3 )l(Mo Ka)F ð0 0 0Þ
Crystal color
Crystal size (mm)Temperatureh range (°)
Index range
Reﬂections collected
Reﬂections uniqueRint
Reﬂections (I > 2rðIÞ)R1 ðF ÞðI > 2rðIÞÞ
wR2 ðF 2 Þ for all reﬂection
Number of parameters
Goodness-of-ﬁt for all
reﬂections (F 2 )Weighting scheme
Maximum shift/esd
3 )qmax (e A3 )qmin (e A

519.7(2)2
2.786
2.78  0.02
15.133396
yellow
0.09  0.08  0.07
298
2.56–27.47
9 6 h 6 9, 10 6 k 6 10,
6 6 l 6 92722
585
0.0442433
0.03340.0836440.944

2600(5)4
2.471
2.45  0.02
12.1181784
yellow
0.12  0.07  0.06
298
2.61–27.52
18 6 h 6 20, 11 6 k 6 7,
22 6 l 6 18
5684
2593
0.05931639
0.05780.1563156
1.062

C20 H20 D3 O4 N4 Au3 Cl12
1402.73
Mo Ka
0.710730
triclinic
P
1
9.079(2)9.142(2)12.504(2)78.250(3)85.110(4)63.438(3)908.8(3)1
2.564
2.55  0.02
12.994644
yellow
0.08  0.07  0.06
296
1.66–27.48
11 6 h 6 11, 11 6 k 6 11,11 6 l 6 16
5316
3617
0.03022422
0.04230.0971198
0.919

w1 ¼ ðr2 Fo2 Þ þ ð0:0632pÞ2 þ 0:00p,
p ¼ ðFo2 þ 2Fc2 Þ=3
0.000
0.960
)3.152w1 ¼ ðr2 Fo2 Þ þ ð0:0721pÞ2 þ 0:00p,
p ¼ ðFo2 þ 2Fc2 Þ=3
0.000
2.115
)2.077w1 ¼ ðr2 Fo2 Þ þ ð0:0443pÞ2 þ 0:00p,
p ¼ ðFo2 þ 2Fc2 Þ=3
0.001
2.044
)1.318110.42(2)were set to be equal. The D and non-disordered H atoms
were found from a diﬀerence Fourier map and includedin the reﬁnement with isotropic displacement parameters (Uiso ). The H atom attached to C1 was ignored in
the reﬁnement.
The asymmetric unit of the 3/2 D-salt contained a[(PyO)D]þ group having a C2 -axis passing through C6
and C9 as shown in Scheme 1 (C2 symmetry) and a[PyO–D–PyO]þ group having an inversion center (Cisymmetry).
For the latter group, the bridging D atom was ﬁxed at
the center of symmetry, and the positions of the H atoms belonging to the PyO moiety were calculated andthey were included in the reﬁnement with Uiso . The C2
symmetry of the [(PyO)D]þ group indicates an orientational disorder of the group. In addition, another disorder was found for the group, because the asymmetricunit of the group had two oxygen atoms (O2 and O20
shown in Scheme 1) with diﬀerent populations (0.205

and 0.295, respectively). Owing to this disorder of the[(PyO)D]þ group, the N atom was divided into two sites(N2 and N20 ), each overlapping C7 and C8, respectively.
The position and Uij of C7 were set equal to those of N2,
the population of which being that of O2 (0.205). The
same treatment was applied to C8 and N20 . The positions of the H atoms attached to C6 and C9 were calculated and they were included in the reﬁnement with
Uiso . The rest of the H atoms, as well as the D atom wereomitted in the reﬁnement.
The asymmetric unit of the 4/3 D-salt was found toinclude [(PyO)D]þ and [PyO–D–PyO]þ with a center ofsymmetry. The positions of H atoms belonging to the
PyO moieties in the latter unit were calculated and they
were included in the reﬁnement with Uiso . The bridging
D atom was treated in the way same as that describedabove. The structure of the PyO moiety of the[(PyO)D]þ unit was considerably distorted and the Hatoms could not be found on the Fourier maps. The

1608

T. Asaji et al. / Polyhedron 23 (2004) 1605–1611

Since the quality of the single crystals of the deuterated salts was better than that of the normal ones, X-ray
crystal structures of the former salts were determined in
the present work.
C6

O2

3.2. Stability of the 2/1 H- and 2/1 D-salts

N2(C7)

C8(N2’)C9

O2’

Scheme 1. Asymmetric unit of the disordered [(PyO)D]þ group. The
vertical broken line indicates 2-fold axis.

positions of the H atoms were calculated and they were
included in the reﬁnement with Uiso .

3. Results and discussion
3.1. Eﬀect of deuteration on the crystal structures of the
1/1, 4/3 and 3/2 salts
As an isotope eﬀect on crystal structure, we have reported a novel structural diﬀerence between the 2/1 Hsalt and the 2/1 D-salt [4]. In relation to this phenomenon,
we examined the possibility of such an isotope eﬀect on
the crystal structures of the 3/2, 4/3 and 1/1 salts. Their Xray powder patterns revealed, however, that the deuteration did not change their crystal structures.

In order to check the possibility that either of the Hor D-salt is in a meta-stable phase, we carried out DTA
measurements to get information on their thermal stability. DTA curves of the two salts measured in the 170–
385 K region showed no exothermic heat anomaly
corresponding to a monotropic transition of a metastable phase. We also examined X-ray powder diﬀraction
patterns measured at room temperature for melt-grown
crystals of the H- and D-salts or the crystals annealed at
ca. 350 K. The diﬀraction patterns of the melt-grown or
the annealed H- and D-salts were identical to those of
the corresponding compounds obtained by crystallization from solutions. These results of the DTA and X-ray
experiments strongly suggest that both of the H- andD-salts are in stable phases.
3.3. Crystal structures of the 1/1, 4/3 and 3/2 D-salts atroom temperature
The 1/1 D-salt contains only [(PyO)D]þ , while there
exist both [(PyO)D]þ and [PyO–D–PyO]þ in the crystalsof the 4/3 D-salt and the 3/2 D-salt. The molar ratio of
the monomeric:dimeric adducts in the crystal is 1:1 inthe 3/2 salt and 2:1 in the 4/3 salt. These results are
consistent with the chemical compositions of the salts.
The geometries of the monomeric and dimeric
adducts are listed in Table 3.
The dimeric adducts in the 4/3 D-salt and the 3/2 Dsalt are centrosymmetric (Ci symmetry), while the dimeric adduct in the 2/1 D-salt is non-symmetric (C1symmetry) [4]. The O  O distance in the dimeric adduct and that in the 3/2 D-saltin the 4/3 D-salt (2.44(1) A) are typical for a symmetric O  H  O
(2.41(2) A)

Table 3
Geometries of the dimeric adduct ([PyO–(H/D)–PyO]þ ) and the monomeric one ([(PyO)D]þ )[PyO–(H/D)–PyO]þ

[(PyO)D]þ
ab

Compounds

2/1 H-salt

2/1 D-salt

3/2 D-salt

4/3 D-salt

3/2 D-saltc

4/3 D-salt

1/1 D-saltc
N–O distance (A)1.362(8)1.32(1)1.35(1)–d

1.36(1)1.36(2)

O  O distance (A)N–O  O bond angle (°)

2.41(1)108.242.41(2)
109(1)

2.44(1)106.9(6)

–
–

–
–

–
–

N–O  O–N torsion angle (°)
References

180
[3]

1.354(9),1.314(9)2.43(1)106.4(5),116.6(6)142.7(8)[4]180This work

180
This work–
This work–
This work–
This work

The crystal contains one non-symmetric [PyO-D-PyO]þ in the asymmetric unit.
The crystal contains centrosymmetric [PyO–D–PyO]þ .
c
The monomeric adduct in the crystal is disordered (see text).
d
The N–O length was not reliable owing to the orientational disorder (see text).
abb

T. Asaji et al. / Polyhedron 23 (2004) 1605–1611

hydrogen bond, being comparable with the corresponding values found, for example, in the 2/1 D-salt [4],2/1 H-salt [3], [(PyO)2 H]ClO4 [5] and [(2,6(CH3 )2 PyO)2 H]- [AuCl4 ] [7]. The N–O bond lengths of
the PyO moiety in [(PyO)D]þ found for the 1/1 and 4/3
D-salts are longer than that of the neutral PyO molecule [8]. This observation indicates the hydrogena(1.30 A)
tion of the oxygen atom and/or its participation in theformation of a hydrogen bond.
The crystal of the 1/1 D-salt is built by alternatingstacking of the layers of [(PyO)D]þ and [AuCl4 ]
(Fig. 1). [(PyO)D]þ has C2h symmetry to result in its
orientational disorder, which can be seen in Fig. 1. Itshould be mentioned that the [(PyO)D]þ groups in the
1/1 salt form a layer structure as shown in Fig. 2. The suggests
short intermolecular O  N contact (3.16(2) A)

q’

q

o

1609

a certain interaction between the adjacent [(PyO)D]þ
groups. There exists a short intermolecular Cl  Cl
 which seems to contribute tocontact of 3.391(6) A,
connect the layers of [AuCl4 ] . The short intermolecu indicates possibility of
lar O1  Cl1 length (3.16(2) A)
N–O–D  Cl hydrogen bonds.
The crystal structures of the 4/3 D-salt and 3/2 D-salt
are depicted in Figs. 3 and 4, respectively. Several aggregation motifs of [AuCl4 ] ions are found in the 1/1, 4/
3, 3/2 and 2/1 D-salts. In the crystal of the 1/1 D-salt, one
can recognize a planar sheet structure formed by[AuCl4 ] ions (Fig. 1). If one takes account of a relatively
 in the sheet structure,long Au  Cl contact of 3.509 (3) A
the formal coordination number of the Au atom is six
and the layer structure is formed by vertex-sharing of the
AuCl6 octahedrons. A similar layer structure has beenreported for [EtC(OEt)NH2 ][AuCl4 ] [9].

c
s

p
r

c
D t
r
M
q

b

p

s

M’

Fig. 1. Crystal structure of the 1/1 D-salt (stereo). Short contacts are O1  Cl1 (3.16(2) A) and Cl2  Au
Cl1(q)  Cl10 (q0 ) (3.391(6) A), Symbols p, q, q0 , r and s denote O1, Cl1, Cl10 , Cl2 and(3.509(3) A).
Au, respectively. Symmetry codes: p : ðx þ 1:5; y  0:5; z þ 1Þ;
q : ðx þ 1; y þ 1; z þ 1Þ; q0 : ðx þ 1; y  1; zÞ; r : ðx þ 0:5; y  0:5;
z þ 1Þ and s : ðx; y  1; z þ 1Þ.

o

a

D’ b
Au2
Cl3
Cl2
Au1

Fig. 3. Crystal structure of the 4/3 D-salt (stereo). D and M indicate[PyO–D–PyO]þ and [(PyO)D]þ , respectively. D and D0 , M and M0 as
well, are related by an inversion center. Symbols p, q, r, s and t are Cl1,
Cl4, O1, Cl2 and Cl5, respectively. Symmetry codes: p : ðx þ 2;
y þ 1; z þ 1Þ; q : ðx; y; zÞ; r : ðx; y; zÞ; s : ðx  1; y; zÞ and t : ðx  1;y; zÞ.

O

N

Fig. 2. Layer structure formed by disordered [(PyO)D]þ units in the 1/1
D-salt. The broken lines indicate short O  N contacts of 3.16(2) A.

Fig. 4. Crystal structure of the 3/2 D-salt (stereo). D and M denote[PyO-D-PyO]þ and [(PyO)D]þ units. Symbols p and q are Cl1 andCl10 . Symmetry codes: p : ðx þ 0:5; y þ 1:5; z þ 0:5Þ and q : ðx þ0:5; y þ 1:5; z þ 1Þ.

T. Asaji et al. / Polyhedron 23 (2004) 1605–1611

A ribbon-like aggregation motif of [AuCl4 ] ions thatwas found in the 4/3 salt (Fig. 3) can be regarded as apart of the sheet structure observed in the 1/1 D-salt.
The Au1  Cl3 and Au2  Cl2 contacts are 3.525(3) and respectively, being comparable with the
3.590(3) A,
Au  Cl contacts found for the chain structures of[AuCl4 ] ions in [EMIM][AuCl4 ] and [BMIM][AuCl4 ] respectively) [10]. In the ribbon(3.356 and 3.452 A,
structure of the 4/3 D-salt, there are six and ﬁve coordinations of the Au atom, and vertexes of AuCl6 octahedrons and AuCl5 distorted square pyramids areshared to result in the ribbon.
The crystals of the 3/2 and 2/1 D-salts consist of another type of ribbon structure, each of which can be also
regarded formally as a part of the layer structure of the
1/1 salt. The ribbon structure of the 3/2 D-salt has an (Fig. 4). The Au  Cl
Au1  Cl4 contact of 3.559(5) A being
contact in the 2/1 D-salt amounts to 3.983(2) A,
comparable to that found for a chain structure of
AuCl4 ions in [(H5 O2 )2 (12-crown-4)2 ][AuCl4 ]2 (3.845 A)[11].
In the crystal of the 4/3 D-salt, the ribbons are packed
along the crystallographic a-axis as shown in Fig. 3. A
 of Cl1  Cl4) is obshort Cl  Cl contact (3.612(4) A
served between the ribbons. A columnar cavity surrounded by four ribbons of [AuCl4 ]n is ﬁlled by a[(PyO)D]þ (M), a PyO moiety of a [PyO–D–PyO]þ (D)and their inversion-related ones (M0 and D0 ) as shown in
Fig. 3. It is interesting to note that one of the two PyO
moieties of a [PyO–D–PyO]þ unit is accommodated in acolumnar cavity, while the other one in an adjacentcavity. There are two short intermolecular contacts between the chlorine atoms in the ribbon and an oxygen andatom in the PyO group: Cl2  O1 (3.31(1) A)Cl5  O1 (3.15(1) A). The planes of the PyO moieties
stack parallel to each other (see Fig. 3).
The crystal structure of the 3/2 D-salt is similar tothat of the 4/3 D-salt as can be seen by comparing Figs.
3 and 4. The cavity of the 3/2 D-salt is ﬁlled by D, D0(related to D by 2-fold symmetry axis) and M. The
number of the PyO moieties accommodated in a cavityof the 3/2 D-salt is smaller than that of the 4/3 D-salt asa result of the decrease in the width of the ribbon
structure. D and M stack almost parallel to each otherand M, which is ﬂanked by D and D0 , exhibits the disorder mentioned above. There is a short Cl  Cl contact

between adjacent ribbons (Cl1  Cl10 of 3.575(9) A).

3.4. Characterization of the 1/1, 4/3 and 3/2 D-salts by
Cl NQR and thermal analysis

rine atoms in the crystal. Previously, we have improperly
assigned these frequencies to the 1/1 D-salt [4].
The temperature dependence of the 35 Cl NQR frequencies of the 4/3 D-salt is shown in Fig. 5. In thetemperature range investigated (77–230 K), m versus T
curves seem to have no anomaly corresponding to aphase transition. The thermal analyses (DTA and DSC)
carried out in 150–300 K region also could not detect
any anomaly due to a phase transition.
For the 3/2 D-salt, a phase transition was detected at
199 K by the thermal analyses with increasing temperature (DH ¼ 1:3 kJ mol1 ). For the 3/2 H-salt, it was
located at 197 K. The transitions exhibited very slighthysteresis, indicating the ﬁrst-order nature of the transition. It was suggested from the temperature dependence of the 1 H NMR spin–lattice relaxation time of the
3/2 D-salt that at least one of the cationic molecules in
the unit cell performs a fast reorientational motion
above the phase-transition point. This mobility of thecation shows that the disordered structure found in the
crystal structure determination is likely to be a dynamic
one. So far, we have found four very broad 35 Cl NQR
lines at 77 K for the 3/2 D-salt. The crystal structure of
the low-temperature phase and details of the NMR/NQR results will be reported in a forthcoming paper.
DTA/DSC curves of the 1/1 D-salt measured in 150–
300 K region exhibited a very small thermal anomaly at
273 K (Tc1 ) indicating a phase transition (DHc1 ¼ 0:4
kJ mol1 ). Hysteresis (super-cooling of ca. 30 K) of thetransition suggested a ﬁrst-order nature of the transition. It is very interesting that we could not detect any
anomaly in the DTA/DSC curves for the 1/1 H-salt in35

30

Cl NQR of [(PyO)4D3][AuCl4]3

ν / MHz

1610

29

28

27

35

For the 4/3 D-salt, we observed the ﬁve NQR frequencies (m) of 29.164, 28.727, 28.625, 28.272 and 27.618
MHz at 77 K. There must be one missing line, since
there exist six crystallographically non-equivalent chlo-

0

100

200

300

T/ K
Fig. 5. Temperature dependences of 35 Cl NQR frequencies of the
4/3 D-salt.

T. Asaji et al. / Polyhedron 23 (2004) 1605–1611

the temperature range 150–300 K. Such a kind of isotope eﬀect has been reported previously [12,13].
Two 35 Cl NQR lines (high- and low-frequency lines)
were observed for the 1/1 D-salt at room temperature.
When the sample temperature was decreased, the highfrequency line was observed to split drastically into fourlines below 70 K (Tc2 ). Thus, from the 35 Cl NQR measurements, we have found another phase transition thatis more drastic than that at Tc1 . Moreover, it is interesting that the low-frequency line disappeared before
reaching to Tc2 .
In the region of T > Tc2 , the frequencies (m) of the twolines decreased monotonously with increasing temperature. No anomaly was detected in the m versus T curves
at Tc1 within the accuracy of the present frequency
measurements. This feature, together with the smallvalue of DHc1 , indicates quite a subtle structural change
at Tc1 . The presence of the two 35 Cl NQR lines is consistent with the crystal structure determined at room
temperature having two crystallographically nonequivalent chlorines.
The temperature dependence of the NQR spin–lattice
relaxation time T1 showed an anomalous decrease withdecreasing temperature in the region of T > Tc2 . This
result suggests a dynamical disorder of the [(PyO)D]þ
cations at T > Tc2 , since the 35 Cl T1 behavior can be
explained by the relaxation mechanism due to a ﬂuctuation of the electric ﬁeld gradient caused by reorientational motions between disordered orientations of
nearby [(PyO)D]þ cations. The presence of such a motion of the [(PyO)D]þ cation was strongly suggested by
the 1 H NMR spin–lattice relaxation measurements aswell. The details of NMR/NQR measurements will bereported elsewhere.
These conclusions from the NQR and NMR investigations are consistent with the disorder model of the[(PyO)D]þ group proposed in the crystal structure determination. An ordering of the [(PyO)D]þ cation is
likely to take place below Tc2 and hence the transition at
Tc2 seems to be of an order–disorder type.
3.5. Origins of compositional variety and isotope eﬀect on
preparation
The present structural analyses revealed that both ofthe monomeric and dimeric adducts of PyO with proton
can be formed in the crystal. In addition, several types ofaggregation of AuCl4 units are possible. These structural possibilities found for the cation and anion seem tobe a reason why so many salts having diﬀerent base-toacid ratio could be formed.
There was a tendency that the base-to-acid molar
ratio of the crystal was lower than that of the mothersolution in the heavy hydrogen system, while it was

1611

found vice versa in the normal hydrogen system (seeTable 1). This observation indicates that equilibrium
processes in PyO + (H/D)AuCl4 + (H/D)2 O systems are
dependent on the isotopic composition. Similar isotope
eﬀects on chemical equilibrium are known, for example,
in the enol-enethiol tautomerism of b-thioxoketones [14]and in an acid–base equilibrium [15].

4. Supplementary material
Crystallographic data for the 1/1, 4/3 and 3/2 D-salts
have been deposited with the Cambridge Crystallographic Data Centre, CCDC Nos. 219958, 219959 and219960, respectively. Copies of the information may beobtained free of charge from The Director, CCDC, 12
Union Road, Cambridge, CB2 1EZ, UK (fax: +44-1223336033; e-mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk).

Acknowledgements
This work was partially supported by a Grant from
the Ministry of Education, Culture, Sports, Science andTechnology to promote advanced scientiﬁc research.

References[1] P.G. Tsoucaris, Acta Crystallogr. 14 (1961) 914.
[2] O. Moers, K. Wijaya, T. Hamann, A. Blaschette, P.G. Jones,
Z. Naturforsch. 56b (2001) 1052.
[3] M.S. Hussain, E.O. Schlemper, J. Chem. Soc., Dalton Trans.
(1982) 751.
[4] T. Asaji, F. Tajima, M. Hashimoto, Polyhedron 21 (2002) 2207.
[5] J. Wasicki, M. Jaskolski, Z. Pajak, M. Szafran, Z. Dega-Szafran,M.A. Adams, S.F. Parker, J. Mol. Struct. 476 (1999) 81.
[6] G.M. Sheldrick, S H E L X L -97 Program for the Solution and theReﬁnement of Crystal Structures, University of G€
ottingen,Germany, 1997.
[7] M.S. Hussain, Polyhedron 15 (1996) 645.
[8] F.H. Allen, O. Kennard, D.G. Watson, L. Brammer, A.G. Orpen,
R. Taylor, J. Chem. Soc., Perkin Trans. II (1987) S1.
[9] R.A. Potts, D.L. Gaj, W.F. Schneider, N.S. Dean, J.W. Kampf,
J.P. Oliver, Polyhedron 10 (1991) 1631.
[10] M. Hasan, I.V. Kozhevnikov, M. Raﬁq, H. Siddiqui, A. Steiner,
N. Winterton, Inorg. Chem. 38 (1999) 5637.
[11] M. Calleja, K. Johnson, W.J. Belcher, J.W. Steed, Inorg. Chem.
40 (2001) 4978.
[12] T. Matsuo, A. Inaba, O. Yamamuro, N. Onoda-Yamamuro, J.
Phys.: Condens. Matter 12 (2000) 8595.
[13] T. Gustafsson, M. Ichikawa, I. Olovsson, Solid State Commun.
115 (2000) 473.
[14] B. Andresen, F. Duus, S. Bolvig, P.E. Hansen, J. Mol. Struct. 552(2000) 45.
[15] S.L.R. Ellison, M.J.T. Robinson, J. Chem. Soc., Chem. Commun.
(1983) 745.