Co-Doped Effect of Afterglow Properties in R
3GaO6:Eu3+ (R = Y, Gd)
Authors: Dazai Takuro , Yasui Shintaro , Taniyama Tomoyasu , Itoh Mitsuru 
Published: February 2020

Afterglow materials have attracted much attention for medical applications and energy storages due to its individual character. These materials are able to be stored energy during irradiation of the excitation light, and then shows a lasting of the luminescence after turning off the excitation light. SrAl2O4:Eu2+, Dy3+ is one of the famous afterglow material which emits green luminescence and has long duration time. A lot of green and blue afterglow materials such as aluminates and silicates have been reported, but there are few reports of red color afterglow materials with superior property. Y2O3:Eu3+ is one of the famous phosphors having red bright luminescence. However, it doesn’t have afterglow property. D. R. Evans et al.1 reported that Ca2+ co-doped Y2O3:Eu3+ had short duration time with a few minutes after cathode-ray excitation. Also, Y. Lin et al. 2 reported that the afterglow in Mg2+ co-doped Y2O3:Eu3+ had longer duration time with several tens of minutes. Among these materials, entity of oxygen vacancy is a key of afterglow showing. However, previous reports have not explained the difference in the role of each co-doped ion effect to duration time in luminescent materials. Therefore, we focused on the origin of afterglow property in co-doped luminescent　materials with oxygen vacancy. 

 We chose R
3GaO6 (R = rare earth elements) phosphor instead of Y2O3-based materials. Eu3+ or Tb3+ doped R
3GaO6 phosphor with bright photoluminescence were already reported.3 As for the electronic structure, top of the valence band and bottom of the conduction band are composed O2p and Y5d, respectively.4 On the other hand, the difference in the crystal structures of Y2O3 and Y3GaO6 gives different number of the oxygen sites. Multi-oxygen vacancy can prepare the multi-energy trap level in R
3GaO6 materials. For this reason, R
3GaO6 phosphors with co-doped ions may show afterglow property. In this study, we prepared co-doped R
3GaO6and investigated relationship between afterglow property and effect of Mg ions. 

 R
2.99-x
GaO6-δ
:Eu0.01, Ax

(R=Y, Gd, A=Mg, Ca, Sr, x = 0 - 0.05) were prepared by solid state reaction using R
2O3, Ga2O3, Eu(NO3)3 and A(NO3)2. All starting materials were mixed with ethanol and sintered at 1723 K for 12 h in air. Powder X-ray diffraction measurement were carried out using Cu Kα radiation (Rigaku, RINT). Emission and excitation properties were measured by spectrofluorometer (JASCO, FP-6500). Afterglow properties were measured after irradiation by handy type UV-light for 10 min. Thermoluminescence measurement (80-600 K) were recorded at 614 nm, which is the emission peak, by changing heating rate (10-50 K/min) in order to estimate the trap depth.

 R
3GaO6 structure without any secondary phases was obtained in all products and red luminescence was observed during irradiation of UV-light (254 nm) by human eyes. Excitation spectrum showed broad absorption band centered at around 250 nm. This result agrees with reported spectra, indicating observed spectrum is originated from Eu-O charge transfer. 3 Emission spectra showed that red color luminescence is originated from 5D0 - 7Fj (j=1-4) transition, which corroborates the valence of Eu3+. 614 nm emission spectrum was assigned to 5D0 - 7F2 transition. All emission and excitation spectra showed the same shape, and the intensity of those were enhanced by co-doping of A ions. And furthermore, all co-doped products showed the afterglow. The duration times were over 10 min for all products, especially Mg2+ co-doped products had longer duration time than that of Ca2+ and Sr2+ ions. The glow curve obtained by thermoluminescence showed the three peaks in Ca2+, Sr2+ co-doped products. On the other hand, four peaks were observed in Mg2+ co-doped product. The number of peaks corresponds to that of trap levels. Excited electrons transfer from conduction band to trap levels, they can travel to couples of trap levels, and finally go to luminescent center. Therefore, we suggest that the increase in the number of trap levels should lead to longer duration time. The increase in the number of trap levels in Mg co-doped product should be related possibly to the ionic radius. Since ionic radius of Mg ion is smaller than that of R ions, Mg ions cannot stay at the center of original Rionic position. Mg ions in the off-center may form defect pair with oxygen vacancy, resulting a new trap levels.

1. D. R. Evans et al., J. Soc. Inform. Disp., 3, 197 (1996)

2. Y. Lin. et al., J. Alloys Compd., 361, 92 (2003)

3. P. Guo. et al., J. Electrochem. Soc.,
150, H201 (2003)

4. A. Jain et al., APL Materials, 1, 011002 (2013)