Growth and Structural Analysis of Silicate Phosphor Single Crystal Using Gas Phase Method
Authors: Hasegawa Shota , Kim Sun Woog , Uematsu Kazuyoshi , Ishigaki Tadashi , Toda Kenji , Sato Mineo , Masaki Takaki , Yoon Dae Ho , Koide Junko , Toda Masako , Kudo Yoshiaki 
Published: February 2020

Silicate materials have been usually used as host materials of phosphors for white LEDs because　there are high chemical stabilities, low cost and these silicate phosphors show good luminescence properties under near-UV and blue light excitation. In the series of silicate phosphors, orthosilicate phosphors, Ba-rich (Ba,Sr)2SiO4:Eu2+
(green emission) and Sr-rich (Ba,Sr)2SiO4:Eu2+ (yellow emission), have been used in white LEDs based blue light LED as commercial phosphors [1]. In addition, (Ca,Sr)2SiO4:Eu2+ phosphor are recently reported by M. Kakihana's which is shown the red emission by blue light excitation when larger amount of Eu2+ doped into the host lattice [2]. However, there are no reported on crystal structure data of alkaline earth silicate phosphors by single crystal structure analysis and the doping site of Eu2+in the host lattice also has not yet been established definitely. In this study, we prepared orthosilicate phosphor single crystals and the detail crystal data of these phosphors are gained by single-crystal structure analysis. The single crystal of orthosilicate phosphor are grew by a novel vapor phase technique (gas and solid phase hybrid synthesis method), which is gas (SiO) and solid phase (Ba/Sr/Ca/Eu compound) hybridized [3].

Figure 1 shows the polarized microscope image of the Ba1.00(Sr0.77Eu0.23)SiO4 and the Ca0.116Sr1.810Eu0.074SiO4 phosphors single crystal under UV light (365 nm). Single crystal of Ba1.00(Sr0.77Eu0.23)SiO4 and Ca0.116Sr1.810Eu0.074SiO4 is plate-like crystal and the single crystal sizes of these phosphors are about 250 μm and 120 μm, respectively. These crystals used for structure analysis to investigate the doping site of Eu2+in the host lattice.

Figure 2 shows the crystal structure of Ba1.00(Sr0.77Eu0.23)SiO4 phosphor single crystal obtained by X‐ray single-crystal diffraction analysis. The Ba1.00(Sr0.77Eu0.23)SiO4 phosphor has a orthorhomic structure and the space group of Pnma (No. 62) with the lattice parameters a = 0.73856 (12) nm, b = 0.57579 (9) nm, and c = 0.99394 (16) nm. Ba2+ is coordinated by 10 oxide anions and Sr2+ is coordinated by 9 oxide anions in Ba1.00(Sr0.77Eu0.23)SiO4 crystal structure. Furthermore, Eu2+ is only substituted into the Sr2+ site because the ionic radius of Sr2+
(0.131 nm; CN = 9) and Eu2+
(0.130 nm; CN = 9) are nearly length while the ionic radius of Sr2+ is much smaller than that of the Ba2+(0.152 nm; CN = 10) [4].

Figure 3 shows the crystal structure of Ca0.116Sr1.810Eu0.074SiO4 phosphor single crystal obtained by X‐ray single-crystal diffraction analysis. The Ca0.116Sr1.810Eu0.074SiO4 phosphor also has a orthorhomic structure and the space group of Pnma (No. 62) with the lattice parameters a = 0.70548 (11) nm, b = 0.56524 (8) nm, and c = 0.96805 (14) nm. Sr2+ is coordinated by 10 and 9 oxide anions in Ca0.116Sr1.810Eu0.074SiO4 crystal structure and small amount of Ca2+ is coordinated by 9 oxide anions. Moreover, Eu2+ is only substituted into the 10 coordination site of Sr2+ in the Ca0.116Sr1.810Eu0.074SiO4 phosphor, which is different from Ba1.00(Sr0.77Eu0.23)SiO4 phosphor. Since ionic radius of Ca2+ (0.118 nm; CN = 9) is smaller than Eu2+, Ca2+ is preferentially substituted into the 9 coordination site of Sr2+ in the case of Ca0.116Sr1.810Eu0.074SiO4 phosphor. To the best of our knowledge, this is the report of the crystal structure of single crystal of orthosilicate phosphors and the definite doping site of Eu2+in the orthosilicate phosphors lattice.

References

[1] X. Zhang, X. Tang, J. Zhang, M. Gong, J. Lumin. 130 (2010) 2288‐2292.

[2] S. Tezuka, Y. Sato, T. Komukai, Y. Takatsuka, H. Kato, M. Kakihana, Appl. Phys. Express, 6 (2013) 072101.

[3] T. Sakamoto, K. Uematsu, T. Ishigaki, K. Toda, M. Sato, Key Eng. Mater.485 (2011) 325.

[4] R.D. Shannon, Acta Cryst. Sect.A, 32 (1976) 751-767.