Growth and Structural Analysis of Silicate Phosphor Single Crystal Using Gas Phase Method

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 Ba_1.00(Sr_0.77Eu_0.23)SiO₄ and the Ca_0.116Sr_1.810Eu_0.074SiO₄ phosphors single crystal under UV light (365 nm). Single crystal of Ba_1.00(Sr_0.77Eu_0.23)SiO₄ and Ca_0.116Sr_1.810Eu_0.074SiO₄ 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 Ba_1.00(Sr_0.77Eu_0.23)SiO₄ phosphor single crystal obtained by X‐ray single-crystal diffraction analysis. The Ba_1.00(Sr_0.77Eu_0.23)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 Ba_1.00(Sr_0.77Eu_0.23)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 Ca_0.116Sr_1.810Eu_0.074SiO₄ phosphor single crystal obtained by X‐ray single-crystal diffraction analysis. The Ca_0.116Sr_1.810Eu_0.074SiO₄ 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 Ca_0.116Sr_1.810Eu_0.074SiO₄ 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 Ca_0.116Sr_1.810Eu_0.074SiO₄ phosphor, which is different from Ba_1.00(Sr_0.77Eu_0.23)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 Ca_0.116Sr_1.810Eu_0.074SiO₄ 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.