Langasite-type (LGS-type) crystal with four elements in A₃BC₃D₂O₁₄ structure has been intensively studied as a group of piezoelectric crystal^[1,2]. Recently, we have found the amount of thermally activated holes in LGS-type crystal depends on metal vacancy (V_m^×), which crucially affects conductivity of bulk crystal^[3]. However, no study on the site occupancy of V_m^× has been carried out in LGS-type crystal with four elements so far.

Site occupancy model (SOM) is able to raise all the possible site-structure of constituent elements and corresponding defects. In LGS-type crystal, Ca₃TaGa₃Si₂O₁₄ (CTGS), 6 site-structures out of 16 are possible but not essential to exist. So,solid solution range (ss range)was determined in CTGS to verify which site-structuresexist.

3 compositional series of CTGS were synthesized: 1. Ca,Ta-stoichiometric (-s), changing Ga and Si ratio; 2. Ca,Si-s, changing Ga and Ta ratio; 3. Ca,Ga-s, changing Ta and Si ratio, as shown in Fig. 1. Secondary phases were observed with backscattering electron image (BEI). In the sintered materials, four kinds of secondary phases in the composition of CaTa₂O₆ (CT2), Ca₃Ta₂Ga₄O₁₄ (CTG), CaSi₂O₅ (CS2) and Ca₃Ga₄Si₂O₁₃ (CGS) were recognized. The lever rule was applied to obtain ss range from composition and relative quantity of secondary phases. In CTGS, ss range locates at Ca-s, Ta, Ga-poor and Si-rich area, the calculated composition of the boundary of ss range was shown in Fig. 1. Only one of the 6 possible site-structures is applicable in CTGS, which shows B site accommodates V_m^×. The quantitative relationship between V_m^×and antisite defect is also deduced by this SOM site-structure: 5Si_Ga- V_Ta=0, where Si_Ga is Si population in Ga site. This result indicates V_m^× in CTGS bulk crystal will generate significant amount of holes at elevated temperature, which is harmful to conductivity stability. This work suggests the method that SOM discussion accompanied with ss range measured by investigating secondary phase is valid to study site of V_m^×in a certain LGS-type crystal. 

[1] S.Zhang, et al., J.Appl.Phys105, 114107 (2009).

[2] B.Mill,et al., Russ.J.Inorg.Chem.43, 1270 (1998).

[3] R. Yaokawa et al. J.Appl.Phys108, 064112 (2010)

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