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Study on Site Occupancy of Metal Vacancy in Langasite-type Crystal with Four Elements

Hengyu Zhao ,  Jun Nozawa ,  Kensaku Maeda ,  Haruhiko Koizumi ,  Kozo Fujiwara ,  Satoshi Uda 

Tohoku University, Institute for Materials Research (IMR), 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan


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 (Vm×), which crucially affects conductivity of bulk crystal[3]. However, no study on the site occupancy of Vm× 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 Vm×. The quantitative relationship between Vm×and antisite defect is also deduced by this SOM site-structure: 5SiGa- VTa=0, where SiGa is Si population in Ga site. This result indicates Vm× 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 Vm×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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Presentation: Oral at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, General Session 7, by Hengyu Zhao
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17

Submitted: 2013-04-05 06:41
Revised:   2013-04-06 14:28