GW calculations on La ₃Ta _0.5Ga _5.5O ₁₄ with oxygen vacancies at non-equivalent sites

Langatate (La ₃Ta _0.5Ga _5.5O ₁₄, LTG) has been widely used in piezoelectric sensors in high temperature applications because of its structural and piezoelectric reliability at high temperature. However, in the low oxygen partial pressure and elevated temperature ranging from 300 to 700 °C, an increase of electrical conductivity originated from intrinsic oxygen vacancies have been observed in a previous experimental result. In this study, to elucidate the oxygen vacancies effects on LTG, formation energy of each electronic state of oxygen vacancies (VO, VO and VO×) at non-equivalent Wyckoff positions (2d and 6g) and their electronic structures were calculated by combining GW quasi-particle scheme and conventional density functional theory (DFT). The formation energy of V _O(6g) was lower than the formation energy of V _O(2d) in each electronic state. Thus, relatively stable oxygen vacancies at 6g sites (V _{O(6g )}) can be major oxygen vacancies in LTG system. Also, calculated band gaps of LTG with thermodynamically stable defects such as VO(6g) or VO(6g)× were higher than band gaps of LTG with VO(2d) or VO(2d)× at Γ-point. By the band interpolation based on Boltzmann transport theory, calculated electrical conductivities per unit relaxation time (σ/τ) of the LTG with VO(2d) and VO(6g) were similar at the same electron energy, however, σ/τ of the LTG with VO(2d)× was higher than that of the LTG with VO(6g)×.