In order to elucidate the oxygen vacancy effects on electronic structures and transport properties of La3Ta0.5Ga5.5O14, formation energy curves of three different states of oxygen vacancy (V••O, V•O and VxO) and their electronic structures and transport properties, such as carrier concentration and electrical conductivity were calculated by utilizing first principles calculations. The calculated band gaps by the GW method were 5.36, 4.34 and 4.66eV for defect-free LTG, LTG with V••O and VxO, respectively. Also, the GW result showed higher transition level of oxygen vacancy compared with the transition level calculated from the conventional DFT method. Although, in a previous experimental result, Ga loss was observed in Langasite (LGS) crystal, however, the formation energy of VO was much lower than the formation energy of VGa in LTG. At 900K, the calculated electrical conductivity of LTG with two stable oxygen vacancy (V••O and VxO) were ten times higher than the conductivity of defect-free LTG around the Fermi levels. Therefore, at intermediate temperature, oxygen vacancy generation can cause an increase of electrical conductivity.

First Principles Calculations of Oxygen Vacancy in Langatate Crystal. C.Y.Chung, R.Yaokawa, H.Mizuseki, Y.Kawazoe: Solid State Ionics, 2012, 206, 1–6