First principles calculations of oxygen vacancy in langatate crystal

Single crystalline langatate (La3Ta0.5Ga5.5O14, LTG) has been widely used in piezoelectric sensors for high temperature applications because of its structural stability at high temperature. Ionic conduction originated from oxygen vacancy diffusion has been reserved at high temperature regime (over 700 °C). Similarly, an increase of electrical conductivity has been also observed at the intermediate temperature region ranges from 300 to 700 °C. Thus, it can be presumed that spontaneously generated point defects influence resistivity degradation of the crystal. In this study, to elucidate the oxygen vacancy effects on electronic structures and transport properties of LTG, formation energy curves of three different states of oxygen vacancy (V••O, V•O and V×O) 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 eV, 4.34 eV and 4.66 eV for defect-free LTG, LTG with V••O and V×O, 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 900 K, the calculated electrical conductivity of LTG with two stable oxygen vacancy (V••O and V×O) 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.