First-principles study of native defects and lanthanum impurities in NaTaO3

The formation energies and electronic structure of lattice vacancies, antisite defects, and lanthanum impurities in NaTaO(3) are investigated using first-principles calculations based on density-functional theory. The Na antisite and the Ta vacancy, which are both multiple acceptors, are energetically favorable under O-rich conditions, whereas the O vacancy, which is a double donor, is preferred under O-poor conditions. The Ta antisite is a quadruple donor but its high formation energy renders it unlikely to be stable under the thermal equilibrium other than p-type and O-poor conditions. In La-doped NaTaO(3), substitutional La at the Na site is likely to form as a double donor under O-poor conditions. It is expected to generate carrier electrons except for heavily doped cases where compensation by acceptorlike La at the Ta site, as well as the native acceptors, is significant. For all the native defects and La impurities, no localized one-electron states are found in the band gap, which is consistent with the shallow donor/acceptor behavior determined using the formation energies.