Hybrid density functional study of oxygen vacancies in KTaO3 and NaTaO3

Using the Heyd-Scuseria-Ernzerhof hybrid functional, we systematically study the energetics and electronic properties of oxygen vacancies in KTaO3 and NaTaO3. The oxygen vacancies in these systems show similar behavior. The 2+ charge state is the most stable for most positions of the Fermi level inside the band gap. The neutral and+ charge states become comparable in formation energy with the 2+ charge state when the Fermi level is close to the conduction band minimum. Therefore, the oxygen vacancies are double shallow donors, which can provide carrier electrons. Two types of off-symmetric configurations, in which the two nearest tantalum atoms of the oxygen vacancies are asymmetrically located, also possibly form as metastable configurations. These metastable configurations show a striking difference in electronic structure from each other; one configuration has a delocalized characteristic as in the case of the stable configuration, while the other induces a deep, localized state in the band gap. On the basis of the predicted formation energies and electronic properties, the previous experimental and theoretical findings relevant to the oxygen vacancies are discussed.