Defect States Induced by Oxygen Vacancies in Cubic SrTiO3: First-Principles Calculations

Using the first-principles calculations with the correction of on-site Coulomb interaction, we found that single O vacancy (V-O) in cubic SrTiO3 takes a magnetic ground state in which V-O induces a singly-occupied localized state in the band gap and a delocalized state in the conduction band. Our calculations showed that V-O energetically prefers the singly-ionized state V-O(+) rather than forming a pair of neutral V-O(0) and doubly ionized V-O(2+) states from 2V(O)(+). Furthermore, we found that two O vacancies of a vacancy pair complex tend to align in a magnetically anti-parallel shoulder-to-shoulder configuration in the xy plane, rather than in a non-magnetic head-to-head configuration along the z direction as predicted in an available theoretical work. For O divacancies in the all five configurations considered here, localized states were found in the band gap. Our results not only clarify the puzzling findings reported in literature, but also can provide a better understanding of the role of O vacancies in the low temperature conductivity and the optical excitation of oxygen-deficient SrTiO3.