Ab initio calculations of the geometry and electronic structure of point defects in ferroelectrics with a perovskite structure

Abstract

Ab initio calculations of the optimized geometry and the electronic structure of lattice defects in incipient perovskite ferroelectrics SrTiO₃ and KTaO₃ are performed in the framework of the density functional theory. The results are presented for the Li⁺ impurity ion at the A site in the KTaO₃ and SrTiO₃ ferroelectrics; the Mn²⁺, Cd²⁺, Ca²⁺, Mg²⁺, and Zn²⁺ ions at the A site and the Mn⁴⁺ and Mg²⁺ ions at the B site in the SrTiO₃ compound; and the MN ²⁺_Ti -V _O and Mg ²⁺_Ti -V _O complexes in the SrTiO₃ ferroelectric. The results are obtained by the cluster method with allowance made for the structural relaxation initiated by the defect and, for nonisovalent substitutional impurities, with due regard for the charge and spin states of the defect. It is established that the Ca ²⁺_Sr , Cd ²⁺_Sr , Mn ⁴⁺_Ti , and Mg ²⁺_Ti ions have a stable central position, whereas the Li ⁺_K ion in the KTaO₃ compound and the Li ⁺_Sr , Mn ²⁺_Sr , and Zn ²⁺_Sr defects in the SrTiO₃ ferroelectric are off-center ions. The shape of the multiminimum adiabatic potential and the parameters of dielectric relaxators (activation barrier, dipole moment) for polar defects are obtained. The electronic impurity levels are determined for the Li ⁺_Sr and Mg ²⁺_Ti neutral defects.