First-principles and semiempirical Hartree-Fock calculations for F centers in KNbO[sub 3] and Li impurities in KTaO[sub 3

Abstract

The linear muffin-tin-orbital method based on the density-functional theory and the semi-empirical method of the Intermediate Neglect of the Differential Overlap based on the Hartree-Fock formalism are used for a supercell study of F centers in cubic and orthorhombic ferroelectric KNbO3 crystals. Two electrons are found to be considerably delocalized even in the ground state of the defect. The absorption energies are calculated by means of the INDO method using the ΔSCF scheme after a relaxation of atoms surrounding the F center. As an example of another type of point defect in perovskite, an isolated Li impurity in KTaO3 as well as interacting Li pairs are considered in the supercell approach, using supercells of up to 270 atoms. The off-center Li displacement, reorientational energy barriers and the lattice relaxation around impurities are calculated. The results are compared with those obtained earlier within the shell model, revealing the relaxation pattern somehow different from the shell model estimations.