First-principles and semiempirical calculations for F centers in KNbO3
arXiv:cond-mat/9707142 · doi:10.1103/PhysRevB.56.8599
Abstract
The linear muffin-tin-orbital method combined with density functional theory (local approximation) and the semiempirical method of the intermediate neglect of the differential overlap (INDO) based on the Hartree-Fock formalism are used for the study of the centers (O vacancy with two electrons) in cubic and orthorhombic ferroelectric KNbO crystals. Calculations for 39-atom supercells show that the two electrons are considerably delocalized even in the ground state of the defect. Their wave functions extend over the two Nb atoms closest to the O vacancy and over other nearby atoms. Thus, the center in KNbO resembles electron defects in the partially-covalent SiO crystal (the so-called center) rather than usual centers in ionic crystals like MgO and alkali halides. This covalency is confirmed by the analysis of the electronic density distribution. Absorption energies were calculated by means of the INDO method using the self-consistent-field scheme after a relaxation of atoms surrounding the center. For the orthorhombic phase three absorption bands are calculated to lie at 2.72 eV, 3.04 eV, and 3.11 eV. The first one is close to that observed under electron irradiation. For the cubic phase, stable at high temperatures, above 708 K, only the two bands, at 2.73 eV and 2.97 eV, are expected.
7 pages, including 4 postscript figures; with minor changes, as will appear in Phys. Rev. B
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