First-principles and semiempirical calculations for F centers in KNbO3

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 $F$ centers (O vacancy with two electrons) in cubic and orthorhombic ferroelectric KNbO${}_3$ 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 $F$ center in KNbO${}_3$ resembles electron defects in the partially-covalent SiO${}_2$ crystal (the so-called $E_1^{\prime }$ center) rather than usual $F$ 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 $\Delta$ self-consistent-field scheme after a relaxation of atoms surrounding the $F$ 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

References in corpus (5)

• First-Principles Determination of Chain-Structure Instability in KNbO${}_3$
• Correlation effects in ionic crystals: I. The cohesive energy of MgO
• Semiempirical Hartree-Fock calculations for KNbO3
• Semiempirical Hartree-Fock calculations for pure and Li-doped KTaO3
• Phonon Properties of Knbo3 and Ktao3 from First-Principles Calculations

Cited by in corpus (2)

• A first-principles study of oxygen vacancy pinning of domain walls in PbTiO3
• First-principles and semi-empirical calculations for bound hole polarons in KNbO3