Semiempirical Hartree-Fock calculations forKNbO3

Abstract

In applying the semiempirical intermediate neglect of differential overlap (INDO) method based on the Hartree-Fock formalism to a cubic perovskite-based ferroelectric material KNbO3, it was demonstrated that the accuracy of the method is sufficient for adequately describing the small energy differences related to the ferroelectric instability. The choice of INDO parameters has been done for a system containing Nb. Based on the parametrization proposed, the electronic structure, equilibrium ground state structure of the orthorhombic and rhombohedral phases, and Γ-TO phonon frequencies in cubic and rhombohedral phases of KNbO3 were calculated and found to be in good agreement with the experimental data and with the first-principles calculations available.

Computer Modeling of Point Defects, Impurity Self-Ordering Effects and Surfaces in Advanced Perovskite Ferroelectrics

“…The calculated frequencies of the transverse-optic (TO) phonons at the 1' point in the Brillouin zone (BZ) of cubic and rhombohedral KNbO3 and the atomic coordinates in the minimum energy configuration for the orthorhombic and rhombohedral phases of KNbO3 are also in good agreement with experiment, thus indicating that a very successful INDO parametrization has been achieved in Refs. [24,25]. FroZen-phonon calculations for Ta, and T2u modes of cubic KTaO3 are also in good agreement with experiment.…”

“…This method is based on the Hartree-Fock formalism and allows for self-consistent calculations of the atomic and electronic structure of pure and defective crystals. The method has demonstrated very good results in previous calculations of defects, both in the bulk and on the surface, in many oxide materials [1], including pure KNbO3 and KTaO3 perovskites, and defects -Li impurities in KTaO3 [24,25]. The calculated frequencies of the transverse-optic (TO) phonons at the 1' point in the Brillouin zone (BZ) of cubic and rhombohedral KNbO3 and the atomic coordinates in the minimum energy configuration for the orthorhombic and rhombohedral phases of KNbO3 are also in good agreement with experiment, thus indicating that a very successful INDO parametrization has been achieved in Refs.…”

First-principles and semiempirical calculations forFcenters inKNbO3

“…A detailed analysis of the KNbO 3 parametrization for the INDO method is presented in Ref. 15. In that work considerable covalency was found of the chemical bonding in pure KNbO 3 .…”

“…Their agreement with the experimental data 24 is very good. 15 The displacements of Nb atoms nearest to V O are given in the same Table II for the both kinds of F centers existing in this phase and, again, with and without atomic orbitals centered at the O vacancy. In fact, Nb displacements are very similar in magnitude (6.6%) and also close to those found for the cubic phase.…”

“…The atomic positions for the perf ect orthorhombic cell shown in Fig. 4 were calculated earlier 15 and included in Table II. Their agreement with the experimental data 24 is very good.…”