arXiv subjects

A. V. Postnikov

Publications and source records attributed to A. V. Postnikov.

44 records · Page 3Linked to original sources

arXiv1997

First-principles calculations for Fe impurities in KNbO3

Nb-substituting Fe impurity in KNbO3 is studied in first-principles supercell calculations by the linear muffin-tin orbital method. Possible ways to account for the impurity charge compensation are discussed. Calculations are done in the local density approximation (LDA) and, for better description of Coulomb correlation effects within the localized impurity states, also in the LDA+U scheme. The achievements and problems encountered in both approaches are analyzed. It is found that the impurity possess either a low-spin configuration (with 0 or 1 compensating electron), or a high-spin configuration (with 2 or 3 compensating electrons), the latter two apparently corresponding to practically relevant rechargeable impurity states.

cond-mat.mtrl-sci↗

arXiv1997

Refined Geometry and Frozen Phonons in KNbO3

In order to arrive at ultimately accurate results available with the LMTO method in the local density approximation, the stability of full-potential LMTO predictions for off-center displacements in KNbO3, as depending on the choice of basis and expansion cutoffs, has been thoroughly investigated. With the calculation setup thus optimized, supercell frozen phonon calculations aimed at the study of the chain-structure instability over the Brillouin zone have been done, and the long-wavelength limit of the LO phonon is discussed.

cond-mat.mtrl-sci↗

arXiv1997

Semiempirical Hartree-Fock calculations for pure and Li-doped KTaO3

In extension of our previous study of KNbO_3 by the semiempirical Hartree-Fock method we present parameterization and total-energy results for non-ferroelectric KTaO_3 as a pure crystal (concentrating on the frozen phonon calculations) and that with Li impurities. The magnitudes of off-center Li displacements and the relaxation energies related to re-orientation of Li are calculated and compared with experimental estimates and earlier calculation results. The spatial extent of lattice relaxation around Li impurities and contributions from different neighbors to the relaxation energy are discussed.

cond-mat.mtrl-sci↗

arXiv1996

Semiempirical Hartree-Fock calculations for KNbO3

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 Gamma-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.

mtrl-th↗

arXiv1995

Electronic Structure and Valence Band Spectra of Bi4Ti3O12

The x-ray photoelectron valence band spectrum and x-ray emission valence-band spectra (Ti K _beta_5, Ti L_alpha, O K_alpha) of Bi4Ti3O12 are presented (analyzed in the common energy scale) and interpreted on the basis of a band-structure calculation for an idealized I4/mmm structure of this material.

mtrl-th↗

arXiv1995

ELECTRONIC STRUCTURE OF FeSi

The full set of high-energy spectroscopy measurements including X-ray photoelectron valence band spectra and soft X-ray emission valence band spectra of both components of FeSi (Fe K_beta_5, Fe L_alpha, Si K_beta_1,3 and Si L_2,3) are performed and compared with the results of ab-initio band structure calculations using the linearized muffin-tin orbital method and linearized augmented plane wave method.

mtrl-th↗

arXiv1994

Gamma phonons and microscopic structure of orthorhombic KNbO3 from first-principles calculations

{}From a series of total energy calculations by the full-potential linear muffin-tin orbital method, the total energy hypersurface as function of atomic displacements from equilibrium positions has been fitted for different Gamma phonon modes in orthorhombic KNbO3. Frequencies and eigenvectors of all TO Gamma phonons have been calculated in the harmonic approximation, and in the quantum oscillator scheme -- for A2 and B2 modes. The microscopic structure of the orthorhombic phase has been analyzed in a series of supercell calculations for different patterns of Nb displacements, providing indications in favour of the chain structure, with oppositely directed neighboring chains.

cond-mat↗

arXiv1994

Phonon Properties of Knbo3 and Ktao3 from First-Principles Calculations

The frequencies of transverse-optical $Γ$ phonons in KNbO$_3$ and KTaO$_3$ are calculated in the frozen-phonon scheme making use of the full-potential linearized muffin-tin orbital method. The calculated frequencies in the cubic phase of KNbO$_3$ and in the tetragonal ferroelectric phase are in good agreement with experimental data. For KTaO$_3$, the effect of lattice volume was found to be substantial on the frequency of the soft mode, but rather small on the relative displacement patterns of atoms in all three modes of the $T_{1u}$ symmetry. The TO frequencies in KTaO$_3$ are found to be of the order of, but somehow higher than, the corresponding frequencies in cubic KNbO$_3$.

cond-mat↗