U.D. Wdowik

Structural stability and thermal properties of BeO from the quasiharmonic approximation

The phase diagram and thermal properties of BeO crystal are calculated within the quasiharmonic approximation and using density functional theory. The wurtzite, zinc blende and rocksalt phases of BeO are considered. Phonons are calculated versus external pressure. The pressure dependence of frequencies for the zone-center phonon modes is analyzed. The resulting free energy provides predictions for the temperature dependence of various quantities such as the equilibrium volume, thermal expansivity and isobaric heat capacity. The mean-squared vibrations of BeO atoms are investigated as well. The calculated (p,T) diagram predicts a structural phase transition from the wurtzite to rocksalt phase solely. The zinc blende structure is energetically not preferred in the investigated range of pressures and temperatures. Theoretical results are compared with the available experimental data and other ab initio calculations. Existing discrepancies between quasiharmonic theory and experiment are discussed and some explanation is given.

Anomalous temperature-induced volume contraction in GeTe

The recent surge of interest in phase change materials GeTe, Ge

Strong effects of cation vacancies on the electronic and dynamical properties of FeO

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Lattice dynamics of Fe-doped CoO from first principles

Sb

Expansion of the Lamb-Mössbauer factor into semi-invariants

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Ab initio study of the lattice dynamics of CsNiF3

Te

Calibration of the Mössbauer Spectrometer Velocity by Optical Methods

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Influence of isolated and clustered defects on electronic and dielectric properties of wüstite

, and related compounds motivated us to revisit the structural phase transition in GeTe in more details than was done before. Rhombohedral-to-cubic ferroelectric phase transition in GeTe has been studied by high resolution neutron powder diffraction on a spallation neutron source. We determined the temperature dependence of the structural parameters in a wide temperature range extending from 309 to 973 K. Results of our studies clearly show an anomalous volume contraction of 0.6\% at the phase transition from the rhombohedral to cubic phase. In order to better understand the phase transition and the associated anomalous volume decrease in GeTe we have performed phonon calculations based on the density functional theory. Results of the present investigations are also discussed with respect to the experimental data obtained for single crystals of GeTe.

Vibrational properties and the stability of the KCuF3 phases

We report pronounced modifications of electronic and vibrational properties induced in FeO by cation vacancies, obtained within density functional theory incorporating strong local Coulomb interactions at Fe atoms. The insulating gap of FeO is reduced by about 50% due to unoccupied electronic bands introduced by trivalent Fe ions stabilized by cation vacancies. The changes in the electronic structure along with atomic displacements induced by cation vacancies affect strongly phonon dispersions via modified force constants, including those at atoms beyond nearest neighbors of defects. We demonstrate that theoretical phonon dispersions and their densities of states reproduce the results of inelastic neutron and nuclear resonant x-ray scattering experiments \emph{only} when Fe vacancies and Coulomb interaction

Structural and thermal properties of LaMnO3 from neutron diffraction and first principles studies.

U