Anomalous temperature-induced volume contraction in GeTe

The recent surge of interest in phase change materials GeTe, Ge$_2$Sb$_2$Te$_5$, 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

Strong Effects of Cation Vacancies on the Electronic and Dynamical Properties of FeO

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 $U$ are both included explicitly in \emph{ab initio} simulations, which also suggests that the electron-phonon coupling in FeO is strong.Comment: 5 pages, 4 figure

Effect of cation-vacancy superstructure on the phonon dynamics in KNi2Se2

Density functional theory investigations of effects arising from ordered structure of cation vacancies in KNi2Se2 are reported. The simulated cation-deficient KxNi2-ySe2 phases with x = 0.8, y = 0.0, and y = 0.4 lie within the stoichiometry range of experimental samples produced by the self-flux method or oxidative deintercalation of a vacancy-free system. Results of the present studies indicate pronounced impact of cation vacancy superstructure on the structural, electronic, and vibrational properties of KNi2Se2. Revealed modifications of the local structure, atomic bond lengths, electronic, and phonon bands, which are especially noticeable in the system with both potassium and nickel deficiencies, are reflected in the simulated neutron pair-distribution functions, the phonon and Raman spectra, which are provided to facilitate both experimental verification of the predicted effects and analysis of the phase composition of a multiphase K-Ni-Se material.Web of Science1014art. no. 04512

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

The influence of intrinsic Fe defects in FeO (either single cation vacancies or prototypical 4:1 vacancy clusters) on electronic and dielectric properties is studied within density-functional theory. The importance of local Coulomb interactions at Fe atoms is highlighted and shown to be responsible for the observed insulating Mott gap in FeO, which is reduced by the presence of defects. We investigate nonstoichiometric configurations of Fe_{1-x}O with x ranging from 3% to 9%, and we find the aliovalent Fe cations in both the regular and interstitial lattice sites of the considered configurations. Furthermore, we show that the trivalent Fe ions, induced by both isolated and clustered Fe vacancies, introduce the empty band states inside the insulating gap, which decreases monotonically with increasing cation vacancy concentration. The Fe_{1-x}O systems with high defect content become metallic for small values of the Coulomb interaction U, yielding an increase in the dielectric functions and optical reflectivity at low energies, in agreement with the experimental data. Due to the crystal defects, the infrared-active transverse optic phonons split and distribute over a wide range of frequencies, clarifying the origin of the exceptionally large spectral linewidths of the dielectric loss functions observed for wüstite in recent experiments