Paweł T. Jochym

Ab initio lattice dynamics and elastic constants of ZrC

Abstract:Ab initio calculations and a direct method are applied to derive the phonon dispersion relations and phonon density of states for the ZrC crystal. The results are in good agreement with neutron scattering data. The force constants are determined from the Hellmann-Feynman forces induced by atomic displacements in a 222 supercell. The elastic constants are found using the deformation method and successfully compare with experimental data.

TiC lattice dynamics from ab initio calculations

Abstract:Ab initio calculations and a direct method have been applied to derive the phonon dispersion curves and phonon density of states for the TiC crystal. The results are compared and found to be in a good agreement with the experimental neutron scattering data. The force constants have been determined from the Hellmann-Feynman forces induced by atomic displacements in a 2x2x2supercell. The calculated phonon density of states suggests that vibrations of Ti atoms form acoustic branches, whereas the motion of C atoms is confined to optic branches. The elastic constants have been found using the deformation method and compared with the results obtained from acoustic phonon slopes.

Elastic properties and phase stability of AgBr under pressure

Silver halide crystals are of considerable interest, since they exhibit deviation from the ideal ionic character due to the presence of the d electrons in Ag 1 ions. Numerous studies of these crystals have been undertaken in the past years using various forms of interaction potentials including threebody interactions and van der Waals forces, 1,2 as well as the ab initio total energy pseudopotential calculations. 3 There are also many experimental works studying various aspects of these solids. 4‐1 3 The results of studies using phenomenological potentials generally show good agreement with experiment. 2 These studies use sophisticated schemes involving three body interactions and van der Waals potentials. The ab initio results for transition pressure seems to be less accurate. 3 There is one ab initio study of phase stability of silver bromide AgBr. 14 Nevertheless, the transition mechanism is still unclear for this compound. Several models have been proposed, 5‐7,14 ‐17 but the transition mechanism remains still an open problem. The ab initio study by Nunes, Allen, and Martins, 14 which investigated the energy of several can

Structure and elastic properties of Mg(OH)2 from density functional theory

The structure, lattice dynamics and mechanical properties of magnesium hydroxide have been investigated by static density functional theory calculations as well as ab initio molecular dynamics. The hypothesis of a superstructure existing in the lattice formed by the hydrogen atoms has been tested. The elastic constants of the material have been calculated with a static deformations approach and are in fair agreement with the experimental data. The hydrogen subsystem structure exhibits signs of disordered behaviour while maintaining correlations between the angular positions of neighbouring atoms. We establish that the essential angular correlations between hydrogen positions are maintained to a temperature of at least 150 K and that they are well described by a physically motivated probabilistic model. The rotational degree of freedom appears to be decoupled from the lattice directions above 30 K.

Comparative ab initio study of lattice dynamics and thermodynamics of Fe2SiO4- and Mg2SiO4-spinels.

Lattice dynamics and thermodynamic properties of antiferromagnetic Fe(2)SiO(4)-spinel have been studied using density functional theory. Phonon dispersions are obtained for several hydrostatic pressures up to 20 GPa. They are used to calculate thermodynamic properties within the quasiharmonic approximation. Comparison with ab initio results obtained for Mg(2)SiO(4)-spinel is made in order to study the effect of the cation exchange on the dynamic and thermodynamic properties of (Mg, Fe)(2)SiO(4)-spinel. The obtained results have been compared with the available experimental data.

Lattice dynamics of Ga1−xAlxAs studied by ab initio calculations

Abstract We have calculated the phonon dispersion and phonon density of states of superlattices of Ga1−xAlxAs for x=0.0, 0.25, 0.75 and 1.0 using the ab initio method within the supercell approach and calculating the Hellmann–Feynman forces. A noticeable difference of force constants between Ga–As and Al–As atomic pairs has been found. In any case, Al atoms vibrate in well-separated high-frequency optic modes.

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 the electronic and dielectric properties is studied within the 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 Fe1-xO with x ranging from 3 to 9 % and 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 are induced by both isolated and clustered Fe-vacancies and introduce the empty band states inside the insulating gap, which decreases monotonically with increasing cation vacancy concentration. The Fe1-xO systems with high defect content become metallic for small values of the Coulomb interaction U, yielding the 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 wustite in recent experiments.

Soft phonon modes in rutile TiO 2

The lattice dynamics of TiO2 in the rutile crystal structure was studied by a combination of thermal diffuse scattering, inelastic x-ray scattering, and density functional perturbation theory. We experimentally confirm the existence of a predicted anomalous soft transverse acoustic mode with energy minimum at q=(1/2 1/2 1/4). The phonon energy landscape of this particular branch is reported and compared to the calculation. The harmonic calculation underestimates the phonon energies but despite this the shape of both the energy landscape and the scattering intensities are well reproduced. We find a significant temperature dependence in the energy of this transverse acoustic mode over an extended region in reciprocal space, which point to a strong role of anharmonicity in line with a substantially anharmonic mode potential-energy surface. The reported low-energy branch is quite different from the ferroelectric mode that softens at the Brillouin zone center and may help explain anomalous convergence behavior in calculating TiO2 surface properties and is potentially relevant for real behavior in TiO2 thin films.

Dynamics and stability of icosahedral Fe–Pt nanoparticles

The structure, dynamics and stability of Fe-Pt nanoparticles have been investigated using DFT-based techniques: total energy calculations and molecular dynamics. The investigated systems included multi-shell and disordered nanoparticles of iron and platinum. The study concerns icosahedral particles with the magic number of atoms (55): iron-terminated Fe43Pt12, platinum-terminated Fe12Pt43, and disordered Fe27Pt28. Additionally, the Fe6Pt7 cluster has been investigated to probe the behaviour of extremely small Fe-Pt particles. Molecular dynamics simulations have been performed for a few temperatures between T = 150-1000 K. The calculations revealed high structural instability of the Fe-terminated nanoparticles and a strong stabilising effect of the Pt-termination in the shell-type icosahedral particles. The platinum termination prevented disordering of the particle even at T = 1000 K indicating very high melting temperatures of these Fe-Pt icosahedral structures. The analysis of evolution of the radial distribution function has shown a significant tendency of Pt atoms to move to the outside layer of the particles - even in the platinum deficient cases.

Vibrational properties and stability of FePt nanoalloys

The structural and dynamical properties of FePt nanoparticles were studied within the density functional theory. The effect of size and chemical composition on the dynamical stability of nanoparticles was investigated for the cuboctahedral and icosahedral symmetries. In cuboctahedra, a structural distortion is observed, which for systems with odd number of Pt layers leads to lowering of the tetragonal symmetry. Significant differences between the vibrational properties of FePt particles and bulk crystals are observed, but similarly to the FePt crystal, cuboctahedral particles exhibit a strong anisotropy of atomic vibrations. The icosahedral particles with perfect shell geometry are unstable due to enlarged distances between Fe atoms. They can be stabilized by removing a central atom or replacing it by a smaller one. The heat capacity and entropy of nanoparticles show typical enhancement due to low-energy vibrations at the surface layers.