M P Belov

Stability of the hcp Ruthenium at high pressures from first principles

The method of calculation of the elastic constants up to third order from the energy-strain relation under pressure for the hcp crystals is given and described in details. The method is applied to the hcp phase of Ruthenium. Elastic constants, lattice dynamics, and electronic structure are investigated in the pressure interval of 0–600 GPa by means of first principles calculations. The obtained parameters are in very good agreement with available experimental and theoretical data. No preconditions for phase transformation driven by mechanical or dynamical instabilities for hcp Ru were found in the investigated pressure range. The reason of stability at such high pressures is explained in the context of electronic structure peculiarities.

Elastic properties and lattice dynamics of ruthenium at high pressures

The elastic properties and structural stability in ruthenium under pressure are investigated. The analysis is performed in the framework of Landau theory and nonlinear elasticity. For this purpose the definition of effective elastic constants (EC) of n-th (n≥2) order characterizing elastic properties of loaded crystal and the relations between effective EC and corresponding EC of Bragger type for hcp crystals is given. The conditions of hcp lattice stability to the uniform shear strain under the pressure P are expressed in terms of the second order effective EC. The method of effective EC calculations for hcp crystals under hydrostatic pressure is presented. The equation of state and EC of second and third order and phonon dispersion relations in high-symmetry directions in the pressure range of 0 – 600 GPa are calculated in the framework of the density functional theory (DFT) and the density functional perturbation theory (DFPT) respectively. EC are in the good agreement with available experimental data and increase monotonically with pressure, no softening or stability condition violation are observed. Softening of phonon frequencies near the Brillion zone center is also not observed.

Correlation between charge state and diffusion of hydrogen in Ti-based quasicrystals

The dependence of the charge state of hydrogen as a function of H content in the 1/1 approximant to icosahedral Ti-based quasicrystals (QCs) was studied using a first-principles method based on the density functional theory and norm-conserving pseudopotentials. It was found that the dependence is strong and non-monotonic. Hydrogen diffusion is significantly hindered in QCs with a charged hydrogen atom.

Mott transition and magnetic collapse in iron-bearing compounds under high pressure

ABSTRACT We discuss the electronic, magnetic, and related structural transitions in the iron-based Mott insulators under high pressures relevant to the Earths lower mantle conditions. The paper focuses on the above-mentioned topics based primarily on our theoretical analysis and various experimental studies employing synchrotron X-ray diffraction,57Fe Mössbauer spectroscopy, and electrical transport measurements. We review the main theoretical tools employed for the analysis of the properties of materials with strongly interacting electrons and discuss the problems of theoretical description of such systems. In particular, we discuss a state-of-the-art method for calculating the electronic structure of strongly correlated materials, the method, which merges standard band-structure techniques (DFT) with dynamical mean-field theory of correlated electrons (DMFT). We employ this method to study the pressure-induced magnetic collapse in Mott insulators, such as wüstite (FeO), magnesiowüstite (FeMg)O (x=0.25 and 0.75) and goethite (FeOOH), and explore the consequences of the magnetic collapse for the electronic structure and phase stability of these materials. We show that the paramagnetic cubic -structured FeO and (Fe,Mg)O and distorted orthorhombic (Pnma) FeOOH exhibit upon compression a high- to low-spin (HS-LS) transition, which is accompanied by a simultaneous collapse of local moments. However, the HS-LS transition is found to have different consequences for the electronic properties of these compounds. For FeO and (FeMg)O, the transition is found to be accompanied by a Mott insulator-to-metal phase transition. In contrast to that, both (FeMg)O and FeOOH remain insulating up to the highest studied pressures, indicating that a Mott insulator to band insulator phase transition takes place. Our combined theoretical and experimental studies indicate a crossover between localized to itinerant moment behavior to accompany magnetic collapse of Fe ions.

Hydrogen in palladium: Anharmonicity of lattice dynamics from first principles

The interaction potentials of the palladium and hydrogen sublattices at different hydrogen concentrations have been obtained in terms of the density functional theory and ab initio pseudopotentials. It has been shown that the anharmonicity of this interaction depends on the hydrogen concentration. The phonon spectrum of palladium hydride PdH has been calculated in the harmonic approximation and taking into account the anharmonic effects. The temperature-dependent effective potential technique accounting for the anharmonic effects of lattice vibrations has been described.

Hydrogen in the approximant i-TiZrHf: The energy state, charge, and diffusion

The energy of hydrogen dissolution in different tetrahedral pores of a 1/1 approximant of the icosahedral TiZrHf quasicrystal has been determined in terms of the density functional theory and ab initio pseudopotentials. At low and high degrees of loading of TiZrHf with hydrogen, the charges of atoms involved in the system and the Mayer bond order have been calculated for all possible M-M and M-H pairs. The diffusion coefficient of a single hydrogen atom in the system under study has been estimated numerically.