Eugene Heifets

Semi-empirical simulations of surface relaxation for perovskite titanates

The (100) and (110) surface relaxations are calculated for SrTiO 3 and BaTiO 3 perovskite thin films. By means of a semi-empirical shell model, the positions of atoms in 16 near-surface layers placed atop a slab of rigid ions are calculated. Surface rumpling and surface-induced dipole moments are calculated for all possible surface terminations. Our results for the (100) surface structure are in good agreement with ab-initio plane-wave pseudopotential calculations and LEED experiments. The surface energy for the Ba-, Sr-, TiO-terminated (110) surfaces is found to be much larger than that for the (100) surface. In contrast, the surface energy for the asymmetric O termination, where outermost O atoms are strongly on-plane-displaced, is the lowest for all (110) terminations and thus the most stable.

The first-principles treatment of the electron-correlation and spin–orbital effects in uranium mononitride nuclear fuels

The DFT+U calculations were employed in a detailed study of the strong electron correlation effects in a promising nuclear fuel-uranium mononitride (UN). A simple method for solving the multiple minima problem in DFT+U simulations and insure obtaining the correct ground state is suggested and applied. The crucial role of spin-orbit interactions in reproduction of the U atom total magnetic moment is demonstrated. Basic material properties (the lattice constants, the spin- and total magnetic moments on U atoms, the magnetic ordering, and the density of states) were calculated varying the Hubbard U-parameter. By varying the tetragonal unit cell distortion, the meta-stable states have been carefully identified and analyzed. The difference in the magnetic and structural properties obtained for the meta-stable and ground states is discussed. The optimal effective Hubbard parameter U(eff) = 1.85 eV reproduces correctly the UN anti-ferromagnetic ordering, and only slightly overestimates the experimental total magnetic moment of the U atom and the unit cell volume.

Hartree - Fock simulation of the Ag/MgO interface structure

The atomic and electronic structure of the Ag/MgO interface are calculated using an ab initio Hartree - Fock computer code and a supercell model of a silver monolayer atop three layers of MgO substrate. The band structure, electronic density distribution and densities of states are analysed in detail for isolated and interacting slabs of a metal and MgO. The energetically most favoured adsorption position for Ag atoms is found to be above the O atoms, with the binding energy of 0.20 eV and the equilibrium Ag - O distance of 2.64 A. Neither appreciable charge transfer in the interfacial region, nor considerable population of bonds between the silver monolayer and the insulating substrate take place. The adhesion energy arises mainly due to the electrostatic interaction of substrate atoms with a complicated charge redistribution in the metal monolayer, characterized by large quadrupole moments and electron density redistribution towards the gap position in the middle of the nearest Ag atoms. This could be a reason for the disagreement of all three SCF theories with the phenomenological image interaction model.

Atomistic simulation of the [001]surface structure in BaTiO3

Abstract We simulate the effect of the surface relaxation on the polarization of the layers of paraelectric phase in the vicinity of the [001] surface in BaTiO 3 in the framework of the shell-model potentials. We observe large polarization of ions in the first two layers of the surface. Our simulations confirm the possibility of existence of Ti- and Ba-containing top layers in [001] BaTiO 3 surfaces.

Atomistic simulation of SrTiO3 and BaTiO3 (110) surface relaxations

Abstract The (110) surface relaxations were calculated for SrTiO 3 and BaTiO 3 perovskites in a cubic phase. Using a shell model, the positions of atoms in 16 near- surface layers placed atop a slab of rigid ions are calculated. The strong surface rumpling and induced surface dipole moments perpendicular to the surface are predicted for both the O-terminated and TiO-terminated surfaces.

Theoretical simulation of localized holes in MgO

The authors propose a consistent approach to the study of hole self-trapping and diffusion in ionic crystals based on the static quantum-chemical calculation of the self-trapping energy and adiabatic barriers or hole diffusion. The calculations made for MgO revealed the possibility that the hole is localized on one anion site in the perfect lattice and predict its high mobility.

Atomistic simulation of surface relaxation

The (001) surface relaxation of the cubic perovskite crystal has been studied using the shell model. The positions of atoms in several surface layers embedded in the electrostatic field of the remainder of the crystal are calculated. We show that , and ions in six near-surface layers are displaced differently from their crystalline sites which leads to the creation of so-called surface rumpling, a dipole moment, and an electric field in the near-surface region. Calculated atomic displacements are compared with LEED experimental data.

Oxygen Incorporation Reaction into Mixed Conducting Perovskites: a Mechanistic Analysis for (La,Sr)MnO3 Based on DFT Calculations

Based on DFT calculations of intermediates and transition states, several hypothetical mechanisms for oxygen incorporation into mixed conducting La1-xSrxMnO3±δ perovskites are discussed. In the most probable mechanism, the rate-determining step comprises the encounter of a highly mobile surface oxygen vacancy and a molecular oxygen adsorbate. Starting from these results, the variation of reaction rates for different materials is explored.

Calculations of the atomic structure of the KNbO3 (110) surface

Abstract The O-terminated KNbO 3 (110) surface is modeled using a semi-empirical shell model and two different short-range interatomic potentials. We find this surface to be unstable with respect to a strong reconstruction and K-termination. This conclusion is confirmed by preliminary calculations using the ab initio linear combination of atomic orbitals (LCAO) formalism.

The calculation of the self-trapping energy in crystals with mixed valence band

The authors present a method of calculation of the localization energy of the hole in the crystal in which the density of states in the valence band has comparable contributions from both cation and anion states. The method is applied to the study of hole self-trapping in AgCl. The quantum-chemical simulations of the different hole structures revealed that the VK-type form of hole self-trapping in AgCl is unstable. The calculated self-trapping energy for the hole on one silver site is equal to -0.1 eV.