E. A. Kotomin

Semi-empirical simulations of the electron centers in MgO crystal

Abstract Semi-empirical quantum chemical simulations of 125-atom clusters have been undertaken to obtain the self-consistent atomic and electronic structure of the two basic electron defects in MgO crystals — F+ and F centers (one and two electrons trapped by an O vacancy). The calculated absorption and luminescence energies agree well with the experimental data, the excited states of both defects are found to be essentially delocalized over nearest-neighbour cations. The mechanism of the F+ → F photoconversion is discussed.

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.

Quantum chemical simulations of the optical properties and diffusion of electron centres in mgo crystals

Semiempirical quantum chemical simulations have been undertaken to obtain the self-consistent atomic and electronic structure of the two basic electron defects in MgO crystals: F+ and F centres (one and two electrons trapped by an 0 vacancy, V,). The calculated absorption and luminescence energies agree well with the experimental data; the excited states of both defects are found to be essentially delocalised over nearest-neighbour cations. The activation energy for diffusion is found to increase monotonically in a series V, --f F+ --f F centre (2.50 eV, 2.72 eV and 3.13 eV, respectively).

Optical properties of silver halide fibres: ageing effects

The relation between the structure of fibres extruded from mixed silver halides and their IR optical losses and luminescence properties is studied. The recently established ageing effect of the considerable growth of mean grain size of the fibres polycrystalline structure (typically from 20 to 1000 nm in two years) is correlated with IR and luminescence measurements. IR spectra measured in the interval show growth of the concentrations of molecular impurities. The luminescence measurements are used for the characterization of the fibres structure ordering. The luminescence decay kinetics is analysed theoretically in terms of the diffusion-controlled recombination of spatially well-correlated Frenkel defects. The considerable difference between the diffusion properties of monocrystals and fibres is established. This is caused by partial disorder (the grain structure effect) of the initially polycrystalline structure arising due to the fibre extrusion.

The kinetics of the bimolecular A+B→0 reaction in condensed matter: Effects of non‐equilibrium charge screening

The kinetics of the bimolecular A+B→0 reaction between charged reactants is studied in two dimensions, i.e., on a surface. The theory is based on the Kirkwood superposition approximation for three‐particle densities and the self‐consistent treatment of the electrostatic interactions defined by the non‐uniform spatial distribution of similar and dissimilar reactants. Special attention is paid to pattern formation and many‐particle effects arising from reaction‐induced formation of loose domains containing similar reactants only. It is shown that the critical exponent α characterizing the algebraic concentration decay law, n(t)∝t−α, differs strongly between symmetric (DA=DB) and asymmetric (DA=0) reactant mobilities. This effect is abnormal from the point of view of standard chemical kinetics. It arises directly from the specific spatial distribution in the system as in ‘‘raisins A in a dough B.’’ At long reaction times the asymptotics of the interaction potentials is of non‐equilibrium type at large relati...

A comparison of the effective medium and modified Smoluchowski equations for the reaction rate of the diffusion-controlled reactions

Abstract We demonstrate the equivalence of the effective-medium approach and the modified Smoluchowski equations for the reaction rate of the bimolecular A + B → B process. This fact encourages us to use the latter, flexible formalism for a study of complicated spatial distributions of traps.

Microscopic theory of colloid formation in solids under irradiation

Results of the first-principles study of diffusion-controlled aggregation of Frenkel defects-interstitial atoms-under irradiation of solids are presented. Conditions of the efficient radiation-induced aggregation of vacancies and interstitials are studied and the scenario of this process is presented.