Hydrogen Interaction with Dislocations in Si

An H plasma has a remarkable effect on dislocation mobility in silicon, reducing its activation energy to 1.2 eV. Applying density functional theory to the interactions of H and H? With the core of the 90 degrees partial dislocation in Si, we have identified a path for motion involving kink formation and migration at hydrogenated core bonds which conforms exactly to the experimentally measured activation energ

Evaluation of the many-body contributions to the interionic interactions in MgO

The predictions of an "extended" ionic interaction model for the phonons and other properties of crystalline MgO are considered. In the extended model, the polarization interactions resulting from the dipoles and quadrupoles induced on the oxide ions by both Coulombic and short-range interactions with other ions are explicitly represented. Furthermore, the shapes of the oxide ions, as perceived through the short-range repulsive interactions with their immediate neighbors, are allowed to respond to their instantaneous coordination environment. Shape deformations of spherical (breathing), dipolar and quadrupolar symmetry are allowed. Each of these many-body effects is independently represented in the simulation model, so that the effect of each term on observed properties can be separately evaluated. It is shown how the polarization and breathing effects may be directly parametrized from ab initio electronic structure calculations. By consideration of the symmetry of the individual phonons at high symmetry points, it can be shown that the remaining effects may contribute only to particular phonons. Consequently, they may be assigned values in an unambiguous manner. The importance of each many-body effect for the different phonon branches is clearly delineated. © 1998 American Institute of Physics

Dipole and quadrupole polarization in ionic systems: Ab initio studies

The results of electronic structure calculations of the induced dipoles and quadrupoles on an anion in a locally distorted rocksalt crystal are described. Such information is of interest in the construction of ionic interaction potentials and for modeling the dielectric behavior of ionic materials. The systems included in the study are LiF, NaF, KF, LiCl, NaCl, KCl, LiBr, MgO and CaO. The results are used to characterize short-range contributions to the induced multipoles -those not included in a point-charge, point-polarizable ionic model (the "asymptotic" model). It is shown that these short-range effects are large, opposing and sometimes reversing the asymptotic contribution. The representation of the short-range effects in a computationally tractable form, suitable for use in computer simulations, is examined. A pairwise additive form, containing a steplike function of the interionic separation, is found to provide a good representation of the ab initio data for both dipoles and quadrupoles. Furthermore, the parameters involved in the fitted functions transfer from one material to another in a physically transparent and predictable way. © 1999 American Institute of Physics

Environmental effects on anion polarizability: Variation with lattice parameter and coordination number

The results of ab initio calculations of in-crystal ionic polarizabilities, α, over a wide range of lattice parameters, R, are presented for LiF, NaF, KF, LiCl, NaCl, KCl, LiBr, NaBr, KCl, and MgO. The derivatives of the mean polarizability with respect to lattice parameters are compared with experimental values obtained from the variation of the refractive index with pressure. The environmental effects on the polarizability of an anion may be viewed as the consequence of imposition of a confining potential on its electron density whose origin includes Coulombic interactions and the exclusion of these electrons from the region occupied by the electron density of the first-neighbor shell of cations. This model suggests scaling relationships, between values of α(R) obtained at different levels of calculation and for a given anion with different cations, which are shown to be semiquantitative. These findings lead to the proposal of a universal representation of the polarizability of a given anion, which predicts the dependence on lattice parameter and crystal form and transfers from one substance to another

Models of environmental effects on anion polarizability

This paper deals with three different approaches to the representation of environmental effects on anion polarizability in cubic crystals of the stoichiometry MX, where M is an alkali metal and X is a halogen. Ab initio embedded cluster calculations of the variation in anion polarizability with pressure in a fixed crystal type are presented and compared with experiment. The results are then used in a scaled ab initio model used to predict further values for the pressure dependence of the in-crystal anion polarizability. This scaled model is compared with a fully empirical 'universal' model due to Batana et al. based on polarizability change with ionic radius [1997, Molec. Phys., 92, 1029]. The assumptions of the two models differ substantially and the central purpose of this paper is to contrast these differences and highlight their consequences for prediction. Although the empirical model typically overestimates the experimental pressure derivatives, and the ab initio calculations somewhat underestimate them, it is shown that the assumption of incompressible cations in the scaled ab initio-derived model has a firmer physical basis than the empirical picture in which all ions are compressible. © 1999 Taylor and Francis Ltd