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.

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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.