High-pressure equation of state for partially ionic solids.

Abstract

Recently, we showed that the cohesive energy of partially ionic solids may be characterized by a two-term energy relationship consisting of a Coulomb term arising from the valence-charge transfer \ensuremath{\delta}Z between the atoms, and a scaled universal energy function ${\mathit{E}}^{\mathrm{*}}$(${\mathit{a}}^{\mathrm{*}}$), which accounts for the partially covalent character of the bond and for the repulsion between the atomic cores for small R; ${\mathit{a}}^{\mathrm{*}}$ is a scaled length. Normalized cohesive-energy curves of alkali halide crystals and of Tl and Ag halide crystals were obtained, and the cohesive-energy-curve parameters were used to generate theoretical equation-of-state (EOS) curves for the Li, Na, K, Cs, and Ag halides. Good agreement was obtained with the experimental isothermal compression curves over a wide pressure range (0--90 kbar). In this paper we verify that the cohesive-energy relationship is valid for divalent partially ionic solids; physically reasonable charge-transfer values (1.80Z2.0) are obtained for MgO, CaO, and CaS. Next, EOS curves for LiF, NaF, NaI, CsCl, CsI, MgO, CaO, and CaS are generated in terms of the cohesive-energy parameters. These EOS's yield excellent fits to experimental isothermal-compression data and to shock-wave data to very high pressures (${\mathit{P}}_{\mathrm{max}}$=250--1350 kbar).