HIGH-PRESSURE EQUATION OF STATE FOR PARTIALLY IONIC SOLIDS

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 deltaZ between the atoms, and a scaled universal energy function E*(a*), which accounts for the partially covalent character of the bond and for the repulsion between the atomic cores for small R; a* 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 (1.80 < deltaZ < 2. 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 (P(max) = 250-1350 kbar).