First-principles equation-of-state calculations for alkali halides

Results of first-principles (parameter-free) equation-of-state calculations are reported for sixteen alkali halides. The theory adopts the methods of Gordon and Kim for computing pair potentials for closed-shell systems. Specifically, the electronic ground-state energy is assumed to be that of an electron gas with the total charge density obtained by rigidly overlapping the free-ion charge densities. A number of properties are treated within this framework: lattice dynamics, elastic behavior, structure determination, thermal expansion, compressibility, and the overall stability of the lattice as a function of temperature and pressure. At high temperatures a lattice instability develops in which above a certain critical temperature ($T_c$) the vibrational and static pressures cannot balance each other at any volume. It is argued that this instability plays a role in causing solids to melt. An attempt is made to ascribe trends in the discrepancies between theory and experiment to particular approximations of the theory. From this analysis it appears that (a) the pair-potential approximation is in greatest need of improvement, (b) anharmonic corrections tend to raise $T_c$ by about 20%, and (c) lattice imperfections are not an important factor for temperatures up to about 90% of the melting temperature.