Theory of the lattice energy, equilibrium structure, elastic constants, and pressure-induced phase transitions in alkali-halide crystals

The interionic forces in alkali-halide crystals are calculated theoretically, using a modified electron-gas treatment including corrections to the kinetic, exchange, and correlation energy contributions. These results are used to predict the equilibrium bond distances and lattice energies, the pressure-volume phase diagrams, and the elastic constants of the lithium, sodium, potassium, and rubidium fluorides, chlorides, bromides, and iodides. Both the $B1\mathrm{}$ (rock salt) and $B2\mathrm{}$ (cesium chloride) lattice types are considered, and the pressure-induced phase transition between them is predicted. First- and second-nearest-neighbor short-range interactions between the ions have been included for the $B1\mathrm{}$ phase, while third-nearest-neighbor interactions were also needed for the $B2\mathrm{}$ phase. The average magnitude of the deviation between the predicted and observed bond distances is 2%, lattice energy 2%, and elastic constants 10%. The pressure variation of the elastic constants for NaCl has also been predicted, up to the onset of a shear instability.