Phase transitions in alkali halide crystals

The paper is devoted to the study of the characteristics of structural phase transitions occurring in ionic crystals under an extremely high pressure. The calculations are made for crystals of infinite size at absolute zero temperature. The pressure of $B1$–$B2$ polymorphic transformation is calculated for a number of alkali halide crystals using pair interaction potentials obtained self-consistently within the framework of inhomogeneous electron gas theory. In so doing, the effects of seven coordination spheres are taken into account in the thermodynamic potential. The changes of the cohesion characteristics of crystals (cohesion energy, lattice constant) are calculated for the transition from the $B1$ structure ($\mathrm{NaCl}$ type) to the $B2$ structure ($\mathrm{CsCl}$ type). Based on the obtained results, conclusions are made on the stability of one or another crystal structure in the given range of external pressure. For crystals in the $B1$ phase, the moduli of elasticity and relative changes of the volume at the phase transition pressure are calculated. It is shown that the inclusion of the effect of higher coordination spheres affects considerably the values of the elastic characteristics of ionic crystals.