LOW-PRESSURE AND HIGH-PRESSURE AB-INITIO EQUATIONS OF STATE FOR THE ALKALI CHLORIDES

We have carried out ab initio perturbed ion calculations in the rocksalt (B1) and cesium chloride (B2) phases of the alkali (A) chloride (ACl) crystals. Zero temperature (T), and pressure (P) lattice energies and equilibrium distances are computed with errors less than 5%. From static calculations, zero-T equations of state (EOS) are reported in the ranges of 0-80 GPa for LiCl, 0-60 GPa for NaCl and KCl, 0-10 GPa for RbCl, and 0-5 GPa for CsCl. Since experimental data are a critical test of the performance of a theoretical methodology, we have placed particular emphasis on (a) the comparison between calculated and experimental trends and (b) the consistency with the behavior observed in real materials. We have found that our theoretically modeled solids obey the Vinet universal EOS and match the experimental behavior in temperature-scaled EOS diagrams. We have also analyzed the phase stability of the ACl crystals from a thermodynamic point of view. The hydrostatic pressure neccessary to produce the B1-B2 phase transition is calculated to decrease with the cation size, in agreement with the experimental observation. Our predicted value of the (not yet measured) B1-B2 transition pressure for LiCl is close to 80 GPa. Finally, our calculations based on the combined kinetic-thermodynamic model proposed by Li and Jeanloz for the NaCl transition phase [Phys. Rev. B 36, 474 (1987)] predict that the hysteresis pressure range of the B1-B2 transition decreases from LiCl to RbCl.