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 rocksa lt (B1) and cesium chloride (B2) phases of the alkali (A) chloride (AC l) crystals. Zero temperature (T), and pressure (P) lattice energies a nd equilibrium distances are computed with errors less than 5%. From s tatic calculations, zero-T equations of state (EOS) are reported in th e 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 te st of the performance of a theoretical methodology, we have placed par ticular emphasis on (a) the comparison between calculated and experime ntal trends and (b) the consistency with the behavior observed in real materials. We have found that our theoretically modeled solids obey t he Vinet universal EOS and match the experimental behavior in temperat ure-scaled EOS diagrams. We have also analyzed the phase stability of the ACl crystals from a thermodynamic point of view. The hydrostatic p ressure 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 trans ition pressure for LiCl is close to 80 GPa. Finally, our calculations based on the combined kinetic-thermodynamic model proposed by Li and J eanloz for the NaCl transition phase [Phys. Rev. B 36, 474 (1987)] pre dict that the hysteresis pressure range of the B1-B2 transition decrea ses from LiCl to RbCl.