First-principles study of the rocksalt–cesium chloride relative phase stability in alkali halides

Abstract

We present a detailed investigation of observable properties associated with the relative stability of the rocksalt $(B1)$ and cesium chloride $(B2)$ phases in the $\mathrm{AX}$ $(A=\mathrm{Li},$ Na, K, Rb, Cs; $X=\mathrm{F},$ Cl, Br, I) crystal family. Thermodynamic $B\stackrel{\ensuremath{\rightarrow}}{1}B2$ transition pressures and $\ensuremath{\Delta}Y=Y(B2)\ensuremath{-}Y(B1)$ differences in total energies, volumes, and bulk moduli at zero and transition pressures are computed following a localized Hartree-Fock method. The arrangement of the data in clear trends is shown to be mainly dominated by the cation atomic number. This behavior is well interpreted in terms of a variety of microscopic arguments that emerge from (i) the evaluation of the energy Hessian at the $B1$ and $B2$ points and (ii) the decomposition of the energy and pressure in anionic and cationic classical and quantum-mechanical contributions.