PRESSURE-INDUCED B1-B2 PHASE-TRANSITION IN ALKALI-HALIDES - GENERAL-ASPECTS FROM FIRST-PRINCIPLES CALCULATIONS

A first principles, general study of the thermodynamic and kinetic aspects of the B1-B2 phase transition in alkali halides is presented. Particular attention is paid to (a) how to construct models of increasing complexity to be used with generic quantum-chemistry techniques and (b) the topological and symmetry-dependent features of the energetic and the Gibbs potential surfaces analyzed. Our results indicate that the transition may be thought of as involving the simultaneous opening of the rhombohedral angle in the primitive B1 crystallographic cell while a contraction of the lattice parameter takes place. Transition paths depend strongly on pressure and show large and asymmetric Gibbs barriers that qualitatively account for many of the empirical facts around the phenomenon of hysteresis.