Universal-binding-energy relations across the rock-salt–cesium chloride phase transition in alkali halides

The fulfillment of universal-binding-energy relations across the rock-salt $(B1)$--cesium chloride $(B2)$ phase transition in alkali halides is analyzed from a first principles point of view. We show that extensive ab initio quantum-mechanical calculations fully support the existence of universality in both intraphase intersystem and intrasystem interphase phenomena. For the latter problems, it is found that the fundamental requirement for a universal law to simultaneously describe both phases and the unstable intermediate steps along the transition path is the topological equivalence of the Gibbs energy profile at the $B1$ and $B2$ points. Several simple relations between thermodynamic quantities of both phases are put forward and discussed in reference to our theoretical data and their interest in experimental research.