Stability of Alkali Metal Halide Polymorphs as a Function of Pressure

We investigated the regions of thermodynamic stability of possible modifications of the alkali metal halides as a function of pressure and type of alkali metal and halogen. Both Hartree-Fock and density functional calculations (for six different functionals) were performed. The results are in good agreement with experiment, and the trends in the computed quantities such as transition pressures and lattice parameters as a function of the ab initio method are similar to those found in earlier studies of the alkali metal sulfides. We predict that in most of these systems the so-called 5-5 modification should be metastable at standard pressure and be thermodynamically stable at slightly negative pressures. The sizes of the pressure ranges over which the various modifications are stable showed characteristic trends as a function of the type of the constituent elements, thus generalizing the traditional pressure-homologue rule for transition pressures and stable phases in ionic solids.