Pressure-induced phase transitions in silver halides

A first-principles study of the relative stability of a few different structures, viz. AgCl, AgBr, and AgI, is undertaken. Modern pseudopotentials, the local-density approximation, and a plane-wave basis are used. The relative stability of the cubic zinc blende, rocksalt, and CsCl structures, as well as the wurtzite, beta-Sn, NiAs, and cinnabar structures is investigated. Ground-state structural parameters are computed and compared with experiment. The optimization of the cinnabar structure leads to a simple rhombohedral phase, which is found to be the most stable among those considered in an intermediate range of pressures for AgCl and AgBr. According to these results, AgCl and AgBr prefer a rhombohedral route from the rocksalt structure to the CsCl structure instead of the usual discontinuous transition which is common in ionic compounds.