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 pseudopote ntials, the local-density approximation, and a plane-wave basis are us ed. The relative stability of the cubic zinc blende, rocksalt, and CsC l structures, as well as the wurtzite, beta-Sn, NiAs, and cinnabar str uctures is investigated. Ground-state structural parameters are comput ed and compared with experiment. The optimization of the cinnabar stru cture leads to a simple rhombohedral phase, which is found to be the m ost stable among those considered in an intermediate range of pressure s 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 ioni c compounds.