Pressure-induced structural instability of cesium halides from ab initio pseudopotential techniques

The structural stability of the cubic phase of CsI versus tetragonal distortions is studied from first principles using state-of-the-art local-density techniques, namely, norm-conserving pseudopotentials and large plane-wave basis sets. The effects of the polarization of the cation are explicitly accounted for using a pseudopotential that sustains the Cs 5s and 5p bands. We find that, in agreement with recent x-ray diffraction experiments and with previous theoretical work, the tetragonal phase is more stable at volumes smaller than 0.54 of the zero-pressure value. The mechanism of transition is revealed in terms of the balance between the Madelung and repulsive interionic interactions. We find that at volumes smaller than the transition volume the cubic phase is metastable, thus indicating that the transition is first order. The electron-charge-density rearrangements following compression and distortion are also examined. No evidence of a further transition to an even lower-symmetry structure has been found in a preliminary search.