Phonon softening and high-pressure low-symmetry phases of cesium iodide

The relative stability of various high-pressure phases of CsI is studied from first principles and analyzed using the Landau theory of phase transitions. We demonstrate that the cubic-to-orthorhombic transition recently observed to occur slightly below 20 GPa is driven by the softening of an acoustic phonon at the M point of the Brillouin zone. The coupling between this mode and anisotropic strain makes the transition slightly first order (with a volume variation of the order of 0.1%), and stabilizes the experimentally observed orthorhombic phase with respect to other competing symmetry-allowed structures.