  The relative stability of different high-pressure phases of various Cesium
Halides is studied from first principles and analyzed using the Landau theory
of phase transitions. We present results for CsI, CsBr, and CsCl up to
pressures of about 100 GPa. A cubic-to-orthorhombic transition, driven by the
softening of an acoustic phonon at the M point of the Brillouin zone, is
competing with the cubic-to-tetragonal martensitic transition typical of these
compounds. The phonon softening takes place only in CsI and CsBr at a residual
volume of 0.64, 0.52 respectively. A cubic-to-tetragonal instability is found
instead to occur at 0.54 for all the compounds considered here. The
orthorhombic phase is stable only in CsI, whereas it is taken over by the
tetragonal one in the case of CsBr. Our analysis reveals the essential role
played by the phonon-strain coupling in stabilizing the orthorhombic phase and
in making the corresponding transition first-order.
