The lattice dynamics of zinc-blende (ZB) CuBr and CuI is studied as a function of pressure using density functional linear-response theory. A pressure-induced soft transverse acoustic (TA) phonon mode is identified for both compounds. Each compound shows a different pressure-induced phonon softening behavior. A TA phonon branch softens along the [ξξ 0] direction in CuBr, resulting in a phase transition from a ZB phase (CuBr-III) to a tetragonal phase (CuBr-IV). A softening TA phonon mode at the zone boundary $L$ point in CuI may induce a phase transition from a ZB phase (CuI-III) to a rhombohedral phase (CuI-IV), although the phonon softening towards zero frequency in CuI is superseded by a first order phase transition occurring at 1.74 GPa. Possible phase transition paths are presented for both compounds.

• Received 12 September 2003

DOI:https://doi.org/10.1103/PhysRevB.69.064102