Raman and infrared study of phase transitions in solid HBr under pressure

Phase transition in solid HBr has been investigated by Raman and infrared s pectroscopy up to 50 GPa at 298 K. The liquid transforms to a cubic structu re (Fm3m) with a bromine fee lattice at 0.5 GPa and further to an orthorhom bic structure (Cmc2(1)) consisting of zigzag chains of hydrogen-bonded mole cules at 13+/-0.5 GPa. At about 39+/-2 GPa, Raman peaks related to the mole cular stretch and rotation disappear and two lattice peaks remain. The Cmc2 (1) orthorhombic structure transforms into a Cmcm orthorhombic structure wi th symmetrized hydrogen bonds. The symmetrized phase is instable; HBr molec ules begin to dissociate after the bond symmetrization and Br-2 molecules a re formed. [S0163-1829(99)08917-1].