Raman and Infrared Study of Phase Transitions in Solid HBr under Pressure

Phase transition in solid HBr has been investigated by Raman and infrared spectroscopy up to 50 GPa at 298 K. The liquid transforms to a cubic structure (Fm3m) with a bromine fcc lattice at 0.5 GPa and further to an orthorhombic structure (Cmc2_I) consisting of zigzag chains of hydrogen-bonded molecules at 13 �} 0.5 GPa. At about 39 �} 2 GPa, Raman peaks related to the molecular stretch and rotation disappear and two lattice peaks remain. The Cmc2_I orthorhombic structure transforms into a Cmcm orthorhombic structure with symmetrized hydrogen bonds. The symmetrized phase is instable; HBr molecules begin to dissociate after the bond symmetrization and Br₂ molecules are formed.