Pressure-induced structural and chemical changes of solid HBr

The structural, dynamical, and electronic properties of solid HBr at high p ressure are investigated using the ab initio constant pressure molecular dy namics method. A detailed analysis of the orientational distribution, and t he reorientational and vibrational dynamics of the disordered phase I at am bient temperature showed that this phase can be described as a rotator phas e with fluctuating hydrogen bonds up to pressures well over 10 GPa. We pred ict that the disorder at higher densities leads to cooperative proton-trans fer dynamics. The approach to hydrogen-bond symmetrization is studied in ph ase I and the high pressure ordered phase III. The simulation results for p hase III also indicate that this phase develops dielectric instabilities at high density. At pressures over 40 GPa we observe spontaneous formation of H-2 with rearrangement of the Br lattice from fcc to hcp. The chemical rea ctivity is rationalized in terms of the electronic structure under conditio ns of near symmetrical hydrogen bonding. (C) 1999 American Institute of Phy sics. [S0021-9606(99)70427-7].