Theoretical aspects of solid hydrogen halides under pressure

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Abstract
The electronic and dynamic properties of the solid phases of HF, HCl, and HBr under pressure are studied theoretically. A simple model is constructed so that the pressure-dependent properties of these systems and possibly other hydrogen-bonded systems can be studied in terms of a few parameters. The model predicts quite simply the pressure dependence of the stretching-mode frequency and the nature of the phase transition from the molecular hydrogen-bonded phase to a new symmetrical hydrogen-bonded (nonmolecular) phase. Quantum effects due to the light hydrogen atom are taken into account within a many-body Hartree approximation. New experimental data on the pressure dependence of the symmetric-stretching-mode frequency in HF is presented. The possibility of soliton formation is discussed and it is shown how pressure may act as a unique tuner to adjust the energetics of these nonlinear excitations. In addition, we report the results of our ab initio calculations of the total energy of ringlike structures of HF and the first ab initio pseudopotential calculation of the band structure and total energy of solid HBr. The calculations for HF are within the Hartree-Fock approximation, while those of solid HBr are within the local-density approximation and have been simplified by considering a linear instead of a zigzag geometry. The use of the local-density approximation for hydrogen is also discussed.

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