VIBRATIONAL-SPECTRA IN FLUORIDE-CRYSTALS AND GLASSES AT NORMAL AND HIGH-PRESSURES BY COMPUTER-SIMULATION

Molecular dynamics computer simulation methods to obtain the infrared and Raman spectra of ionic liquids and glasses are described. These methods are applied to two problems: (1) the simulation and interpretation of the vibrational spectra of heavy metal fluoride glasses, and (2) the problem of reversible collapse of network glasses and the dramatic spectroscopic consequences of this collapse observed in recent experiments on SiO2-like glasses. As a contribution to this latter problem, the case of BeF2, for which our rigid ion potentials should be good approximations to the actual interactions, is studied. In the spectra, an initial network stiffening with increase of pressure, followed by rather sudden collapse to a spectrum characteristic of a higher coordinated state, is observed. This is correlated with a sudden increase in average coordination numbers. The changes are mostly reversible on decompression, so the behavior is reminiscent of the transition between 'vitreous polymorphs' observed in the case of vitreous water and vitreous silica.