Dynamics of β-Quartz Structures of Vitreous SiO2 and BeF2

The optically active vibrational modes of vitreous SiO2 and BeF2 were calculated using the β-quartz models determined for these materials from recent x-ray diffraction experiments. The assumption of regular β-quartz lattice models with the appropriate unit cell parameters resulted in good agreement between observed and calculated frequencies. The crystal potential functions of SiO2 and BeF2 were derived from a modified valence force field, and several force fields which differed in the type of interaction terms allowed were studied. Calculated mean amplitudes for Si–O and Be–F stretching motion and for O ··· O, Si ··· Si, F ··· F, and Be ··· Be nonbonded displacements were in good agreement with the respective mean amplitudes derived from the x-ray data.