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 $\mathrm{Si}–\mathrm{O}$ and $\mathrm{Be}–\mathrm{F}$ stretching motion and for $\mathrm{O}\mathrm{···}\mathrm{O}\mathrm{,\hspace{0.17em}}\mathrm{Si}\mathrm{···}\mathrm{Si}\mathrm{,\hspace{0.17em}}\mathrm{F}\mathrm{···}\mathrm{F}\mathrm{,\hspace{0.17em}}\mathrm{and}\hspace{0.17em}\mathrm{Be}\mathrm{···}\mathrm{Be}$ nonbonded displacements were in good agreement with the respective mean amplitudes derived from the x‐ray data.