Vibrational normal modes of SiO2. II. Cristobalite and tridymite

Abstract

The valence force field previously obtained on α and β polymorphs of quartz is applied to the other principal crystalline forms of SiO₂. A correlation is found between certain vibrations belonging to different symmetry species. The calculated frequencies of cristobalite fit the whole experimental spectra with as much accuracy as for α‐quartz, proving that the forces acting on the atoms in the two polymorphs are quite the same. A detailed study of β‐cristobalite showed a complete correlation between the α and β forms. Information about the phonon dispersion curves of this material is given and the existence of a soft mode near 230 cm⁻¹ is proved. However, the calculations made on tridymite, based on the only structural form known, the β variety, do not explain satisfactorily enough the Raman spectra of the α form.