PHASE-TRANSITIONS IN CRISTOBALITE AND RELATED STRUCTURES STUDIED BY VARIABLE-TEMPERATURE INFRARED-EMISSION SPECTROSCOPY

The results of a temperature-dependent, infra-red spectroscopic study of the C9, cristobalite-related phases SiO2-Cristobalite, AlPO4-cristobalite, NaAlSiO4-carnegicite and NaAlO2 are presented. The effect of increasing Al substitution for Si and of Na stuffing cations are thereby investigated. Comparison of the room temperature diffuse reflectance spectra (2000-400 cm-1) shows a decrease in mean frequency in all but one of the bands with increasing Al substitution and is consistent with the increased bond length and ionic character of the Al-0 bond relative to the Si-O bond. The high temperature phase transitions in each of these materials are investigated via i.r. emission spectroscopy. Dramatic changes are observed in the 550-850 cm-1 region across the phase transition in each case. For SiO2- and AlPO4-cristobalite these changes are interpreted in terms of a Rigid Unit Mode model.