Structural phase transitions of tridymite: A ²⁹Si MAS NMR investigation

Changes in ²⁹Si MAS NMR spectra of synthetic tridymite from room temperature to 504 °C are consistent with the structural phase transitions previously observed by X-ray diffraction and by optical and calorimetric methods. The results also provide evidence for a disordered or incommensurate structure for the OP and OS phases (stable from about 110 to 200 °C) and for dynamical disordering of the O atoms at greater than kilohertz frequencies for at least the higher temperature OC and HP phases. The time-averaged Si-O-Si bond angles of the OC and HP phases inferred from the ²⁹Si chemical shifts are identical (154.1 ± 1°) and similar to that of β-cristobalite (153.3 ± 1°, Phillips et al., in preparation). The Si-O-Si bond angles for the OS, OC, and HP phases inferred from the NMR data are significantly less than those obtained from the mean atom positions determined by diffraction methods.