Abstract
Thermal vibrational parameters of lower temperature phases of tridymite and melanophlogite, and their related quantities were used to characterize their higher temperature phases and phase transitions. The orthorhombic tridymite appearing right above the monoclinic room temperature phase and β-melanophlogite are all subject to atom disorder. Raman spectroscopic data of the orthorhombic tridymite and their simulations in lattice dynamical calculations suggested that the motions causing the disorder are triggered by a low frequency phonon mode via coupling between themselves having the same symmetry. Higher-order thermal motion tensor analysis of β-melanophlogite indicated that the probability density functions of O atoms are strongly distorted from ellipsoidal ones, directly proving atom disorder, which were further analyzed in terms of 12 α-domains.
