High-temperature phase transitions in Steinbach tridymite

Abstract

Steinbach tridyrnite has been studied by high-resolution X-ray powder diffraction, differential scanning calorimetry and hard-mode infrared spectroscopy. Two phase transitions have been analysed in detail at 350 and 465 °C. The higher-temperature transition is second-order in character and the coefficients in a Landau potential have been determined as: G = 1.2(T-738)Q 2 + 443Q 4 Jmol −1 . This phase change appears to be driven by a volume decrease and the excess free energy is relatively small. The lower-temperature transition also appears to be continuous and could be close to second-order in character, depending on the precise change in symmetry involved. Although Landau coefficients can not be determined precisely, the excess free energy is again small. Two mechanisms are presented to account for the thermal behaviour. Firstly, a dynamic model involving the «flipping» of apical oxygen atoms is described. Secondly, a purely displacive model involving shifts in atomic positions and the appearance of an intermediate phase is considered