X-ray, DTA and Raman studies of monoclinic tridymite and its higher temperature orthorhombic modification with varying temperature

Unit-cell parameters, Raman scattering profiles and DTA curves were measured on monoclinic tridymite from fired silica brick. Ten peaks appear in the Raman measurements from 100 to 1250 cm−1 at 25°C, and their positions and widths show no significant change up to 110°C. The monoclinic unit-cell parameters, a, b, c and β, slightly increase or remain nearly constant with increasing temperatures up to 110°C. Both the Raman spectra and the unit-cell parameters undergo significant changes in the range from 110 to 115°C. Only two endothermic peaks, at 117 and 166°C, appear in the DTA curve measured on a powdered sample of nearly pure monoclinic phase in a heating run from 25 to 300°C. In X-ray single-crystal refinements, the structural parameters including anisotropic temperature factors of atoms were fully determined for the monoclinic phase at 25, 80, 90 and 100°C, and the orthorhombic phase at 140°C. The difference between the temperature factors on both sides of the transition point is the most remarkable: 0.0286 Å2 in average for all O atoms at 100°C in the monoclinic phase and 0.0713 Å2 at 140°C, both for Beq. The averaged Si-O distances calculated for the atomic mean positions decrease with increasing temperatures: 1.599 Å at 25°C, 1.595 Å at 100°C and 1.576 Å at 140°C, whereas the values corrected for the thermal displacement parameters remain unchanged at 1.61 Å through all the temperatures studied.