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

Abstract

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 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 , 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 A in average for all O atoms at 100°C in the monoclinic phase and 0.0713 A at 140°C, both for Beq. The averaged Si-O distances calculated for the atomic mean positions decrease with increasing temperatures: 1.599 A at 25°C, 1.595 A at 100°C and 1.576 A at 140°C, whereas the values corrected for the thermal displacement parameters remain unchanged at 1.61 A through all the temperatures studied.