Crystal structure and thermal expansion of α‐quartz SiO2 at low temperatures

The crystal structure of α‐SiO2 (low quartz) has been refined at 296, 78, and 13 K from time‐of‐flight neutron powder diffraction data. The major effect of temperature from 296 to 78 K is a nearly rigid body rotation of the SiO4 tetrahedra. Below 78 K, tetrahedral rotation is substantially reduced giving rise to a much smaller thermal expansion. A comparison of the volume dependence of the Si‐O‐Si angle, the rotation angle for SiO4 tetrahedra and the c/a ratio suggests that the mechanism for expansion may be changing from tetrahedral rotation to tetrahedral distortion at the lowest temperatures. The inverse linear relation between the mean Si‐O bond distance and −sec(Si‐O‐Si) which has been observed for silica minerals at both ambient and high‐temperature conditions appears to be consistent with the structural variations that occur at low temperatures.