Crystal structure of low cristobalite at 10, 293, and 473 K: Variation of framework geometry with temperature

Abstract

The crystal structure of low cristobalite (SiO2) was refined at 10, 293, and 473 K from time-of-flight neutron powder diffraction data. One of the Si-O distances and one Si-O-Si angle change considerably: 10 K, 1.602(1) and 1.617(1) Å, 144.7(1)°; 473 K, 1.605(2) and 1.590(2) Å, 148.4(1)°. The angular distortion of the SiO4 tetrahedron is greater at 473 K than at the lower temperatures. Root-mean-square displacements of Si and O are greater for cristobalite than for quartz and coesite at room temperature. The negative correlation between Si-O distance and Si-O-Si angle for cristobalite is similar to that for quartz held at a higher temperature (∼350 K). Any adjustment of the Si-O distance for riding motion between Si and O atoms would reduce the variation of Si-O distance with temperature and with Si-O-Si angle. Furthermore, adjustment of the Si-O-Si angle for rocking of the oxygen atom out of the Si-O-Si plane could improve the correlation between secant (Si-O-Si) and the isotropic chemical shift for 29Si nuclear magnetic resonance in silica polymorphs. The temperature variation of the crystal structure of low cristobalite cannot be modeled by simple tilting of rigid tetrahedra.