High-temperature single-crystal study of the cristobalite inversion *

Abstract

Abstract Single crystals of low cristobalite from Ellora Caves, India, were studied using single-crystal x-ray diffraction methods as a function of temperature through the inversion to high cristobalite. The equilibrium inversion temperature in these crystals was approximately 220°C. The inversion was sluggish with transformations occurring up to 25°C from the equilibrium temperature, and at varying temperatures for different crystal subvolumes, apparently as a function of variable impurity content. The crystal structure of low cristobalite was refined for data obtained at seven temperatures between 28 and 230°C, and that of high cristobalite at 221, 248, 273 and 310°C. The oxygen atom of high cristobalite was found to be on a circle of radius 0.45 Å, oriented normal to the Si–Si separation. There is one-sixth occupancy of six different equally spaced positions on the circle. Interatomic distances and angles are nearly equivalent to those of low cristobalite. As temperature is increased, both oxygen and silicon atoms of low cristobalite shift continuously and regularly toward the positions finally reached through the inversion to high cristobalite.