X-ray studies of orthorhombic-I tridymite, especially focussed on atomic mean-square displacements (m.s.d.'s) at 443, 493, 573, 653 and 693 K respectively, were carried out on the basis of the usual second-order refinement of the Dollase model.
The m.s.d.'s for O are highly anisotropic, and large in the planes perpendicular to Si – Si axes. Plots of temperature vs. m.s.d.'s point to a disorder of O atoms in the orthorhombic-I form. The transition, from the orthorhombic-I to the hexagonal high form or vice versa, is an example of a displacive structure transition from a disordered structure to another disordered one. When temperature falls from 693 K, any two SiO4 tetrahedra joined by O on the two-fold axes parallel to a move away from the hexagonal positions, rotating progressively around the two-fold axes, and then symmetry is lowered to orthorhombic. The pair-wise rotations are accompanied by anisotropic changes of the thermal ellipsoids for two kinds of O atoms forming the basal planes of silica tetrahedra, nearly normal to c; the thermal ellipsoids are distorted to elliptical shapes in the sections perpendicular to Si – Si axes when temperature falls. On the other hand, the thermal ellipsoid for the apical O atom is approximately circular in that section.