Analysis of the incommensurately modulated OS phase of SiO2 tridymite

The structure, symmetry and origin of the incommensurately modulated OS phase of tridymite (SiO₂) and its lock-in to the OP three-fold superlattice structure are discussed in a computational study. The structure of the OS phase (which has not been determined experimentally) is deduced as the only geometrically possible structure derived from the parent OC phase by rotation and translation of the SiO₄ tetrahedra without significant distortion of these units. It can be visualised conveniently in terms of the McConnell formalism of two component difference structures C₁ and C₂ whose space group symmetries are derived. The results are in accordance with the known lock-in structure at wave vector Q= a ^*. In the latter, the ±C₁ regions expand and the structure can square up in a very general way to take advantage of the lost symmetries (lost compared with the OC phase). Received: 4 August 1997 / Revised, accepted 24 November 1997