LATTICE MECHANICAL STUDY OF THE STRUCTURE OF DODECASIL-3C

A recently developed1 partial charge potential model for SiO2 polymorphs, derived from quantum chemical calculations, is applied to the calculation of the lattice energy minimized structure and physical properties of the low-density SiO2 crystal Dodecasil-3C. Calculations are performed using Ewald summations and without symmetry constraints. Results are compared with calculations using the shell model of Sanders et at.2,3 Best agreement between experimental and theoretically predicted elastic constants is achieved assuming a triclinic structure. In order to establish lattice stability, the vibrational frequency spectrum is calculated. For structures with imaginary frequencies, the corresponding atomic displacements are used to deform the quasi-stationary geometry. Upon lattice energy minimization, the deformed structure transforms to a stable energy minimum. Computation of thermal ellipsoids indicates that the large anisotropic temperature factors observed experimentally are mainly due to static disorder in atomic positions.