Modeling the properties of silicates

Assuming a simple force field involving only short range non-Coulombic molecular energy terms along with P1 symmetry, a variation of the SQLOO model (Boisen and Gibbs, 1993) successfully generates the structure types of both {dollar}alpha{dollar} and {dollar}beta{dollar} quartz along with at least five alternative structure types of silica not yet observed to our knowledge. These structure types are identified by the existence of symmetry elements represented in the optimized atomic coordinates and cell parameters that define a minimizer in the model. A family of minimizers is discovered through the combined use of Monte Carlo simulated annealing followed by quasi-Newton minimization techniques. By varying the parameters used in the minimization process, different families of structure types are discovered. Several structure types were found to have high symmetries. The results reported here are for calculations involving Z = 3 and 6 formula units. This strategy may be useful in the prediction of possible high silica zeolite structure types.; An examination of the atomic displacement parameters (ADPs) obtained for TO{dollar}sb4{dollar} tetrahedra (T = Si, Al) suggest rigid TO bonds are more common in non-framework than in framework silicates. Correlated motion is found among the ADPs that is consistent with TLS rigid body motion. For these data, the translational motion is represented by the ADPs of the central T atom while both the librational and translational motion is contained in those of the surrounding O atoms. The libration angle for rigid tetrahedra is linearly dependent on the difference between the isotropic equivalent displacement parameter of the T and O atoms, B(T) and B(O), respectively. The value of B(O) is on average twice that of B(T) with a maximum value of {dollar}sim{dollar}2.0A{dollar}sp2.{dollar} Variations in the SiO bond lengths of rigid tetrahedra in the silica polymorphs is related only to {dollar}fsb{lcub}s{rcub}(O).{dollar} Rigid TO and OO bonds are a necessary but not sufficient condition for rigid body motion. Nonrigid tetrahedra may represent crystals containing disorder or problems with the refinement. (Abstract shortened by UMI.)...