A crystal chemical study of stishovite

The crystal structure of stishovite, SiO₂, a = 4.1773(1), c = 2.6655(1)Å, space group $\text{P4}{}_2\text{mnm}$, with Z = 2, has been refined with 217 graphite-monochromatized MoKα data (2θ max = 120°) to R = 0.012 (R_w = 0.014). Electron deformation density maps show a modest accumulation of charge density ascribed to partial covalent bonding in both the equatorial and axial bonds together with a delocalization of density in and around the shared octahedron edges with the shorter equatorial bonds showing higher peaks (0.47 eÅ^?3) than the longer axial bonds (0.29 eÅ^?3). Net atomic charges for Si and O determined by a population and κ refinement by varying the occupancy and shape of their valence shells are +1.71 and ?0.86(15) e, respectively. The observed structural distortions and charge distributions conform with the results of ab initio molecular orbital calculations undertaken on molecules designed to mimic the local geometry of the structure of stishovite. The SiO bond length (1.73 Å) calculated for a neutral molecule with 6-coordinated Si is slightly shorter than that observed on the average (1.76 Å) for silicates and molecular crystals with 6-coordinated Si. In addition, the calculated geometrical parameters of two edge-sharing octahedra agree to within ～5% of the observed geometry of stishovite. The calculated charge on Si is indicated to increase with coordination number and bond length in agreement with the larger net charge recorded for stishovite as compared with that recorded for α-quartz, Q(Si) = +1.0 e. Observed deformation maps are compared with theoretical ones calculated in the equatorial plane of the 6-coordinated molecule using molecular orbital theory with a split valence s,p basis supplemented with d-type polarization functions on Si.