A crystal chemical study of stishovite
- CSIRO, Part Melbourne, Victoria, Australia
The crystal structure of stishovite, SiO/sub 2/, a=4.1773(1), c=2.6655(1) angstrom, space groups P4/sub 2//mnm, with Z=2, has been refined with 217 graphite-monochromatized MoK..cap alpha.. data (2THETA max=120/sup 0/) to R=0.012 (R/sub 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 denstity in and around the shared octahedron edges with the shorter equatorial bonds showing higher peaks (0.47 e angstrom/sup -3/) than the longer axial bonds (0.29 e angstrom/sup -3/). Net atomic charges for Si and O determined by a population and KAPPA 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 angstrom) calculated for a neutral molecule with 6-coordinated Si is slightly shorter than that observed on the average (1.76 angstrom) for silicates and molecular crystals with 6-coodinated Si. In addition, the calculated geometrical parameters of two edge-sharing octahedra agree to within approx.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 ..cap alpha..-quartz, Q(Si)= +1.0e.Observed deformation maps are compared with theoretical ones calculated in the equatorial plan of the 6-coodinated molecule using molecular orbital theory with a split valence s, p basis supplemented with d-type polarization functions on Si.
- DOE Contract Number:
- AC02-76CH00016
- OSTI ID:
- 5609470
- Journal Information:
- J. Solid State Chem.; (United States), Vol. 47:2
- Country of Publication:
- United States
- Language:
- English
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