Crystal structure of coesite, a high-pressure form of SiO/sub 2/, at 15 and 298 K from single-crystal neutron and x-ray diffraction data: test of bonding models

Journal Article · Thu Feb 12 00:00:00 EST 1987 · J. Phys. Chem.; (United States)

DOI:https://doi.org/10.1021/j100288a043· OSTI ID:6772613

Smyth, J R; Smith, J V; Artioli, G; Kvick, A

The crystal structure of a natural coesite from an eclogite rock fragment in the Roberts Victor kimberlite, South Africa, was determined at 15 K by neutron diffraction (a = 7.1357 (13) A, b = 12.3835 (26) A, c = 7.1859 (11) A, ..beta.. = 120.375 (16)/sup 0/, C2/c), and at approx.298 K by X-ray diffraction. Cell dimensions measured by neutron diffraction at 292 K (7.1464 (9), 12.3796 (19), and 7.1829 (8) A, 120.283 (9)/sup 0/) differed from those determined by X-ray diffraction, probably because of a systematic absorption error for the latter. The strongly anisotropic nature of the thermal expansion is explained qualitatively by the relatively large changes (approx.1%) in the distances between the nonbonded oxygen neighbors and the relatively small changes of Si-O-Si and O-Si-O angles in the compact three-dimensional framework. There is a good, but not perfect, negative correlation between the eight independent Si-O distances and the five independent values for sec theta(Si-O-Si) at 15 K. It is weaker than that for 298 K, and the scatter from a straight-line prediction from molecular-orbital models for small clusters (e.g., H/sub 6/Si/sub 2/O/sub 7/) implies that it is desirably to consider additional forces, including repulsive forces between nonbonded oxygen neighbors. The combined at a for Si-O and Si-O-Si in coesite, quartz, and cristobalite at 10-15 K show less scatter than those for approx.298 K, in accordance with the greater thermal response of framework geometry in the more open structures.

• Research Organization: Univ. of Colorado, Boulder
• DOE Contract Number: AC02-76CH00016
• OSTI ID: 6772613
• Journal Information: J. Phys. Chem.; (United States), Vol. 91:4
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
SILICA
CRYSTAL STRUCTURE
BOND ANGLE
BOND LENGTHS
EXPERIMENTAL DATA
LATTICE PARAMETERS
MEDIUM TEMPERATURE
NEUTRON DIFFRACTION
VERY LOW TEMPERATURE
X-RAY DIFFRACTION
CHALCOGENIDES
COHERENT SCATTERING
DATA
DIFFRACTION
DIMENSIONS
INFORMATION
LENGTH
MINERALS
NUMERICAL DATA
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
SCATTERING
SILICON COMPOUNDS
SILICON OXIDES
400201* - Chemical & Physicochemical Properties