| Autor: |
Bartelmehs, K. L., Downs, R. T., Gibbs, G. V., Boisen, M. B., Birch, J. B. |
| Zdroj: |
American Mineralogist (De Gruyter); July 1995, Vol. 80 Issue: 7-8 p680-690, 11p |
| Abstrakt: |
The atomic displacement parameters (ADPs) obtained from published refinements of silicate crystals measured at or below room temperature are examined to determine if TO4(T = Al,Si) tetrahedra display rigid-body thermal motion. In many cases, the correlation found among the ADPs is consistent with the TLS model of rigid-body motion. For these data, the translational motion is described by the ADPs of the central T atom, whereas both librational and translational motion are displayed in those of the surrounding 0 atoms. The libration angle for rigid tetrahedra is quadratically dependent on the difference between the isotropic equivalent displacement parameter of the T and 0 atoms, Beq(T) and Beq(O), respectively. The value of Beq(O) is on average twice that of Beq(T), with an observed maximum value of ~ 2.0 Å2. Variation in the observed Si-O bond lengths of rigid tetrahedra in the silica polymorphs is related only to the fractional s-character of O, fS(O). ADPs that do not indicate rigid-body motion for a given tetrahedron may be used to identify crystals containing disorder or suggest problems with the refinement. |
| Databáze: |
Supplemental Index |
| Externí odkaz: |
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