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Publication Type
Journal Article
UWI Author(s)
Author, Analytic
Antao, Sytle M.; Hassan, Ishmael; Mulder, Willem H.; Lee, Peter L.; Toby, Brian H.
Author Affiliation, Ana.
n/a
Article Title
In situ study of the R3c ›R3m orientational disorder in calcite.
Medium Designator
n/a
Connective Phrase
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Journal Title
Physics and Chemistry of Minerals
Translated Title
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Reprint Status
Refereed
Date of Publication
2009
Volume ID
36
Issue ID
3
Page(s)
159-169
Language
n/a
Connective Phrase
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Location/URL
http:; link.springer.com/content/pdf/10.1007%2Fs00269-008-0266-y.pdf
ISSN
n/a
Notes
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Abstract
The temperature dependences of the crystal structure and intensities of the (113) and (211) reflections in calcite, CaCO3, were studied using Rietveld structure refinements based on synchrotron powder X-ray diffraction data. Calcite transforms from R[`3] cR3c to R[`3] mR3m at about T c = 1,240 K. A CO3 group occupies, statistically, two positions with equal frequency in the disordered R[`3] mR3m phase, but with unequal frequency in the partially ordered R[`3] cR3c phase. One position for the CO3 group is rotated by 180° with respect to the other. The unequal occupancy of the two orientations in the partially ordered R[`3] cR3c phase is obtained directly from the occupancy factor, x, for the O1 site and gives rise to the order parameter, S = 2x - 1. The a cell parameter shows a negative thermal expansion at low T, followed by a plateau region at higher T, then a steeper contraction towards T c, where the CO3 groups disorder in a rapid process. Using a modified Bragg–Williams model, fits were obtained for the order parameter S, and for the intensities of the (113) and (211) reflections.....
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