In situ study of the $$ R\overline{3} c \to R\overline{3} m $$ orientational disorder in calcite |
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Authors: | Sytle M Antao Ishmael Hassan Willem H Mulder Peter L Lee Brian H Toby |
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Institution: | (1) Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA;(2) Department of Chemistry, University of the West Indies, Mona, Kingston 7, Jamaica |
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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 to at about T
c = 1,240 K. A CO3 group occupies, statistically, two positions with equal frequency in the disordered phase, but with unequal frequency in the partially ordered 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
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.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. |
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Keywords: | Calcite CaCO3 Orientational disorder transition High-temperature studies Rietveld refinement Synchrotron X-ray diffraction |
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