| Abstrakt: |
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 $$ R\overline{3} c $$ to $$ R\overline{3} m $$ at about T c = 1,240 K. A CO3 group occupies, statistically, two positions with equal frequency in the disordered $$ R\overline{3} m $$ phase, but with unequal frequency in the partially ordered $$ R\overline{3} c $$ 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 $$ R\overline{3} c $$ 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. [ABSTRACT FROM AUTHOR] |
orientational disorder in calcite.
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