Neutron powder diffraction study of the orientational order–disorder phase transition in calcite, CaCO3
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Neutron powder diffraction studies of calcite on heating towards the orientational order–disorder phase transition show that the phase transition is not a simple analogue of an Ising-like transition, but more similar to a rotational analogue of Lindemann melting. The transition is precipitated by the librational amplitude of the carbonate molecular ions exceeding a critical value rather than a result of a statistical entropy of ‘wrong’ orientations. Using tested interatomic potentials the single-particle orientational potential and nearest-neighbour orientational interactions have been calculated.
MTD wishes to thank EPSRC for financial support. We are pleased to acknowledge the collaboration with Mark Hagen and Mark Harris. We would also like to thank Ruth Lynden-Bell for discussions on the molecular dynamics simulation.
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