Neutron powder diffraction study of the orientational order-disorder phase transition in calcite, CaCO3

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.