Neutron diffraction study of the tricritical orientational order/disorder phase transition in calcite at 1260 K

Abstract

Neutron powder diffraction measurements of the temperature dependence of superlattice reflections in calcite have shown that there is a continuous phase transition at 1260 K. The change in space group symmetry $({\text{R}}\overline {\text{3}} {\text{c}} \to {\text{R}}\overline {\text{3}} {\text{m}})$ and the halving of the unit cell size on heating indicate that this transition is an orientational order/disorder transition. The intensities of the superlattice reflections show that the temperature dependence of the order parameter, Q, is of the form (T _c −T)^β, where β is 0.25, indicating that the transition is tricritical. The transition is accompanied by a large contraction along the c axis on cooling, defining a spontaneous strain e₃ which is related to the order parameter (and hence temperature) via e₃ αQ ². No evidence for critical lowering of the value of β was found. These measurements confirm that, apart from the detailed critical behaviour, the phase transition in calcite is similar to that observed in NaNO₃.