Neutron diffraction study of the tricritical orientational order/disorder phase transition in calcite at 1260 K期刊界 All Journals 搜尽天下杂志传播学术成果专业期刊搜索期刊信息化学术搜索

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

Authors:	M T Dove B M Powell

Institution:	1. Department of Earth Sciences, University of Cambridge, Downing Street, CB2 3EQ, Cambridge, England 2. Atomic Energy of Canada Limited, Chalk River Nuclear Laboratories, Chalk River, KOJ 1JO, Ontario, Canada

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), where 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 Q ². No evidence for critical lowering of the value of was found. These measurements confirm that, apart from the detailed critical behaviour, the phase transition in calcite is similar to that observed in NaNO₃.

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