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

✍ Scribed by M. T. Dove; B. M. Powell

• Publisher: Springer-Verlag
• Year: 1989
• Tongue: English
• Weight: 467 KB
• Volume: 16
• Category: Article
• ISSN: 0342-1791
• DOI: 10.1007/bf00197019

✦ Synopsis

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 (R3c~R3m) 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~--T) p, where fi 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 e 3 which is related to the order parameter (and hence temperature) via e 3 aQ2. No evidence for critical lowering of the value offi was found. These measurements confirm that, apart from the detailed critical behaviour, the phase transition in calcite is similar to that observed in NaNO3.