ISSN:
1432-2021
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
,
Geosciences
,
Physics
Notes:
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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00197019
Permalink
