ISSN:
1432-2021
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
,
Geosciences
,
Physics
Notes:
Abstract The orientational ordering transition R $$R\bar 3m - R\bar 3c$$ in NaNO3 near 552 K has been investigated using x-ray diffraction techniques. NaNO3 is a model system for CaCO3 and other minerals with orientational disorder of triangular molecules in a simple NaCl-type matrix. The temperature evolution of the integrated intensities of the superlattice reflection $$\bar 1$$ 23 and the fundamental reflection 110 are discussed in terms of Landau theory of two coupled order parameters. It is shown that the known phenomenological critical exponent (Poon and Salje 1988) and the anomalous thermal expansion at T 〉 T tr (Reeder et al. 1988) can be understood as the result of a Z point instability which mainly describes the NO 3 - disorder, and a second order parameter linked with the spontaneous strain of this phase transition.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00209703
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