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Publication Type
Journal Article
UWI Author(s)
Author, Analytic
Antao, Sytle M.; Hassan, Ishmael; Mulder, Willem H.; Lee, Peter L.
Author Affiliation, Ana.
n/a
Article Title
The R3c › R3m transition in nitratine, NaNO3, and implications for calcite, CaCO3
Medium Designator
n/a
Connective Phrase
n/a
Journal Title
Physics and Chemistry of Minerals
Translated Title
n/a
Reprint Status
Refereed
Date of Publication
2008
Volume ID
35
Issue ID
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Page(s)
545-557
Language
n/a
Connective Phrase
n/a
Location/URL
http:; link.springer.com/content/pdf/10.1007%2Fs00269-008-0232-8.pdf
ISSN
n/a
Notes
n/a
Abstract
The temperature dependences of the crystal structure and superstructure intensities in sodium nitrate, mineral name nitratine, NaNO3, were studied using Rietveld structure refinements based on synchrotron powder X-ray diffraction. Nitratine transforms from R[`3] c to R[`3] mR3ctoR3m at T c = 552(1) K. A NO3 group occupies, statistically, two positions with equal frequency in the disordered R[`3] mR3m phase, but with unequal frequency in the partially ordered R[`3] cR3c phase. One position for the NO3 group is rotated by 60° or 180° with respect to the other. The occupancy of the two orientations in the R[`3] cR3c phase is obtained from the occupancy factor, x, for the O1 site and gives rise to the order parameter, S = 2x - 1, where S is 0 at T c and 1 at 0 K. The NO3 groups rotate in a rapid process from about 541 to T c, where the a axis contracts. Using a modified Bragg–Williams model, a good fit was obtained for the normalized intensities (that is, normalized, NI1/2) for the (113) and (211) reflections in R[`3] c NaNO3,R3c\,NaNO3 and indicates a second-order transition. Using the same model, a reasonable fit was obtained for the order parameter, S, and also supports a second-order transition.....
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