Chiral open-framework uranyl molybdates. 3. Synthesis, structure and the C222(1) -> P2(1)2(1)2(1) low-temperature phase transition of C6H16N (2) (UO2)(6)(MoO4)(7)(H2O)(2) (H2O)(2)

Krivovichev, S. V., Armbruster, T., Chernyshov, D. Y., Burns, P. C., Nazarchuk, E. V. and Depmeier, W. (2005) Chiral open-framework uranyl molybdates. 3. Synthesis, structure and the C222(1) -> P2(1)2(1)2(1) low-temperature phase transition of C6H16N (2) (UO2)(6)(MoO4)(7)(H2O)(2) (H2O)(2) Microporous and Mesoporous Materials, 78 (2-3). pp. 225-234. DOI 10.1016/j.micromeso.2004.10.020.

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Abstract

A new chiral 6:7 open-framework uranyl molybdate, [C6H16N](2)[(UO2)(6)(MoO4)(7)(H2O)(2)](H2O)(2), has been synthesized by hydrothermal methods. The structure has been refined using single-crystal X-ray diffraction data collected at 20 and -127degreesC. The 20degreesC structure [orthorhombic, C222(1), a = 11.3045(14), b = 19.962(6), c = 24.416(5) Angstrom, V= 5510(2) Angstrom(3)] has been refined to R-1 = 0.046 on the basis of 6093 unique observed reflections. The -127degreesC structure [orthorhombic, P2(1)2(1)2(1), a = 11.211(4), b = 19.880(10), c = 24.421(8) Angstrom, V = 5443(4) Angstrom(3)] has been refined to R-1 = 0.047 on the basis of 6951 unique observed reflections. The structures are based upon topologically identical frameworks of corner-sharing UO7 pentagonal bipyramids and MoO4 tetrahedra. The extra-framework H2O groups and protonated triethylamine molecules reside in the framework cavities. In the C222(1) structure at 20degreesC H2O and [C6H16N](+) molecules filling the chiral channels along [0 0 1] are disordered, whereas, in the P2(1)2(1)2(1) structure at -127degreesC, they are perfectly ordered. The symmetry difference between structures at 20degreesC and -127degreesC is the result of a C222(1) P2(1)2(1)2(1) second order phase transition that involves ordering of extra-framework protonated amine molecules and H2O groups, and distortion of the flexible [(UO2)(6)(MoO4)(7)(H2O)(2)](2-) uranyl molybdate framework. On the basis of measurements of intensities of reflections that violate absence conditions of C-centering cell, the temperature of the phase transition is estimated as -11degreesC. (C) 2004 Elsevier Inc. All rights reserved.

Document Type: Article
Keywords: uranyl molybdates open framework crystal structure phase transition order-disorder
Research affiliation: Kiel University
DOI etc.: 10.1016/j.micromeso.2004.10.020
ISSN: 1387-1811
Date Deposited: 18 Jan 2012 05:10
Last Modified: 08 Oct 2012 10:26
URI: http://eprints.uni-kiel.de/id/eprint/16066

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