The layered uranyl silicate mineral uranophane-β: crystal structure, mechanical properties, Raman spectrum and comparison with the α-polymorph.

Author(s) Colmenero, F.; Plášil, J.; Sejkora, J.
Journal Dalton Trans
Date Published 2019 Nov 12
Abstract

The crystal structure, elastic properties and the Raman spectrum of the layered calcium uranyl silicate pentahydrate mineral uranophane-β, Ca(UO2)2Si2O6(OH)2·5H2O, are studied by means of first-principles solid-state methods and compared with the corresponding information for the α polymorph. The availability of the energy optimized full crystal structure of uranophane-β, including the positions of the hydrogen atoms, made possible the computation of its elastic properties and the Raman spectrum by using the theoretical methodology. An extended set of relevant mechanical data is reported. Uranophane-β is shown to be a weak and ductile mineral and, consequenty, is mechanically very different from the α polymorph which is a hard and brittle material. Uranophane-β exhibits the important negative Poisson's ratio (NPR) and negative linear compressibility (NLC) phenomena. The experimental Raman spectrum of uranophane-β obtained from a natural mineral sample from pegmatite Perus, São Paulo, Brazil, is compared with the spectrum determined theoretically. Since both spectra are in very good agreement, the theoretical methods are employed to assign the Raman spectrum. Three weak bands of the experimental spectrum of this mineral, located at the wavenumbers 2302, 2128 and 2042 cm-1, are identified as combination bands. The Raman spectrum of uranophane-β is also compared with that of the α polymorph. While they are rather similar, a detailed analysis reveals a significant number of differences. Finally, the relative thermodynamic stability of the α and β polymorphs is evaluated. The α polymorph is more stable than the β polymorph at zero pressure and temperature by -12.0 kJ mol-1.

DOI 10.1039/c9dt03256h
ISSN 1477-9234
Citation Colmenero F, Plášil J, Sejkora J. The layered uranyl silicate mineral uranophane-β: crystal structure, mechanical properties, Raman spectrum and comparison with the α-polymorph. Dalton Trans. 2019;48(44):16722-16736.

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