Crystal Structures and Infrared Spectra of the Tutton Salts Mꞌ2Mg(CrO4)2·6H2O (Mꞌ = Rb, Cs) and Rb2Mg(SO4)2·6H2O, and of Solid Solutions Rb2Mg(Cr1-xSxO4)2·6H2O

Author(s): Manfred Wildner, Delyana Marinova, Donka Stoilova
Abstract: The crystal structures and crystal chem. of Cs2Mg(CrO4)2·6H2O and of twelve representatives along the Rb2Mg(Cr1-xSxO4)2·6H2O solid soln. series, including the two endmembers, were studied using single crystal X-ray diffraction data. The Rb2Mg(Cr1-xSxO4)2·6H2O system seems to be the first structurally investigated continuous sulfate-chromate solid soln. series in inorg. chem. to date, and it turned out that the choice of neutral vs. ionic X-ray scattering curves for S and Cr is crucial for reliable results of the Cr,S site occupancy refinements. All structures belong to the large group of Tutton-type compds. In spite of the strong expansion of the tetrahedral unit from SO4 to CrO4, the overall tetrahedral shape and that of the Mg(H2O)6 octahedron remain nearly const., with very weak bond length and angle distortions. This is attributed to the flexibility of the structure type of Tutton salts, comprising easily adaptable linking alkali cations and hydrogen bonds within a three-dimensional framework structure type, which does not exhibit evident tendencies to form chain- or sheet-dominated assemblies. IR spectra of the Tutton compds., Rb2Mg(CrO4)2·6H2O and Cs2Mg(CrO4)2·6H2O, are discussed with respect to both the normal modes of the tetrahedral ions and the water mols. IR spectra of the solid solns. Rb2Mg(Cr1-xSxO4)2·6H2O (0<×<1) are discussed in the region of the stretching modes 3 and 1 of the anions. The spectroscopic expts. show that the matrix-isolated guest ions (about 5 mol%) exhibit three bands for 3 and one band for 1 in good agreement with the low site symmetry C1 of the host ions. The extent of energetic distortion of the isomorphously included ions as deduced from the values of 3 (site group splitting) and 3/c (where c is the centro-frequency value of the asym. stretches) is discussed. The hydrogen bond strength of the water mols. in the chromate compds. as deduced from the OD modes of matrix-isolated HDO is compared to that of the water mols. in the resp. selenate and sulfate compds. It has been established that the hydrogen bonds in the chromates are stronger than those in the resp. sulfates and selenates. The thermal dehydration of Rb2Mg(CrO4)2·6H2O and Cs2Mg(CrO4)2·6H2O was studied by TG, DTA and DSC methods and the resp. dehydration schemes are proposed. The enthalpies of dehydration and formation have been calcd.
Organisation(s): Department of Mineralogy and Crystallography
External organisation(s): Bulgarian Academy of Sciences (BAS)
Journal: International Research Journal of Pure and Applied Chemistry
Volume: 13
Pages: 1-23
No. of pages: 24
ISSN: 2231-3443
DOI: https://doi.org/10.9734/IRJPAC/2016/29171
Publication date: 10-2016
Peer reviewed: Yes
Austrian Fields of Science 2012: 105113 Crystallography, 104026 Spectroscopy, 104011 Materials chemistry
Keywords
Portal url: https://ucrisportal.univie.ac.at/en/publications/87919ff4-99f4-4ac8-b942-9ccd24cdc808

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