A correlation between structural and vibrational spectroscopic data of some beryllium sulfates and selenates (review)
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2016-01-01
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Abstract
The present paper summarizes experimental results on the study of some beryllium compounds and is a part of a dissertation entitled ``Synthesis, structure and properties of some beryllium salts - sulfates and selenates``. The solubility diagrams of the three-component systems BeSeO4-K2SeO4-H2O, BeSO4-Rb2SO4-H2O, and BeSeO4-MSeO4-H2O (M = Co, Ni, Cu, Zn) at 25°C are presented and the crystallization field widths of the solid phases are determined. The experimental results are discussed with respect to the complex formation processes in the ternary solutions and the solubility of the salt components in their binary solutions. New compounds, Rb2Be(SO4)2·2H2O and K2Be(SeO4)2·2H2O, have been obtained as a result of co-crystallization processes in the systems pointed above. The crystal structures of BeSeO4·4H2O, M2Be(SO4)2·2H2O (M = K, Rb), and K2Be(SeO4)2·2H2O are determined from single crystal X-ray diffraction data. It has been established that the beryllium selenate tetrahydrate crystallizes in the orthorhombic space group Cmca (D18 2h). The double beryllium compounds are isostructural and crystallize in the monoclinic space group P21/c (C52h). Vibrational (infrared and Raman) spectra are recorded in the regions of the normal vibrations of the different motives building up the structures of the beryllium compounds studied. The vibrational spectra are discussed in the light of both the crystal structures of the beryllium salts and the chemical properties of the different entities. Special attention is paid to the influence of different crystal chemical factors on the strength of the hydrogen bonds formed in the beryllium compounds. These are the Be-O interactions (synergetic effect), the proton acceptor capacity of the selenate and sulfate ions, the proton acceptor capacity of the different oxygen atoms as determined according to the Brown's bond valence theory, the compositions of the beryllium tetrahedra (acidity of the water molecules), the anti-cooperative effect (proton donor and proton acceptor competitive effect), the repulsion potential of the lattice sites of water molecules location, the size of M+ cations. The intramolecular O-H bond distances are derived from the νOD vs. rOD correlation curve [H.D. Lutz, C. Jung, J. Mol. Struct., 404, 1997, 63-66]. Water librations of protiated and deuterated samples are also discussed. Crystal matrix infrared spectroscopy was applied to analyze: (i) the distribution of SO42- ions included in the structure of K2Be(SeO4)2·2H2O at two crystallographically different positions; (ii) the molecular symmetry of SeO42- ions in BeSeO4·4H2O determined from the vibrational behavior of matrix-isolated SO42- guest ions; (iii) the strength of the hydrogen bonds (with the application of the isotopic dilution method and that of the matrix-isolated HDO molecules).