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

Abstract

The crystal structures and crystal chemistry of Cs2Mg(CrO4)2·6H2O and of twelve representatives along the Rb2Mg(Cr1‑xSxO4)2×6H2O solid solution 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 solution series in inorganic chemistry 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 compounds. 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 constant, 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. Infrared spectra of the Tutton compounds, 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 molecules. Infrared spectra of the solid solutions Rb2Mg(Cr1-xSxO4)2×6H2O (0 < x < 1) are discussed in the region of the stretching modes n3 and n1 of the anions. The spectroscopic experiments show that the matrix-isolated guest ions (about 5 mol%) exhibit three bands for n3 and one band for n1 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 Dn3 (site group splitting) and Dn3/nc (where nc is the centro-frequency value of the asymmetric stretches) is discussed. The hydrogen bond strength of the water molecules in the chromate compounds as deduced from the nOD modes of matrix-isolated HDO is compared to that of the water molecules in the respective selenate and sulfate compounds. It has been established that the hydrogen bonds in the chromates are stronger than those in the respective 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 respective dehydration schemes are proposed. The enthalpies of dehydration and formation have been calculated.