Infrared spectroscopic study of Rb2M(XO4)2·6H2O (M = Mg, Co, Ni, Cu, Zn; X = S, Se) and of guest ions included in rubidium Tutton selenates

The solubility in the three-component systems Rb2SeO4–MSeO4–H2O (M = Mg, Ni, Cu) is studied at 25 °C by the method of isothermal decrease of supersaturation. It has been established that double compounds, Rb2M(SeO4)2·6H2O (M = Mg, Ni, Cu), crystallize from the ternary solutions within wide concentration ranges. ed spectra of rubidium Tutton compounds, Rb2M(XO4)2·6H2O (X = S, Se; M = Mg, Co, Ni, Cu, Zn) (in the region of 1300–400 cm−1) as well as those of SO 4 2 - ions included in selenate matrices (up to 15 mol%) in the region of the stretching modes ν3 and ν1 are presented and discussed. The matrix-isolated SO 4 2 - ions exhibit three bands for ν3 and one band for ν1 in good agreement with the low site symmetry C1 of the SeO 4 2 - host ions. The extent of energetic distortion of the isomorphously included ions as deduced from the values of Δν3 (site group splitting) and Δν3/νc (where νc is the centro-frequency value of the asymmetric stretches) is discussed. The comparison of the vibrational behavior of the matrix-isolated SO 4 2 - ions (about 2 mol%) in different Tutton selenate matrices show that the distortions of the sulfate guest ions increase on going from potassium selenates to ammonium ones. The stronger distortion of the SO 4 2 - guest ions in ammonium compounds as compared to that of the same ions included in the potassium and rubidium selenates is due to the formation of hydrogen bonds between the guest ions and the NH4+ host cations additionally to those with water molecules of the host compound. The matrix-isolated sulfate guest ions are stronger distorted in the case of the Rb2Cu(SeO4)2·6H2O, (NH4)2Cu(SeO4)2·6H2O, (NH4)2Zn(SeO4)2·6H2O, and (NH4)2Ni(SeO4)2·6H2O as compared to those in the respective magnesium, cobalt and nickel selenate matrices is due probably to the formation of stronger hydrogen bonds in the former compounds (strong synergetic effect of the copper, zinc and nickel ions) as well as to the formation of hydrogen bonds between the guest ions and the ammonium host ions. The water librations are also briefly discussed.