Infrared Spectroscopic Study of Cs2Ni(XO4)2•6H2O (X = S, Se) and of NH4+ Ions Included in M2Ni(XO4)2∙6H2O (M = Rb, Cs; X = S, Se) and Crystal Structures of (M,NH4)2Ni(XO4)2∙6H2O (M = Rb, Cs; X = S, Se) Mixed Crystals

The solubility in the three-component Cs2SO4-NiSO4-H2O system was studied at 25°C. It has been established that a double salt, Cs2Ni(SO4)2·6H2O, crystallizes within a wide concentration range. Infrared spectra of neat Tutton compounds Cs2Ni(XO4)2•6H2O (X = S, Se) as well as those of ammonium doped rubidium and cesium sulfate and selenate matrices are presented and discussed with respect to the normal modes of the tetrahedral ions and water librations. The ammonium ions included in the sulfates exhibit three bands corresponding to the asymmetric bending modes n4 in agreement with the low site symmetry C1 of the host cesium and rubidium cations. However, the inclusion of ammonium ions in the rubidium and cesium selenates leads to the appearance of four bands in the region of n4. At that stage of our knowledge we assume that some kind of disorder of the ammonium ions included in the selenates occurs due to the strong proton acceptor capability of the SeO42- (stronger than that of SO42-), thus facilitating the formation of polyfurcate hydrogen bonds by the ammonium ions in the selenate matrices. The strength of the hydrogen bonds formed in the mixed crystals M1.85(NH4)0.15Ni(XO4)2·6H2O (M = Rb, Cs; X = S, Se) as deduced from the frequencies of the water librations is discussed. The spectroscopic experiments reveal that the water molecules in the mixed crystals form weaker hydrogen bonds than those in the neat rubidium and cesium Tutton salts due to decreasing in the proton acceptor strength of the SO42- and SeO42- ions as a result of the formation of hydrogen bonds between the host anions and the NH4+ guest cations (anti-cooperative or proton acceptor competitive effect). Crystal structure investigations of several (M,NH4)2Ni(XO4)2∙6H2O (M = Rb, Cs; X = S, Se) mixed crystals reveal significant changes in the environment of the monovalent cations as well as in the hydrogen bonding systems of the water molecules upon incorporation of ammonium ions. Disorder of NH4 groups and the formation of polyfurcate N–H···O hydrogen bonds have not been observed, but neither can be excluded by the X-ray diffraction experiments, especially not for rather low ammonium contents.