Infrared spectroscopic study of SO42− ions included in M′2M′′(SeO4)2⋅6H2O (Me′ = K, NH4+; M′′ = Mg, Co, Ni, Cu, Zn) and NH4+ ions included in K2M(XO4)2⋅6H2O (X = S, Se; M′′ = Mg, Co, Ni, Cu, Zn)

Abstract

Infrared spectra of Tutton compounds, M′2M′′(XO4)2⋅6H2O (M′=K, NH4+; M′′=Mg, Co, Ni, Cu, Zn; X=S, Se), as well as those of SO42− guest ions included in selenate host lattices and of NH4+ guest ions included in potassium host lattices are presented and discussed in the regions of ν3 and ν1 of SO42− guest ions, ν4 of NH4+ guest ions and water librations.The SO42− guest ions matrix-isolated in selenate matrices (approximately 2mol%) exhibit three bands corresponding to ν3 and one band corresponding to ν1 in good agreement with the low site symmetry C1 of the host selenate ions. When the larger SeO42− ions are replaced by the smaller SO42− ions the mean values of the asymmetric stretching modes ν¯3 of the included SO42− ions are slightly shifted to lower frequencies as compared to those of the same ions in the neat sulfate compounds due to the smaller repulsion potential of the selenate matrices (larger unit-cell volumes of the selenates). It has been established that the extent of energetic distortion of the sulfate ions matrix-isolated in the ammonium selenates as deduced from the values of Δν3 and Δν3/νc is stronger than that of the same ions matrix-isolated in the potassium selenates due to the formation of hydrogen bonds between the SO42− guest ions with both the water molecules in the host compounds and the NH4+ host ions (for example, Δν3 of the sulfate guest ions have values of 30 and 51cm−1 in the nickel potassium and ammonium compounds, and 33 and 49cm−1 in the zinc potassium and ammonium compounds, respectively).The infrared spectra of ammonium doped potassium sulfate matrices show three bands corresponding to Δν4 of the included ammonium ions in agreement with the low site symmetry C1 of the host potassium ions. However, the inclusion of ammonium ions in selenate matrices (with exception of the magnesium compound) leads to the appearance of four bands in the region of ν4. At that stage of our knowledge we assume that some kind of disorder of the ammonium ions included in selenate lattices occurs due to the different proton acceptor capability of the SO42− and SeO42− ions. The latter ions are known to exhibit stronger proton acceptor abilities. This fact will facilitate the formation of polyfurcate hydrogen bonds of the ammonium ions in the selenate matrices, thus leading to increasing in the coordination number of these ions, i.e. to a disorder of the ammonium guest ions.The strength of the hydrogen bonds formed in the title Tutton compounds as well as that of the hydrogen bonds in potassium compounds containing isomorphously included ammonium ions as deduced from the wavenumbers of the water librations are also discussed. The bands corresponding to water librations in the spectra of the mixed crystals K1.8(NH4)0.2M(XO4)2⋅6H2O (M=Mg, Co, Ni, Cu, Zn; X=S, Se) broaden and shift to lower frequencies as compared to those of the potassium host compounds, thus indicating that weaker hydrogen bonds are formed in the mixed crystals. These spectroscopic findings are owing to the decrease in the proton acceptor capacity of the SO42− and SeO42− ions due to the formation of hydrogen bonds between the host anions and the guest ammonium cations additionally to water molecules (anti-cooperative or proton acceptor competitive effect). Furthermore, the band shifts in the spectra of the selenate matrices are generally larger than those observed in the spectra of the respective sulfates due to the stronger proton acceptor ability of the selenate ions.