AbstractInfrared spectra of K2Ni(SeO4)2·6H2O and (NH4)2Ni(SeO4)2·6H2O containing SO42‐ ions and those of K2Ni(SO4)2·6H2O and K2Ni(SeO4)2·6H2O containing NH4+ ions are presented and discussed in the region of ν3 and ν1 of the sulfate ions and in the region of ν4 of the NH4+ ions, respectively. The SO42‐ ions matrix‐isolated in the selenate matrices (approximately 1 mol%) exhibit three bands for ν3 and one band for ν1 in agreement with the low site symmetry C1 of the host selenate ions. The NH4+ guest ions included in the potassium sulfate matrix are characterized also with three bands for ν4. However, the ammonium ions in (NH4)2Ni(SeO4)2·6H2O as well as those included in K2Ni(SeO4)2·6H2O display four infrared bands corresponding to ν4 due probably to some kind of disorder of the ammonium ions. The extent of energetic distortion of the isomorphously included sulfate ions as deduced from the values of Δν3 and Δνmax is commented. The spectroscopic experiments reveal that the SO42‐ guest ions are weaker distorted in the selenate matrices as compared to the same ions in the neat sulfates due to the larger unit‐cell volumes of the selenate compounds. The band positions of the water librations in the host potassium compounds are affected by the included ammonium cations. The formation of hydrogen bonds between the NH4+ guest ions and the XO42‐ host ions leads to a decrease in the proton acceptor capabilities of the anions and as a result the hydrogen bonds weaken on going from the neat potassium compounds to the mixed crystals. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)