Crystal structure and vibrational study of phase transition in the new material Cs0.86(NH4)0.14(HSO4)0.66(HSeO4)0.34 Original Research Article

Solid solution investigations in cesium–ammonium hydrogen sulphate–selenate were carried out as part of an ongoing effort to elucidate the relationship between hydrogen bonding and phase transitions. The new centrosymmetric mixed compound has been structurally characterized by single crystal X-ray diffraction methods. Cs0.86(NH4)0.14(HSO4)0.66(HSeO4)0.34 consists of (HSO4−) and (HSeO4−) groups interconnected via O–H…O bonds surrounding large one-dimensional channels occupied by 11 co-ordinate Cs+ and NH4+ cations. The structure of the mixed compound is stabilised by a three dimensional N–H…O hydrogen-bonding network linking anionic entities to (NH4+) in addition to the one dimensional O–H…O ordered hydrogen bonds connecting (HSO4−/HSeO4−). The partially (dis)ordered system has been studied by optical spectroscopy at 300 and 77 K. The transition at 238 K has a profound effect on the Raman spectra. The room temperature phase shows broad Raman bands which split in a number of relatively narrow components at 77 K, mainly because of orientational disorder of ammonium, sulphate and selenate ions at 300 K. The degree of the structural disorder in the solid solution Cs0.86(NH4)0.14(HSO4)0.66(HSeO4)0.34 in the two phases is discussed.