Crystal structure and vibrational study of phase transition in the new material Cs-0.86(NH4)(0.14)(HSO4)(0.66)(HSeO4)(0.34)

Solid solution investigations in cesium-ammonium hydrogen sulphate-selenate were carried out as part of an ongoing effort to elucidate the relationshi p between hydrogen bonding and phase transitions. The new centrosymmetric m ixed compound has been structurally characterized by single crystal X-ray d iffraction methods. Cs-0.86(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 NW cations . The structure of the mixed compound is stabilised by a three dimensional N-H... O hydrogen-bonding network linking anionic entities to (NW) in addit ion to the one dimensional O-H... O ordered hydrogen bonds connecting (HSO4 -/HSeO4-). The partially (dis)ordered system has been studied by optical sp ectroscopy at 300 and 77 K. The transition at 238 K has a profound effect o n the Raman spectra. The room temperature phase shows broad Raman bands whi ch split in a number of relatively narrow components at 77 K, mainly becaus e of orientational disorder of ammonium, sulphate and selenate ions at 300 K. The degree of the structural disorder in the solid solution Cs-0.86(NH4) (0.14)(HSO4)(0.66)(HSeO4)(0.34) in the two phases is discussed. (C) 2001 El sevier Science Ltd. All rights reserved.