Phase transitions in triammonium hydrogen disulphate. Crystal structures at -110° c and 140° c

Structural phase transitions in triammonium hydrogen disulphate have been characterised by determining the crystaI structures at -110°C by single crystal X-ray diffraction studies and at +140°C by X-ray powder diffraction studies. The structure at -110°C is monoclinic C2/c (a=15.578(4), b=5.816(1), c=10.050(4) Å, β= 101.58(3)° and volume= 892.0(4) Å3), whereas the structure at the higher temperature (+140°C) is rhombohedral R3m (a=5.906(1), c=22.602(1) Å, volume = 682.75 Å3 and Z=3). The starting model for Rietveld refinement was derived from that of the corresponding selenium analogue. The effect of reorientation of SO4 groups in terms of intra and intermolecular interactions is analysed