LOW-TEMPERATURE STRUCTURAL PHASE-TRANSITIONS OF (NH4)3H(SEO4)2 CRYSTALS STUDIED BY THE X-RAY-DIFFRACTION

The structural phase transitions in triammonium hydrogen diselenate have been studied by X-ray diffraction. The space groups in four different phases below 40-degrees-C are identified as R3mBAR, C2/c, A2/m and A2 with decreasing temperature. (NH4)3H(SeO4)2 crystal has high symmetry in the high-temperature phase, the reorientation and/or the distortion of SeO4 groups are the possible mechanisms to cause the low-temperature structural phase transitions. A ferroelectric phase transition from phase IV to phase V due to order-disorder of the hydrogen is preliminarily confirmed in this study.