LOW-TEMPERATURE HEAT-CAPACITIES OF AMMONIUM HEXACHLOROSELENATE AND OF ITS DEUTERATED ANALOG

Ammonium hexachloroselenate (NH4)2SeCl6 and its deuterated analogue (ND4)2SeCl6 were studied by calorimetry at temperatures T between 12 K and 300 K. The normal salt and deuterated analogue underwent λ-type phase transitions at T = 24.5 K and T = 48.2 K, respectively. The latter was accompanied by a subsidiary peak in the heat capacity at T = 45.7 K. The molar enthalpy and molar entropy changes associated with the phase transition are 33.8 J·mol-1 and 1.50 J·K-1·mol-1 for the normal salt and 546 J·mol-1 and 12.1 J·K-1·mol-1 for the deuterated salt. These values suggest different mechanisms (displacive and order-disorder) for the phase transitions in the normal and deuterated compounds. Standard thermodynamic functions were calculated and are tabulated. © 1994 Academic Press, Inc.