Low-temperature heat capacities of ammonium hexachloroselenate and of its deuterated analogue

Abstract

Ammonium hexachloroselenate (NH 4) 2SeCl 6 and its deuterated analogue (ND 4) 2SeCl 6 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.