Thermodynamics of ammonium halogenated tellurides II. Heat capacity, phase transitions, and thermodynamic properties of (ND4)2TeCl6 at temperatures from 5 K to 325 K

Abstract

The heat capacity of deuterated ammonium hexachlorotellurate (ND4)2TeCl6 was measured at temperatures T from 5 K to 325 K by adiabatic calorimetry. A small anomaly in the curve of heat capacity against temperature was found at T=(89.5±0.3)K with a maximum Cp,m = 23.84·R, ΔtrsS°m = (0.234±0.006)·R. The discovery of this anomaly supports the presence of a rotative displacive transition similar to that involving TeCl2-6 in the undeuterated salt. In addition, two λ-shaped transitions were discovered at T = (31.4±0.1) K, with a maximum Cp,m = 34.7·R, ΔtrsS°m = (0.861±0.007) R, and at T = (47.6±0.1)K, with a maximum Cp,m = 18.6 R, ΔtrsS°m = (0.247±0.006) R. Smoothed values of the standard thermodynamic quantities for pure (ND4)2TeCl6 are tabulated up to T = 320 K.