Heat capacities of dihydrogen and dideuterium thallium (I) orthophosphates. Thermodynamic properties of solid-phase transitions

Abstract The heat capacity of dihydrogen and dideuterium thallium (I) orthophosphates has been measured by adiabatic calorimetry over the range 20 to 300 K. Thermodynamic functions have been evaluated and the values of C p , { S o ( T ) − S o (0)}, and −{G o (T) − H o (0)} T at 298.15 K are respectively 124.14, 174.4, and 97.06 J K −1 mol −1 for TlH 2 PO 4 , and 137.79, 174.7, and 95.94 J K −1 mol −1 for TlD 2 PO 4 . Lambda-shaped maxima were observed at 228.71 K corresponding to an excess enthalpy of 225 J mol −1 for TlH 2 PO 4 , and at 126.72 K corresponding to an excess enthalpy of 69 J mol −1 for TlD 2 PO 4 . Occurence of a phase transition at about 360 K was suggested by d.t.a. of thallium (I) dideuterium orthophosphate.