Thermodynamic investigations on K3Ln5(PO4)6 (Ln = Eu and Gd) compounds

Complex phosphates, such as K(3)Ln(5)(PO4)(6) are considered as promising materials for immobilization of radioactive waste as well as for selective separation of radio nuclides from nuclear waste. In this paper, we report detail thermodynamic properties of phosphate compounds viz., K3Eu5(PO4)(6) and K3Gd5(PO4)(6) measured by calorimetric techniques. The standard molar heat capacity of these compounds has been measured in the temperature range 2-300 K by PPMS and 325 to 935 by high temperature Calvet calorimeter. The heat capacities and standard molar entropies of K3Eu5(PO4)(6) and K3Gd5(PO4)(6) at 298 K are: 688.0, 560.3 J K(-1)mol(-1) and 792.7, 683.7 J K(-1)mol(-1), respectively. The standard molar enthalpy of formation of K3Eu5(PO4)(6), and K3Gd5(PO4)(6) determined employing high temperature solution calorimeter are found to be - 12234.8 +/- 61.5 and - 11790.9 +/- 58.2 kJ mol(-1), respectively. The temperature dependent standard molar free energy of formation and other thermodynamic functions for the compounds have been derived from above heat capacity and enthalpy of formation data. (c) 2018 Elsevier B.V. All rights reserved.