Thermodynamic studies on Cs 4U 5O 17(s) and Cs 2U 2O 7(s) by emf and calorimetric measurements

Abstract

The oxygen potentials over the phase field: Cs 4U 5O 17(s)+Cs 2U 2O 7(s)+Cs 2U 4O 12(s) was determined by measuring the emf values between 1048 and 1206 K using a solid oxide electrolyte galvanic cell. The oxygen potential existing over the phase field for a given temperature can be represented by: Δ μ(O 2) (kJ/mol) (±0.5)=−272.0+0.207 T (K). The differential thermal analysis showed that Cs 4U 5O 17(s) is stable in air up to 1273 K. The molar Gibbs energy formation of Cs 4U 5O 17(s) was calculated from the above oxygen potentials and can be given by, Δ f G 0 (kJ/mol)±6=−7729+1.681 T (K). The enthalpy measurements on Cs 4U 5O 17(s) and Cs 2U 2O 7(s) were carried out from 368.3 to 905 K and 430 to 852 K respectively, using a high temperature Calvet calorimeter. The enthalpy increments, ( H 0 T − H 0 298), in J/mol for Cs 4U 5O 17(s) and Cs 2U 2O 7(s) can be represented by, H 0 T − H 0 298.15 (Cs 4U 5O 17) kJ/mol±0.9=−188.221+0.518 T (K)+0.433×10 −3 T 2 (K)−2.052×10 −5 T 3 (K) (368 to 905 K) and H 0 T − H 0 298.15 (Cs 2U 2O 7) kJ/mol±0.5=−164.210+0.390 T (K)+0.104×10 −4 T 2 (K)+0.140×10 5(1/ T (K)) (411 to 860 K). The thermal properties of Cs 4U 5O 17(s) and Cs 2U 2O 7(s) were derived from the experimental values. The enthalpy of formation of (Cs 4U 5O 17, s) at 298.15 K was calculated by the second law method and is: Δ f H 0 298.15=−7645.0±4.2 kJ/mol.