THERMODYNAMIC STUDIES ON CS4U5O17(S) AND CS2U2O7(S) BY EMF AND CALORIMETRIC MEASUREMENTS

The oxygen potentials over the phase field: Cs4U5O17(s) + Cs2U2O7(s) Cs2U4O12(s) was determined by measuring the emf values between 1048 a nd 1206 K using a solid oxide electrolyte galvanic cell. The oxygen po tential existing over the phase field for a given temperature can be r epresented by: Delta mu(O-2) (kJ/mol) (+/-0.5)= -272.0 + 0.207T (K). T he differential thermal analysis showed that Cs4U5O17(s) is stable in air up to 1273 K. The molar Gibbs energy formation of Cs4U5O17(s) was calculated from the above oxygen potentials and can be given by, Delta (f)G(0) (kJ/mol) + 6 = -7729 + 1.681T (K). The enthalpy measurements o n Cs4U5O17(s) and Cs2U2O7(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-T(0)-H-298(0)), in J/mol for Cs4U5O17(s) and Cs2U2O7(s) can be represented by, H-T(0)-H-298.15(0) (Cs4U5O17) k J/mol +/- 0.9 = -188.221 + 0.518T (K) + 0.433 x 10(-3)T(2) (K) - 2.052 X 10(-5)T(3) (K) (368 to 905 K) and H-T(0)-N-298.15(0) (Cs2U2O7) kJ/m ol + 0.5 = -164.210 + 0.390T 298.15 (K) + 0.104 x 10(-4)T(2) (K) + 0.1 40 X 10(5)(1/T (K)) (411 to 860 K). The thermal properties of Cs4U5O17 (s) and Cs2U2O7(s) were derived from the experimental values. The enth alpy of formation of (Cs4U5O17, s) at 298.15 K was calculated by the s econd law method and is: Delta(f)H(298.15)(0) = -7645.0 +/- 4.2 kT/mol . (C) 1997 Elsevier Science B.V.