Cesium diuranate Cs2U2O7: heat capacity (5 to 350 K) and thermodynamic functions to 350 K. A re-evaluation of the standard enthalpy of formation and the thermodynamics of (cesium + uranium + oxygen)

The low-temperature (5 to 350 K) heat capacity of a well-characterized sample of β-Cs2U2O7 has been measured by adiabatic calorimetry. No anomalies were observed in the plot of heat capacity against temperature, the curve having the normal sigmoid shape. These measurements were used to calculate thermodynamic functions to 350 K. At T = 298.15 K results were obtained for the following and are presented: C0sub(p); S0(T); (H0(T)-H0(0)); (G0(T)-H0(0))/T. Because of recent critical re-evaluations of thermochemical quantities for UO2Cl2, the published value for ΔH0sub(f)(Cs2U2O7, c, β, 298.15 K) has been revised to -(3220.2 +- 1.8)kJ mol-1. A discussion of the thermodynamics of (cesium + uranium + oxygen) at high temperatures is included. (author)