Heat Capacity and Thermodynamic Properties of Plutonium Dioxide

Changes in the heat content of PuO2 were measured from 192° to 1400°K with an isothermal drop calorimeter. The enthalpy curve, which was a smooth function of temperature, is described by the equation HT − H298 = − 8468 + 22.18T + 1.040 × 10−4T2 + 4.935 × 105T−1. The first derivative of this equation gave the heat-capacity relation, Cp = 22.18 + 2.080 × 10−4T−4.935 × 105T−2. At 298°K this equation agrees with previously reported low-temperature measurements made with an adiabatic calorimeter. These high- and low-temperature data were combined to obtain standard-state heat content, heat capacity, entropy, and Gibbs free energy functions to 1800°K. Measurements by other investigators show that PuO2 has a temperature-independent magnetic susceptibility below 1000°K. This behavior explains the comparatively low entropy of 16.34 cal mole−1·deg−1 for stoichiometric PuO2 at 298°K, which is somewhat greater than the value found for ThO2 and considerably less than the values for UO2 and NpO2.