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 $H_T{\mathrm{\hspace{0.17em}-\hspace{0.17em}H}}_{298}{\text{\hspace{0.17em}=\hspace{0.17em}−\hspace{0.17em}8468\hspace{0.17em}+\hspace{0.17em}22.18T\hspace{0.17em}+\hspace{0.17em}1.040\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−4}}{T}^2{\text{\hspace{0.17em}+\hspace{0.17em}4.935\hspace{0.17em}×\hspace{0.17em}10}}^5{T}^{\text{−1}}$ . The first derivative of this equation gave the heat‐capacity relation, $C_p{\text{\hspace{0.17em}=\hspace{0.17em}22.18\hspace{0.17em}+\hspace{0.17em}2.080\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−4}}{\text{T−4.935\hspace{0.17em}×\hspace{0.17em}10}}^5{T}^{\text{−2}}$ . 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.