The Heat Capacity of Thorium Dioxide from 10 to 305°K. The Heat Capacity Anomalies in Uranium Dioxide and Neptunium Dioxide

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Abstract
The heat capacity of thorium dioxide was determined from 10 to 305°K in an adiabatic calorimeter. As expected, no anomaly was observed in the heat capacity. The enthalpy, entropy, and free energy were calculated from the heat‐capacity data and are tabulated for several temperatures. At 298.16°K the entropy and the enthalpy are, respectively, 15.593±0.02 cal deg−1 mole−1 and 2524.4±3 cal mole−1. By assuming the lattice entropy of uranium dioxide and of neptunium dioxide to be equal to the entropy of thorium dioxide, the magnetic entropy contributions at 298.16°K were evaluated as 3.0 cal deg−1 mole−1 for uranium dioxide and 3.6 cal deg−1 mole−1 for neptunium dioxide. These values are somewhat greater than the spinonly values (R ln 3=2.18 cal deg−1 mole−1 for uranium dioxide and R ln 4=2.75 cal deg−1 mole−1 for neptunium dioxide). The entropies of the other isomorphous actinide dioxides at 298.16°K are estimated to be (in cal deg−1 mole−1) 17.8 for PaO2, 19.7 for PuO2, 20.0 for AmO2, 20.3 for CmO2, 20.6 for BkO2, and 20.3 for CfO2.