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

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.