Low-temperature heat capacity of normal and superconducting thorium hydride and thorium deuteride

The low-temperature heat capacities of several bulk samples of ${\mathrm{Th}}_4$ ${\mathrm{H}}_{15}$ and ${\mathrm{Th}}_4$ ${\mathrm{D}}_{15}$ have been measured over the temperature range from 1.4 to 20 K in both the normal and superconducting states. Average values of the electronic-heat-capacity coefficient $\gamma$ and Debye temperature ${\Theta }_D$ are found to be 8.07 ± 0.14 mJ ${\mathrm{K}}^{-2\mathrm{}}$/mole Th and 211.5 ± 1.1 K, respectively, for ${\mathrm{Th}}_4$ ${\mathrm{H}}_{15}$. Corresponding values for ${\mathrm{Th}}_4$ ${\mathrm{D}}_{15}$ are 7.84 ± 0.25 mJ ${\mathrm{K}}^{-2\mathrm{}}$/mole Th and 216.5 ± 6.6 K. A relatively large density of states at the Fermi level suggests the possibility of occupied $5f$ electronic states. Anomalies in the heat capacities of several samples were observed at the melting temperatures of molecular hydrogen and deuterium.