Specific Heat of Uranium Mononitride from 1.3 to 4.6°K

The specific heat of uranium mononitride was measured at temperatures from 1.3 to 4.6°K and was found to be of the form $\gamma T+\beta {\left(\frac{T}{{\Theta }_D}\right)}^3$, where $\gamma$ is 24.79±0.05 mJ${\mathrm{}(\mathrm{g}\mathrm{at}.\mathrm{wt})\mathrm{}}^{-1\mathrm{}}$ ${\mathrm{}\left(\mathrm{deg}\right)\mathrm{}}^{-2\mathrm{}}$ and the Debye $\Theta$ is 324±7°K. The large value of $\gamma$ (35 times the value for copper) apparently results from the Fermi surface intersecting narrow $5f$ electronic bands. The above values refer to a UN sample heat-treated in an atmosphere of nitrogen for 6.5 h at 2250°C. The same sample, before this heat treatment, had a measured $\gamma =24.07\mathrm{}\pm 0.08$ mJ ${\mathrm{}(\mathrm{g}\mathrm{at}.\mathrm{wt})\mathrm{}}^{-1\mathrm{}}$ ${\mathrm{}\left(\mathrm{deg}\right)\mathrm{}}^{-2\mathrm{}}$ and $\Theta =322\mathrm{}\pm 13°$K. The shift in $\gamma$ may be due to a small change in stoichiometry caused by the heat treatment. Although the Debye temperature obtained from the present work, 324°K, is larger than previous specific-heat estimates of 276-289°K, it is lower than the value of 364°K obtained from velocity-of-sound measurements. No changes were observed in the specific heat upon application of magnetic fields up to 35 kG, suggesting the absence of any appreciable magnetic contribution to the total specific heat for temperature below 4.7°K.