Magnetic properties of neptunium and plutonium hydrides

The magnetic susceptibility and magnetization of $\mathrm{Np}{\mathrm{H}}_x$ and $\mathrm{Pu}{\mathrm{H}}_x$ ($2.0\le x\le 3.0$) compounds have been measured between 4 and 700 K. The susceptibility of the $\mathrm{Np}{\mathrm{H}}_x$ compounds is weakly temperature dependent and a crystal-field calculation based on the $5f^4\left({\mathrm{Np}}^{3+}\right)$ ground-state configuration was in reasonable agreement with the experimental results for the dihydride (cubic Ca${\mathrm{F}}_2$-type structure). The cubic plutonium dihydride has a susceptibility maximum, indicative of antiferromagnetic ordering, at 30 K. Cubic plutonium compounds with higher hydrogen concentrations order ferromagnetically with transition temperatures that increase with an increase in $x$ and reach a maximum of 66 K at $x=2.7\mathrm{}$. The hexagonal plutonium trihydride becomes ferromagnetic at 101 K. Although the details of the ordering scheme and a complete crystal-field calculation await a neutron-scattering study, it appears that the most likely ground-state configuration is $5f^5\left({\mathrm{Pu}}^{3+}\right)$.