Pure hydride of the $\alpha \text{−}\mathrm{U}{\mathrm{H}}_3$ type without any $\beta \text{−}\mathrm{U}{\mathrm{H}}_3$ admixture was prepared by high-pressure hydrogenation of bcc U stabilized by Zr. Such material, characterized by a general formula ${\left(\mathrm{U}{\mathrm{H}}_3\right)}_{1\text{−}x}\mathrm{Z}{\mathrm{r}}_x$, is stable in air at ambient and elevated temperatures. H release is observed in the range of 400–600 °C similar to $\beta \text{−}\mathrm{U}{\mathrm{H}}_3$. Its stability allowed us to measure the magnetic properties, specific heat, and electrical resistivity in a wide temperature range. Despite the rather different crystal structure and inter-U spacing, the electronic properties are almost identical to $\beta \text{−}\mathrm{U}{\mathrm{H}}_3$. Its ferromagnetic ground state with Curie temperature $T_{\mathrm{C}}\approx 180\mathrm{K}$ (weakly and nonmonotonously dependent on Zr concentration) and U moments of $1.0{\mu }_{\mathrm{B}}$ indicate why mixtures of α- and $\beta \text{−}\mathrm{U}{\mathrm{H}}_3$ exhibited only one transition. Magnetic ordering leads to a large spontaneous magnetostriction ${\omega }_{\mathrm{s}}=3.2\times {10}^{-3}$, which can be explained by the increase of the spin moment between the paramagnetic (disordered local moment) and the ferromagnetic states. The role of orbital moments in magnetism is indicated by fully relativistic electronic structure calculations.

• Received 23 December 2014

DOI:https://doi.org/10.1103/PhysRevB.91.115116