Stabilization of $α$-UH$_3$ in U-Hf Hydrides: Structural, Magnetic, Thermodynamic, and Transport Properties
Hf substitution stabilizes the metastable body-centered cubic (bcc) $α$-UH$3$ phase in uranium hydrides, enabling systematic measurements of its magnetic, thermodynamic, and transport properties. (UH$3$)${1-x}$Hf$x$ hydrides ($x = 0.10, 0.15, 0.30, 0.40$) were obtained by hydrogenation of U${1-x}$Hf$x$ precursor alloys. Powder X-ray diffraction shows a progressive suppression of $β$-UH$3$ phase with increasing $x$, with $α$-UH$3$ domination at $x = 0.30$ and $β$-UH$3$ nearly fully suppressed at $x = 0.40$. Magnetization measurements show ferromagnetic behavior for all compositions with Curie temperatures in the range $T\mathrm{C} \approx 178$--$185$ K and a maximum near $x = 0.15$; however, the spontaneous magnetization is strongly reduced with Hf content, decreasing from $1.0,μ\mathrm{B}$/U in pure $β$-UH$3$ to $0.46,μ\mathrm{B}$/U for (UH$3$)${0.60}$Hf${0.40}$. Specific-heat data show a broadened Curie anomaly in the $α$-UH$3$ rich hydride samples, consistent with a distribution of $T\mathrm{C}$ values arising from ferromagnetic inhomogeneities. Specific heat also reveals a monotonic decrease in the Sommerfeld coefficient with increasing Hf concentration, reflecting a reduction in the electronic density of states at the Fermi level, especially in (UH$3$)${0.60}$Hf${0.40}$. The resistivity of (UH$3$)${0.60}$Hf${0.40}$ is very large, exhibits a robust negative temperature coefficient over 2--300 K, and shows only weak magnetoresistance, placing transport in a strongly incoherent, disorder-dominated regime.