Stabilization of $\alpha$-UH${}_3$ in U-Hf Hydrides: Structural, Magnetic, Thermodynamic, and Transport Properties

Abstract

Hf substitution stabilizes the metastable body-centered cubic (bcc) $\alpha$-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 $\beta$-UH$3$ phase with increasing $x$, with $\alpha$-UH$3$ domination at $x=0.30$ and $\beta$-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 $1.0,\mu \mathrm{B}$/U in pure $\beta$-UH$3$ to $0.46,\mu \mathrm{B}$/U for (UH$3$)$0.60$Hf$0.40$. Specific-heat data show a broadened Curie anomaly in the $\alpha$-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.