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

TL;DR

This study demonstrates how Hf substitution stabilizes the metastable $ extalpha$-UH$_3$ phase in uranium hydrides, enabling detailed analysis of its magnetic, thermodynamic, and transport properties, revealing reduced magnetization and electronic density of states with increasing Hf content.

Contribution

It provides the first systematic investigation of stabilized $ extalpha$-UH$_3$ in U-Hf hydrides, highlighting structural stabilization and property evolution due to Hf substitution.

Findings

Hf stabilizes the $ extalpha$-UH$_3$ phase in U-Hf hydrides.

Magnetization shows ferromagnetism with decreasing spontaneous magnetization as Hf increases.

Transport properties indicate a strongly incoherent, disorder-dominated regime at high Hf content.

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.