Intermediate-valence behavior in U2Rh2Sb

TL;DR

This study reports the discovery of U2Rh2Sb as a new intermediate-valence uranium compound, characterized by magnetic, electronic, and thermoelectric measurements, supported by first-principles calculations, expanding understanding of actinide-based intermediate-valence systems.

Contribution

The paper introduces U2Rh2Sb as a novel actinide compound exhibiting intermediate-valence behavior, with comprehensive experimental and theoretical analysis.

Findings

Magnetic susceptibility peak at 50 K indicating valence fluctuations

Energy difference between U3+ and U4+ states approximately 400 K

Metallic electrical resistivity with thermoelectric sign change around 100 K

Abstract

Intermediate-valence behavior is sometimes observed in lanthanide-based materials containing Ce, Yb, Sm or Eu. However, the number of actinide-based systems that exhibit this type of behavior is rather limited. In this work, we present the discovery and characterization of a uranium compound U2Rh2Sb, which turns out to be a candidate for the intermediate-valencebehavior. The material shows a characteristic feature in the magnetic susceptibility around T = 50 K, which can be described within the interconfiguration-fluctuation model of intermediate valence systems. We find the energy difference between the 5f3(U3+) and 5f2(U4+) states to be $\delta$Eex/kB $\approx$ 400 K and the corresponding valence fluctuation temperature to be Tvf $\approx$ 140 K. The value of the electronic specific heat coefficient $\gamma$ = 50 mJ mol-1 U K-2 signals a modest electron effective mass enhancement. The electrical resistivity indicates metallic behavior, albeit with a small residual resistivity ratio. Measurements of thermoelectric properties indicate a change of sign in the Seebeck coefficient around T = 100 K, with a minimum achieved at T = 50 K, which coincides with the broad peak observed in magnetic susceptibility. The experimental results are compared with the theoretical analysis, based on the first-principles calculations, including lattice dynamics.