The Interplay Between Phonon and Magnetic Excitations and Its Impacts on Low-Temperature Thermal Transport in Mixed Uranium Oxides

The low-temperature anomalous magnetic and thermal properties of uranium dioxide along with its nuclear fuel applications, make it one of the most extensively studied. In this study, we investigate the low-temperature thermal transport properties of uranium-thorium mixed oxide (U-ThO₂) with a wide range of varying thorium concentrations. The thermal conductivity was measured down to cryogenic temperatures using the spatial domain thermoreflectance technique. The interaction between electron spin-waves and magnetoelastic coupling of the electric quadrupoles to phonons results in a unique low-temperature thermal conductivity profile. The presence of magnetic excitation effects above Neel temperature is suggestive of a dynamical Jahn-Teller effect and the phonon-magnon coupling in the paramagnetic phase. The potential interplay between spins and phonons is also captured using high-resolution inelastic X-ray scattering (IXS) measurements. A first principle-based thermal transport model is implemented to isolate the influence of magnetic excitations on phonon lifetime from the experimental results. Analysis of thermal transport and T_2g Raman mode behavior indicate two distinct spin-phonon scattering mechanisms regimes. Our results provide new insights into the spin-lattice interaction that dominates the low-temperature thermal transport processes in UO₂ and have implications for the optimization of nuclear fuel materials.