Impact of Dynamic Jahn-Teller Effect on the Thermal Transport in U_xTh_1-xO₂ System

The complex interplay between phonon and magnetic excitations results in intriguing thermal transport properties, particularly at low temperatures where spin-lattice interactions dominate. These phenomena are critical for understanding materials in applications that require precise thermal management, from sustainable energy systems to space technologies. In this work, we explore the impact of phonon interaction with the spin wave of paramagnetic ions. We investigate the thermal transport properties of U_xTh_1-xO₂ (with varying 0<x<1), where the presence of paramagnetic UO₂ results in unique low-temperature thermal conductivity profiles, obtained through spatial domain thermoreflectance technique. Along with magnetic susceptibility data, these results suggest the presence of a dynamic Jahn-Teller effect that persists well above the Néel temperature. A strong perturbation of the Raman-active T_2g mode further hints at the presence of Jahn-Teller distortions. To develop an accurate description of this phenomenon we developed a first-principle based thermal conductivity model that validates our experimental findings. Phonon linewidth data from high-resolution Inelastic X-ray scattering and the thermal conductivity model prediction, both suggest phonon coupling to the low-energy spin transition of the paramagnetic ion. Furthermore, our analysis across varying U-Th concentrations indicates two contrasting spin-phonon scattering mechanisms near the critical paramagnetic doping concentration (~58% U). Our findings offer new insights into the influence of the dynamic Jahn-Teller effect and the fundamental spin-lattice interactions that govern low-temperature thermal transport.