Phonon-Spin Scattering in U-Doped ThO₂ and its Suppression on Thermal Conductivity

We report on an investigation of thermal conductivity suppression in thorium dioxide with low-level uranium doping. Thermal conductivity of 6%, 9%, and 16% uranium-doped thorium dioxide is experimentally measured using a spatial domain thermoreflectance technique in the temperature range of 77K-300K, and the results are compared to the predictions of an analytical Klemens-Callaway model. With the impacts from different phonon scattering mechanisms isolated in the analytical model, we observed that phonon-spin scattering, i.e., the lattice distortion induced by the spin of paramagnetic electrons, has a stronger impact than the Rayleigh-type point defect scattering on reducing the phonon-mediated thermal conductivity in this temperature range. Moreover, the relative thermal conductivities among our samples with different uranium doping percentages and pure uranium dioxide suggest that the intensity of phonon-spin scattering is not linearly correlated to the doping percentage. This work shows the profound effect of introducing atoms with 5f electrons, populated in uranium but not in thorium, on the thermal transport behavior of fluorite oxides.