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In this work, the thermal transport properties of thorium dioxide (ThO$_2$,thoria) with low levels of substitutional uranium (U) doping are explored. Weobserve strong indications of resonant phonon scattering, an interactionbetween phonons and electronic degrees of freedom, induced by this doping inaddition to common ``impurity'' scattering due to mass and interatomic forceconstant differences. Uranium doping levels of 6\%, 9\%, and 16\% were studiedin a single hydrothermally synthesized U-doped thoria crystal withspatially-varying U doping levels. Within this crystal, isoconcentrationregions with relatively uniform doping were located for local thermalconductivity measurements using a thermoreflectance technique. The measuredthermal conductivity profiles in the temperature range of 77--300~K arecompared to predictions of an analytical Klemens-Callaway thermal conductivitymodel to identify impacts from different phonon scattering mechanisms. Highlysuppressed thermal conductivity at cryogenic temperatures at these dopinglevels suggests that phonon resonant scattering plays an important role inthermal conductivity reduction in U-doped thoria.

Thermal conductivity suppression in uranium-doped thorium dioxide due to phonon resonant scattering