Impact of dynamic Jahn-Teller effect on magnetic excitations, lattice vibration, and thermal conductivity in U x T h 1 − x O 2 system

Vibrational and magnetic properties of single-crystal uranium-thorium dioxide (${\mathrm{U}}_x\mathrm{T}{\mathrm{h}}_{1\ensuremath{-}x}{\mathrm{O}}₂$) with a full range of $0<x<1$ are investigated. Thorium dioxide is a diamagnet whose thermal properties are governed by lattice vibration. The addition of paramagnetic uranium ions leads to the emergence of magnetic effects that alter the thermophysical properties noticeably even at room temperature. The interaction of phonons with magnetic moments of uranium $5f$ electrons mediated by magnetoelastic coupling results in an anomalous low-temperature thermal conductivity profile. Analysis of the magnetic susceptibility measurements indicates a departure from the Curie-Weiss relationship characteristic of noninteracting paramagnetic ions, previously associated with the dynamic Jahn-Teller (DJT) effect characterized by coupling between spin and the oxygen sublattice. The ${\mathrm{T}}_{2\mathrm{g}}$ Raman peak position follows a nonlinear trend as a function of uranium concentration and hints that these Raman active optical modes play a role in either DJT or in mediating quadrupole-quadrupole interactions. A first-principle-based thermal transport model is implemented to explain the low-temperature transport measurements, where the anomalous reduction is attributed to phonon-spin resonant scattering. The interplay between spins and phonons is also captured using high-resolution inelastic x-ray scattering (IXS) measurements of phonon linewidths. Our results provide insights into the phonon interactions with the magnetic excitations governing DJT effect and impacting the low-temperature thermal transport processes in this material system. These findings have implications for understanding low-temperature thermal transport and magnetic properties in advanced materials for information processing and energy applications.