Thermal transport in defective uranium nitride: Effects of point defects, anharmonicity, and electronic contributions

The impact of point defects on thermal transport in uranium nitride (UN) is investigated using a machine learning interatomic potential combined with Green–Kubo (GK) and normal-mode analysis methods over 300–1500 K. In pristine UN, temperature-dependent calculations of lattice thermal conductivity reveal that four-phonon scattering is essential yet sufficient to accurately capture high-temperature anharmonic phonon transport, as evidenced by close agreement between GK and ShengBTE calculations, including three- and four-phonon processes. In defective systems, all types of point defects significantly reduce thermal conductivity at low temperature. Mode-resolved analysis further shows that interstitial defects introduce new phonon states due to a stronger local strain effect. Notably, the uranium interstitial leads to strong defect-phonon scattering over a broad phonon spectrum, while the other point defects produce more selective scattering, with even reduced phonon scattering for some acoustic modes. The optical contribution to thermal conductivity remains nearly constant in the presence of IU, but decreases with increasing temperature for pristine and the other defect types. The total thermal conductivity, incorporating electron–phonon coupling and an estimated electronic contribution from the Wiedemann–Franz law, yields excellent agreement with experiment in the pristine system, with electronic contributions dominating thermal transport above ∼600 K. Moreover, with defect-electron contribution introduced through a semiclassical electron–defect scattering model, it is found that (i) the total conductivity degradation follows the order of uranium interstitials > uranium vacancies > nitrogen interstitials > nitrogen vacancies and (ii) electron–phonon coupling becomes negligible in defective systems. These results provide a unified understanding of defect-dependent thermal transport in UN.