Pressure induced renormalization of thermal conductivity of Uranium Nitride: A first principles study

The high thermal conductivity of Uranium nitride (UN) makes it one of the best fuels for future fast neutron reactors. Combining density functional perturbation theory and semi classical Boltzman approach, we obtained the anharmonic properties of phonons and electrons, quantitatively. The electrical and thermal properties such as electrical conductivity (sigma), electron thermal conductivity (K-e), lattice thermal conductivity (K-L) and Seebeck coefficient (S) under various external hydrostatic pressures have been calculated as an effective controlling probe. We showed that the application of high compressive hydrostatic pressure increases the electrical and thermal conductivity due to the significant coupling within the deformation potential. Furthermore, this enhancement is addressed by the weakly correlated nature of U-5f electron that treats as the itinerant carriers. The large contribution of U-5f states at Fermi level is accompanied by the Wiedemann-Franz coefficient (L) and reaches to the value of free electron model, exhibits that this model is much applicable at extreme pressures. It is found that Seebeck coefficient depends on the temperature and pressure due to the disparate contribution of electron-phonon coupling at higher temperature. For this complex material, figure of merit is only determined by S-2 parameter. Our results suggest that the external pressure can be exploited as a significant probe to control K-L/K-e ratio for Uranium Nitride based structures on identifying materials with higher figure of merit efficiency than available at present.