Ab initio investigations on lattice dynamics and thermal transport properties of ThO2

Abstract

Thorium dioxide (ThO2) is considered as a potential substitute for uranium dioxide due to its higher melting point, higher thermal conductivity and lower thermal expansion coefficient. In this paper, based on first-principles calculations and Boltzmann transport equations (BTE), we calculate the temperature-dependent phonon dispersion and lattice thermal conductivity using self-consistent phonon (SCP) theory and the results are in good agreement with experimental values. It is worth noting that the optical vibration modes in the range of 5.5 THz to 10 THz harden significantly with the increase of temperature, leading to the gradual separation of the overlapping of the longitudinal acoustic branch and the transverse optical branch, and finally generate a band gap of 0.2 THz at 1500K, which was not found in previous studies on ThO2. After verifying the accuracy of SCP + BTE method in calculating the lattice thermal conductivity of ThO2, we predict the lattice thermal conductivity of cubic phase ThO2 in the range of 0 GPa to 30 GPa at 300K. The lattice thermal conductivity of ThO2 is 14.4 W/mK at 0 GPa and will increases to 61.9 W/mK at 30 GPa. Analysis of pressure-related thermal transport parameters reveals that the large increase in lattice thermal conductivity is mainly due to a nearly threefold increase in phonon relaxation time from 0 GPa to 30 GPa. Our analysis is of great significance for understanding the thermodynamic behavior of this new nuclear fuel at high pressure. It makes up for the lack of study on the thermal transport properties of cubic phase ThO2 under high pressure.