A comparative analysis of the phonon properties in UO2 using the Boltzmann transport equation coupled with DFT + U and empirical potentials

Abstract

The vibrational and thermal properties of UO2 are determined by the solution of the Boltzmann transport equation (BTE) for phonons. The BTE approach relies on interatomic force constants derived from atomic force calculations. Density functional theory (DFT) calculations provide a reliable approach to compute interatomic forces. However, the calculation of interatomic forces using DFT is computationally demanding, and thus significantly limiting its applications. In such cases, empirical potential (EP) calculations are an efficient alternative. However, EPs are not usually constructed to correctly describe lattice vibration properties. In this work, we perform a comparison of lattice vibrations and thermal properties of UO2 solving the BTE using DFT + U and EP computed forces. The experimental lattice thermal conductivity of UO2 is found to be correctly described by DFT + U, while it is only approximated by the Cooper and Potashnikov potentials. However, the heat capacity is only closely reproduced by DFT + U and the Potashnikov potential. In contrast, significant errors were obtained with the Arima and Yamada potentials in all the computed results. The discrepancies in the predicted thermal properties by interatomic potentials were determined to be related to inaccuracies in the description of phonon properties.