Elastic constants, phonon density of states, and thermal properties of UO₂

The elastic properties and phonon density of states of UO₂ have been studied by first-principles spin-polarized electronic-structure calculations in both the local density approximation (LDA) and the generalized-gradient approximation (GGA) for the experimentally determined antiferromagnetic spin configuration. Calculations have also been done both with and without Hubbard corrections (LDA + U and GGA + U). The elastic properties and phonon density of states are in very good agreement with experimental measurements when the Hubbard correction is included. The elastic constants and low-frequency (acoustic mode) phonons are in reasonably good agreement with experiment for all the different calculations. However, when Hubbard corrections are not included, the high-frequency phonons are pushed to lower frequencies and the optical phonons are significantly underestimated. The melting temperature is approximated by using an empirical equation, which uses elastic constants as input parameters, and is in good agreement with experiment. The first-principles calculations are also used to obtain the specific heat and entropy within the harmonic approximation at finite temperatures. It is shown that harmonic approximation is valid up to room temperature. The Debye temperature is estimated using two different methods. The predicted values are in excellent agreement with experimental results. It is shown that inclusion of the spin-orbit interaction does not significantly alter either the elastic or thermal properties.