First-principles study of elastic and thermodynamic properties of UO₂, γ-UO₃ and α-U₃O₈

A detailed solid-state density functional theory study of uranium dioxide (UO₂), gamma uranium trioxide (γ-UO₃), and alpha triuranium octoxide (α-U₃O₈), which are very common materials throughout the nuclear fuel cycle, was carried out. DFT simulations of these three oxides were presented using both PBE+U and LDA+U functionals to examine their mechanical and thermodynamic properties. The computed lattice parameters were found to be in very good agreement with previous experimental results. The values of elastic properties like bulk, shear, and Young's modulus, as well as Poisson's ratio, were predicted, and the calculated Debye temperature was reported. The elastic anisotropy of UO₂, γ-UO₃, and α-U₃O₈ was calculated for the first time, from which the anisotropic degrees of these oxides were revealed by the values of anisotropic parameters and three-dimensional surface constructions of elastic properties. In addition, thermodynamic properties of UO₂, γ-UO₃, and α-U₃O₈, including heat capacity, entropy, enthalpy, phonon free energy, and Gibbs free energy, were determined as a function of temperature in the range of 0 - 2000 K. The computed thermodynamic values show an overall good agreement with available experiments, especially at low temperatures. The obtained physical parameters can provide meaningful data for further experimental investigations and promote the development of nuclear fuel cycles.