Density Functional Theory Study of the Thermodynamic and Raman Vibrational Properties of γ-UO₃ Polymorph

Abstract

Gamma uranium trioxide, γ-UO3, is one of the most important polymorphs in uranium trioxide system, which is common throughout the nuclear fuel cycle and used industrially in the reprocessing of nuclear fuel and uranium enrichment. In this work, a detailed theoretical solid-state density functional theory study of this material was carried out. The computed lattice parameters, bond lengths, bond angles, and X-ray powder pattern were found in very good agreement with their experimental counterparts determined by X-ray diffraction. The equation of state of γ-UO3 was obtained, and therefore, the values of the bulk modulus and its derivatives, for which there are not experimental data to compare with, were predicted. The computed bulk modulus differs from that of a previous density functional theory calculation by only 4.4%. The thermodynamic properties of this material, including heat capacity, entropy, enthalpy, free energy, and Debye temperature were also determined as a function of temperature in the range 0–1000 K. The computed low- and high-temperature thermodynamic functions are in excellent agreement with the experimental ones determined from calorimetric measurements. At ambient temperature, the computed values of heat capacity, entropy, enthalpy, and free energy differ from the experimental values by 5.3, 3.3, 3.9, and 2.6%, respectively. Finally, the Raman spectrum was determined and compared with the experimental one and was found to be in good agreement. A normal-mode analysis of the theoretical spectra was carried out and used in order to resolve the uncertainty of the assignment in the observed Raman bands. The assignment permits to attribute the different bands to vibrations localized in the different distorted octahedra associated with the two nonequivalent uranium atom types present in the structure of γ-UO3.