Ab initio calculations of the uranium-hydrogen system: Thermodynamics, hydrogen saturation of α-U and phase-transformation to UH₃

Total energy calculations based on density functional theory (DFT) have been performed for various uranium-hydrogen configurations relevant to the uranium hydriding reaction. Herein, we investigate the transformation of the supersaturated α-U lattice to the α-UH₃ lattice, where α-UH₃ is believed to be a precursor to the formation of β-UH₃, the stable phase of UH₃. The total energy DFT calculations for α- and β-UH₃ were validated by comparing the predicted and measured decomposition temperatures of the hydride at standard pressure. Calculated energies also confirm the metastability of α-UH₃ vs. β-UH₃. Computational group theory and DFT calculations elucidate this transition, and indicate that the transformation itself is kinetically facile. On the basis of this work, it is proposed that the formation of the volume-expanded, H-saturated α-U phase is the primary kinetic barrier to hydride formation.