Ab initio calculations of the uranium–hydrogen system: Thermodynamics, hydrogen saturation of α-U and phase-transformation to UH 3

Abstract

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-UH 3 lattice, where α-UH 3 is believed to be a precursor to the formation of β-UH 3, the stable phase of UH 3. The total energy DFT calculations for α- and β-UH 3 were validated by comparing the predicted and measured decomposition temperatures of the hydride at standard pressure. Calculated energies also confirm the metastability of α-UH 3 vs. β-UH 3. 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.