The hydrogen-induced phase transition of α-uranium to uranium hydride proceeds through a metastable intermediate $\alpha \text{−}\mathrm{U}{\mathrm{H}}_{3-x}$ phase, bridging the structural gap between α-U and $\alpha \text{−}\mathrm{U}{\mathrm{H}}_3$. Ab initio molecular-dynamics simulations combined with machine-learned smooth overlap of atomic positions descriptors reveal that elevated hydrogen concentrations promote a stepwise reorganization of uranium and hydrogen sublattices. This intermediate phase exhibits dynamical stability, with hydrogen preferentially occupying tetrahedral interstitial sites and uranium lattice symmetry evolving from orthorhombic to tetragonal and ultimately cubic. Moreover, the energy barriers for each phase transition step and the influence of hydrogen partial pressure and temperature on this process have been clarified. The study provides direct dynamical evidence of the metastable intermediate, elucidating the kinetic and symmetry-breaking pathways underlying the hydrogen-induced transformation. These insights advance the fundamental understanding of uranium hydride formation under realistic conditions and emphasize the critical role of intermediate phases in phase transitions.

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• Phase transitions
• Actinides
• DFT+U
• First-principles calculations
• Molecular dynamics