Theoretical Study on Structural Stability and Superionic Phase Transition of UH₅ under High Pressure

The thermodynamic, mechanical, and dynamical stability, along with the electronic properties of UH₅ within 30 GPa, are systematically investigated using first-principles calculations. The experimentally synthesized orthorhombic, hexagonal, and cubic phases are all found to be magnetic materials, with spin polarizations of 82%, 100%, and 100%, respectively, and their thermodynamic stability decreases sequentially. Elastic constant and phonon calculations demonstrate that all three phases are mechanically and dynamically stable. Chemical bonding analysis indicates that this stability primarily originates from the prevalent covalent U-H interaction within the lattice. Furthermore, it is predicted that the orthorhombic phase, which has been experimentally quenched to 1 GPa, transforms into a superionic state at 1200 K, where hydrogen ions undergo rapid diffusion within the uranium sublattice interstices, achieving a diffusion coefficient of 1.2×10⁻⁴ cm²/s.