Pressure- and temperature-driven local structural stability and 5f/d hybridization in a metastable U-5.28 wt% Nb...

Metastable U-Nb alloys near 6 wt% Nb are U-rich actinide metallic materials in which Nb substitution, lattice distortion, vibrational stability and U-5f electron delocalization are strongly coupled. While high-pressure experiments and simulations have clarified important aspects of U-6Nb and ordered U-Nb compounds, the atomistic stability response of dilute U-5.28 wt% Nb under pressure and temperature perturbations remains less resolved. Here, low-enthalpy structure searches, phonon spectra, projected density of states (PDOS) and AIMD-derived finite-temperature configurations are combined to reveal how compression and heating reorganize local stability. The pressure-selected structures show a re-entrant monoclinic-to-orthorhombic-to-monoclinic sequence between 50 GPa and 300 GPa. Monoclinic cells are obtained at 50 GPa, 100 GPa, 250 GPa and 300 GPa, whereas orthorhombic cells occur at 150 GPa and 200 GPa. This non-monotonic symmetry evolution indicates that intermediate compression promotes more efficient packing, while stronger compression restores the energetic role of local monoclinic distortion. Phonon calculations support these pressure-selected structures as dynamically stable local minima and show overall pressure-induced mode hardening. Finite-temperature AIMD relaxation gives a low-symmetry configuration at 773 K, a dynamically unstable configuration at 1023 K, and a higher-symmetry dynamically stable configuration at 1273 K. PDOS analyses show broadening of U-5f-derived states and enhanced U-5f/U-6d/Nb-4d hybridization under compression and heating. These results establish an atomistic stability framework linking structural selection, phonon response and 5f/d hybridization in metastable U-5.28 wt% Nb under extreme conditions.