Local Coordination and Site-Resolved Charge Distribution in Amorphous Ni0.75Fe0.25O from First-Principles Molecular Dynamics

Amorphous NiFe oxides are highly active precursors for the alkaline oxygen evolution reaction, yet their local coordination and the site-to-site variation of the Ni charge remain poorly characterized at the first-principles level. Melt-quench ab initio molecular dynamics (AIMD) and spin-polarized DFT+U calculations on amorphous Ni0.75Fe0.25O show pronounced Ni undercoordination relative to Fe. The Ni Bader charges vary continuously and increase with the Ni–O coordination number, whereas the Fe charges form a single population. The two-means boundaries of the Ni and Fe distributions place nearly identical fractions of sites above them (63% for Ni), indicating that these boundaries are cuts in continuous charge distributions rather than divisions between discrete valence states. ΔSCF calculations predict an O 1s binding energy about 0.9 eV lower for under-coordinated oxygen than for lattice oxygen. A machine-learned interatomic potential trained on the AIMD trajectories reproduces DFT forces to within twice its training error up to x = 0.625 in Ni1–xFexO. Simulations based on it indicate that Ni undercoordination and the coordination dependence of the Ni charge persist over a 16-fold range of quench rates and across 0 ≤ x ≤ 0.5, where the O coordination stays nearly constant while both cations lose coordination as Fe increases.