First-PrinciplesInvestigate the Oxygen VacanciesAffected Reversible Metal-to-Insulator Transition in NonstoichiometricNbO_2–<i>x</i>

The metal-to-insulator transition (MIT) in niobium oxide (NbO₂) regulated by oxygen vacancies (V_Os) is of limited understanding. Here, we systematically investigate the reversible V_Os-MIT, focusing on structural, polaron, electronic, and chemical bonding characteristics. Interestingly, the V_Os modulate Nb–Nb dimers and renormalize the charges on small polarons. V_Os strongly perturb the high-symmetry metal phase and split d_∥ orbitals while only broadening local dimers on the insulating phase. Driven by the cooperative Mott–Peierls transition, both phases structurally and electrically change toward the metastable NbO_2–<i>x</i>(BCT-like) as increasing V_Os, possibly responsible for the decreased voltages and transition temperatures. This work motivates further defects engineering on modulating MIT for NbO₂-based neuron devices.