First-Principles Investigate the Oxygen Vacancies Affected Reversible Metal-to-Insulator Transition in Nonstoichiometric NbO2–x

The metal-to-insulator transition (MIT) in niobium oxide (NbO2) regulated by oxygen vacancies (VOs) is of limited understanding. Here, we systematically investigate the reversible VOs-MIT, focusing on structural, polaron, electronic, and chemical bonding characteristics. Interestingly, the VOs modulate Nb–Nb dimers and renormalize the charges on small polarons. VOs 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 NbO2–x(BCT-like) as increasing VOs, possibly responsible for the decreased voltages and transition temperatures. This work motivates further defects engineering on modulating MIT for NbO2-based neuron devices.