Oxygen point defect stabilized metastable M3-phase VO₂ films

Point defects can stabilize various metastable polymorph phases of complex oxides showing unique physical properties, even though their role in the stabilization of the metastable phase is unclear. In this study, we examined the role of oxygen point defects in stabilizing a polymorph M3-VO₂ phase and its transition characteristics. Raman spectroscopy revealed the M3-VO₂ phase at the ambient environment for the VO₂ film grown on c-Al₂O₃ substrate at a higher oxygen flow rate. High-resolution XRD and DFT calculation exhibited that the M3-VO₂ phase was stabilized by oxygen point defect incorporated into the tetrahedral interstitial site, the most stable defect type in the monoclinic VO₂ system. The M3-phase had a higher IMT temperature and a sharper IMT than the M1-phase due to the decrease in interdimer hopping energy and the small energy barrier for the IMT by the slightly zigzag V-V chain. Furthermore, the temperature-dependent Raman spectra showed the unprecedented structural phase transition from the M3-VO₂ phase to the R-phase without an intermediate state. Overall, these findings provide fundamental information on the role of oxygen point defects in realizing metastable polymorph phases in promising new technological materials.