Topotactic phase transformation in correlated vanadium dioxide through oxygen vacancy ordering

Controlling the insulator-metal transition (IMT) in correlated oxide system through oxygen vacancy ordering opens up a new paradigm for exploring exotic structural transformation and physical functionality. Oxygen vacancy serves as a powerful tuning knob for adjusting the IMT property in VO2, though driving topochemical reduction to V2O3 remains challenging due to structural incompatibility and competing phase instability. Here we unveil consecutive oxygen-vacancy-driven VO2-VO2-x-V2O3 topotactic phase transformation route with enticing facet-dependent anisotropy, engendering tunable IMT properties over an extended temperature range. Remarkably, topochemically reduced V2O3 inherits the crystallographic characteristics from parent VO2, enabling emergent lattice framework and IMT behavior inaccessible via direct epitaxial growth. Analogous electron doping arising from hydrogenation and oxygen vacancy contributes cooperatively to drive the Mott phase transition in VO2 through band-filling control. Our work not only unveils sequential topotactic phase transformations in VO2 through oxygen vacancy ordering but also provides fundamentally new insights for defect-mediated Mott transitions.

💡 Research Summary

In this work the authors demonstrate a systematic route to control the insulator‑metal transition (IMT) of vanadium dioxide (VO₂) by engineering oxygen vacancies and exploiting their ordering to drive topotactic phase transformations. Using high‑vacuum annealing (≈10⁻⁵ Pa) they first create oxygen‑deficient VO₂₋ₓ at temperatures ≤ 400 °C. X‑ray diffraction shows a leftward shift of the (002) peak from 40.11° to 38.88°, corresponding to a ~3 % out‑of‑plane lattice expansion, while Raman spectroscopy reveals the suppression of V‑V dimer (ω₁ = 194 cm⁻¹, ω₂ = 223 cm⁻¹) and V‑O (ω₃ = 612 cm⁻¹) modes, indicating electron donation to the conduction band and a move toward metallicity.

When the annealing temperature is raised to 700 °C, the oxygen‑deficient VO₂₋ₓ undergoes a second, topotactic transformation into corundum‑structured V₂O₃. XRD peaks associated with the (110) and (006) planes of V₂O₃ appear, and high‑resolution TEM together with FFT analysis confirms that the V₂O₃ domains inherit the crystallographic orientation of the parent VO₂ (specifically the (01̅0)‑oriented domains become (110)‑oriented V₂O₃ twins). This inheritance is a key advantage over direct epitaxial growth, where V₂O₃ typically adopts a single out‑of‑plane orientation.

Electrical transport measurements illustrate the functional impact of these structural changes. Pristine VO₂ displays a sharp IMT at ~339 K. Introducing oxygen vacancies progressively lowers the transition temperature, eventually suppressing the IMT altogether for heavily reduced films. The topotactically reduced V₂O₃ exhibits a thermally driven IMT at 132 K with a resistivity change exceeding five orders of magnitude—far larger than that observed in electron‑beam‑reduced V₂O₃ and lower than the bulk V₂O₃ transition (≈156 K) because the pre‑existing (110) domains relieve strain. X‑ray photoelectron spectroscopy tracks the vanadium valence shift from V⁴⁺ toward V³⁺ and shows an initial increase in defect‑related oxygen species that diminishes once the corundum phase is established.

A striking facet‑dependent anisotropy is reported. VO₂ films grown on c‑plane and r‑plane Al₂O₃ substrates convert to V₂O₃ under the same high‑vacuum conditions, whereas VO₂ on a‑plane TiO₂ only forms VO₂₋ₓ and does not undergo the second transformation. The authors attribute this to a higher oxygen‑vacancy formation energy on the a‑plane TiO₂, which suppresses vacancy concentration and thus blocks the topotactic reduction. Consequently, the IMT behavior on TiO₂ remains that of VO₂, albeit with a reduced transition temperature.

To explore the synergy between oxygen vacancies and hydrogen doping, the authors perform a mild hydrogenation (70 °C, 30 min) on oxygen‑deficient VO₂₋ₓ using a Pt catalyst to promote H₂ dissociation (hydrogen spillover). The combined electron donation from vacancies and protons fully metallizes the film, as evidenced by a further leftward shift of the (020) diffraction peak and pronounced changes in soft X‑ray absorption spectra. The V‑L edge shifts to lower energy and the O‑K edge shows a reduced t₂g/e_g intensity ratio, confirming increased occupation of the t₂g band and a cooperative band‑filling effect that drives the Mott transition to lower temperatures.

Overall, the paper delivers three major insights: (1) a two‑step, vacancy‑mediated topotactic pathway VO₂ → VO₂₋ₓ → V₂O₃ that enables precise tuning of the IMT across a wide temperature window; (2) a clear dependence of the reduction pathway on substrate orientation, highlighting the role of facet‑specific vacancy formation energies; and (3) a cooperative electron‑doping mechanism where oxygen vacancies and hydrogen intercalation act together to modify orbital occupancy and trigger a filling‑controlled Mott transition. These findings open new avenues for defect‑engineered correlated oxides, offering a versatile toolbox for designing low‑temperature switches, energy‑conversion devices, and novel electronic phases that are inaccessible through conventional epitaxial growth alone.