Characterisation of the temperature-dependent M1 to R phase transition in W-doped VO2 nanorod aggregates by Rietveld refinement and theoretical modelling

Abstract

Understanding the mechanism of the insulator–metal transition (IMT) in VO₂ is a necessary step in optimising this material's properties for a range of functional applications. Here, Rietveld refinement of synchrotron X-ray powder diffraction patterns is performed on thermochromic V_1−xW_xO₂ (0.0 ≤ x ≤ 0.02) nanorod aggregates over the temperature range 100 ≤ T ≤ 400 K to examine the effect of doping on the structure and properties of the insulating monoclinic (M₁) phase and metallic rutile (R) phase. Precise measurement of the lattice constants of the M₁ and R phases enabled the onset (T_on) and endset (T_end) temperatures of the IMT to be determined accurately for different dopant levels. First-principles calculations reveal that the observed decrease in both T_on and T_end with increasing W content is a result of Peierls type V–O–V dimers being replaced by linear W–O–V dimers with a narrowing of the band gap. The results are interpreted in terms of the bandwidth-controlled Mott–Hubbard IMT model, providing a more detailed understanding of the underlying physical mechanisms driving the IMT as well as a guide to optimising properties of VO₂-based materials for specific applications.