New-phase VO2 micro/nanostructures: investigation of phase transformation and magnetic property

Abstract

New-phase VO₂ micro/nanostructures built by nanoflakes have been first synthesized by a hydrothermal method with NH₄VO₃ as precursor in the presence of poly(vinyl pyrrolidone) (PVP). The combined structural analysis of X-ray powder diffraction (XRD) and X-ray absorption fine structure (XAFS) spectroscopy determined the crystal structure as a new-phase vanadium dioxide, which is the isostructure of monoclinic NiWO₄ and designated as VO₂(D). In particular, electron spin resonance (ESR) measurements provides the direct evidence of vanadium ion in the four oxidation state. The formation energy of VO₂(D) was estimated and showed a very close value to rutile-type VO₂(R), which guided the preparation of VO₂(R/M) by making use of the structural transformation from VO₂(D) to VO₂(R) at 320 °C, which was a comparatively lower temperature than other vanadium oxide, such as VO₂(B). The obtained VO₂(R) shows the reversible metal-to-insulator transition (MIT) near critical temperature (T_c) which is associated with clear changes in differential scanning calorimetry (DSC) curves. In addition, the temperature-dependent DC electrical conductivity of the new-phased VO₂(D) exhibits Arrhenius-type behaviour, which reveals its semiconducting character with a band gap of 0.33 eV. ESR and temperature-dependent magnetic susceptibility measurements were employed to obtain information on the magnetic properties of VO₂(D), which correspond to one-dimentional Heisenberg system.