Investigation of phase transition in self-flux grown VO2 single crystals

Abstract

We report the growth and phase transition of VO₂ single crystals. Millimeter-sized VO₂ single crystals were obtained by a self-flux method, in which the molten phase of V₂O₅ at 1000 °C was slowly cooled to room temperature at a rate of 5 °C/h. Rietveld analysis of X-ray powder diffraction shows a low-temperature monoclinic structure with the V–V dimer spacings of 2.57 Å and 3.20 Å and a high-temperature rutile structure with the V–V spacing of 2.85 Å. Raman scattering measurement suggests that the VO₂ single crystal is highly stoichiometric with negligible oxygen deficiency. Electrical resistivity, magnetic susceptibility, specific heat, and heat flow measurements show drastic changes during heating (~ 338 K) and cooling (~ 334 K) with a narrow temperature range (~ 1 K). The low-temperature insulating phase with negligible magnetic susceptibility changes to a high-temperature metallic phase with Pauli paramagnetic susceptibility. The enthalpy change during the phase transition was estimated to be ~ 87 J/g by specific heat and differential calorimetry measurements. This work provides useful quantitative information for the phase transition of VO₂ single crystals.