On the Phase Transformation of VO2

Abstract

Phase transformation of VO₂ was studied by the optical microscopy, the X-ray difraction and the electrical resistance. Growth of transformed regions from sites of nucleation was observed in micrographs. The critical temperature of the transition and the rate of the growth were dependent on the mechanical strain. Apparent growth velocities were 0.01~1 mm./sec. at heating rates of 0.1~6°C/min. The intermediate phase suggested by Umeda et al with NMR was identified with a phase observed by our experiment. The lattice constants are a=5.80 AA, b=4.52 AA. c=5.38 AA, α=91.5₅°, β=122.7₈°, γ\doteqdot90°. Both the impurities of Al, Fe, and Cr and the reduction were effective for the creation of the triclinic phase. The low temperature phase which is normally monoclinic was found to be triclinic for those crystals which have the wider temperature range of the intermediate phase. The intermediate phase was explained assuming a metastable structure stabilized by the Magneli defect.