The intermediate steps of the phase transition between the metastable monoclinic VO 2 (B) phase and the stable tetragonal rutile VO 2 (R) phase have been studied by in situ electron microscopy. A crystallographic interpretation based on the static concentration waves theory is proposed: as temperature increases, the long-range order in the complex monoclinic VO 2 (B) phase is lost and gradually a first intermediate ill crystallized phase with a drastically reduced symmetry is formed as evidenced from the diffraction patterns. Next, a new tetragonal phase is generated that corresponds to a state where some of the vanadium atoms are now in a disordered state. When annealing inside the microscope, this phase grows out into a superstructure, with coexistence of two possible orientation variants. In all these phases the VO 6 octahedra remain virtually parallel, but for the final transition around 450°C into the rutile stable phase, half of the octahedra have to reorient; the transition therefore has the aspects of a reconstructive one as is evident from the in situ experiment.

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