FromVO2(B)toVO2(R): Theoretical structures ofVO2polymorphs andin situelectron microscopy

The intermediate steps of the phase transition between the metastable monoclinic ${\mathrm{VO}}_2\mathrm{}\left(B\right)\mathrm{}$ phase and the stable tetragonal rutile ${\mathrm{VO}}_2\mathrm{}\left(R\right)\mathrm{}$ 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 ${\mathrm{VO}}_2\mathrm{}\left(B\right)\mathrm{}$ 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 ${\mathrm{VO}}_6$ 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.