From VO2(B) to VO2(R):: Theoretical structures of VO2 polymorphs and in situ electron microscopy

The intermediate steps of the phase transition between the metastable monoclinic VO2(B) phase and the stable tetragonal rutile VO2(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 VO2(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 VO6 octahedra remain virtually parallel, but for the final transition around 450 degrees 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.