A melting-like process and the local structures during the anatase-to-rutile transition

Abstract

Phase transition of titanium oxide from anatase to rutile was investigated by using transmitted electron microscope (TEM), scanning electron microscope (SEM) and X-ray diffraction (XRD), in combination with the atomic pair distribution function (PDF) technique. The transition range is about 528–805 °C and the critical size is ~73 nm for the sol-gel samples calcined for 6 h. A melting-like process was evidently observed in the transition samples by SEM and it will speed up the phase transition. PDF reveals that anatase crystallites are packed by neighbouring TiO6 octahedra in a cis-coordinated way. Anatase crystallites larger than the critical size will be unstable and transform into rutile in a trans-coordinated way by torsion and rotation of part of octahedra and it will rapidly spread the entire crystallites driven by a melting-like process. We suggest the maximum size of anatase will be more technically useful for the description of the phase stability from anatase to rutile because unstable small anatase crystals are consumed during longer calcination process to grow the larger anatase crystals.