The mechanism of phase transformation in nanocrystalline TiO 2 powders at ambient temperature during high energy ball milling and the role of brookite phase in anatase-to-rutile phase transformation were investigated by the use of Rietveld analysis of X-ray diffraction patterns and Raman spectroscopy methods. The milling process was performed on a fully anatase phase nanocrystalline TiO 2 powder via a high energy planetary ball mill with different ball-to-powder weight ratios followed by annealing of the as-milled samples. Some transformation from anatase-to-brookite was observed in all as-milled powders by high resolution transmission electron microscopy. It was proposed that brookite occurs at the {112} twin surfaces of anatase phase and therefore promotes anatase-to-rutile phase transformation. Based on the XRD and Raman results, it was proposed that brookite appears as an essential intermediate phase in mechanically induced anatase-to-rutile phase transformation which facilitates the phase transformation at ambient temperatures and also at higher temperatures during the post-annealing step.
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