Impurity effects on the phase transformation from anatase to rutile were studied under normal atmospheric conditions.
The rate equation of this phase transformation is explained by the Avrami model. Impurities of metal oxides having MO₆ octahedra in a structural unit plays a very important role on anatase-rutile transformation, and V₂O₅ reacts as an accelerator, while WO₃, Nb₂O₅, Ta₂O₅ and Cr₂O₃ are of inhibitors.
The activation energy of this phase transformation was given to be 50 kcal/mol from pure anatase, 35 kcal/mol from antase stabilized by H₃PO₄, and 125 kcal/mol from anatase added W0₃ as an impurity. The value of the activation energy from pure antase was about half of the smallest one reported previously. The variation of the activation energy reflects the difference in the inhibiting mechanism of anatase stabilized by H₃PO₄ or WO₃.