Extended versus small defect equilibria in non-stoichiometric rutile. II. Structural mechanisms for dissolution and reprecipitation of extended defects

High-resolution electron micrographs are presented of a specimen of TiO1.9966, which is previously heated at 750 °C in an electron microscope, in order to dissolve extended defects. This reveals the size and distribution of the small (⪅ 1 nm diameter) defects produced following dissolution of crystallographic shear planes, and also during cooling of the specimen to room temperature. The observed dissolution and reprecipitation phenomena are discussed in terms of new models for the small defects, traditionally cation interstitial and/or oxygen vacancy defects, responsible for the nonstoichiometric phase TiO2−x. Asymmetry between precipitation and dissolution mechanisms should lead to temperature hysteresis.