Transmission electron microscope study of defect structures accommodating non-stoichiometry in rutile

Rutile is the thermodynamically stable form of TiO2 which is the highest oxide of the titanium-oxygen system, Due to the ability of rutile to depart from stoichiometry under reduced oxygen pressure, which is correlated with interesting optical and electrical properties, study of defect structures accomodating non-stoichiometry in rutile has appeared of considerable interest, Here is presented the prominent role that transmission electron microscopy has played in identifying those defect structures, Conventional TEM observations of rutile single crystals reduced under equilibrium to well controlled compositions have definitely established the fact that sight departures from the TiO2 stoichiometric composition are accommodated at sufficiently high temperatures by small defects (traditionally called "point" defects) forming a solid solution with the perfect rutile matrix, whereas extended defects, mainly crystallographic shear planes (CSP), form at lower temperatures, After numerous studies concerned with thermodynamical, and transport properties of TiO2-x the nature of the majority defect in the solid solution TiO2-x namely titanium interstitial or oxygen vacancy, could be still regarded as a matter of debate, It is shown here how high resolution TEM imaging of fine structure of CSP and of small defect aggregates have suggested new structural models for titanium interstitial and oxygen vacancy defects, in light of results from recent physicochemical studies and theoretical calculations.