DIRECT OBSERVATION OF DISORDER WITHIN CRYSTALLOGRAPHIC SHEAR DEFECTS IN DEFORMED AND NON-STOICHIOMETRIC RUTILES

Abstract

High resolution electron microscope images have been obtained at ~2.3 Å point resolution showing mechanical twin lamellae with associated twin dislocations in stoichiometric TiO2.0000 and also a variety of novel crystallographic shear (CS) behaviours in a deformed non-stoichiometric rutile TiO1.9966. In many cases these images proved to be directly interpretable in terms of the positions of Ti atom rows viewed along the [100]r and [1[MATH]1]r projection axis. Thus structural models for a variety of defects could be deduced from the images. In situ observations of the annealing of mechanical twins, obtained at lower resolution using an environmental cell in a 1 MeV electron microscope are also described. New types of CS behaviour include : identification of (143)r CS planes (CSP) lying between the well-known (132)r and (011)r orientations ; disorder within CSP with respect to orientation, both uniform and non-uniform mixing of (121)r - and (011)r - type steps occur along individual boundaries ; the observation of closely-spaced pairs of CS boundaries containing varying degrees of disorder with regard to both orientation and the structure within the pairs ; evidence that defects other than CSP occur in slightly reduced rutile, which give rise to modulations of lattice fringes along both k (011)r and k (110)r, and other as yet uninterpreted contrast, but suggestive of poinf defect clustering