POINT, LINEAR AND EXTENDED DEFECT STRUCTURES IN NONSTOICHIOMETRIC RUTILE.

TiO//x// minus //x crystalss, prepared at 1323 K and given controlled thermomechanical tretment, were examined by high-resolution electron microscopy (HREM). Crystallographic shear planes (CSP) were not present in non-deformed specimens quenched from 1323 K but appeared, mostly as closely-spaced pairs, in slowly-cooled or deformed and reduced specimens. Lateral and longitudinal disorder in the fine structure of the CSP, which was strongly dependent upon cooling rate was observed. Platelet defects, approximately parallel to left brace 100 right brace , formed at about 400-600 degree C. Defects intermediate in size between point defects and CSP were also discovered. In situ observations revealed some details of the mechanisms of formation and dissolution of CSP and platelets. New interstitial cation structural models were derived using the above observations and electrostatic energy arguments. Linear defects, consisting of two face-shared pairs of octahedra containing trivalent cations are proposed. These have very much lower formation and migration energies than the traditional model. Diffusion mechanisms were derived which allow the linear defects to aggregate and readily form pairs of CSP or platelets without the necessity to nucleate dislocation loops.