High-resolution electron microscopy of (210) defect pairs in gallia-doped rutile

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Abstract
The (210)[½ ¼ ½] crystallographic shear planes (CSP) which occur in gallia-doped rutile have been examined at 3.2 Å resolution. At low dopant levels isolated pairs of (210) boundaries having well-defined spacings of 4.8 Å, and stoichiometry Ga4TiO8, occur. At higher dopant levels single boundaries, regularly spaced, and up to 35 Å apart, form members of the homologous series of structures Ga4Tim−4O2m-2. However, even in the ordered structures low concentrations of Ga4TiO8 defect pairs occur. The latter are sometimes associated with voids in the structure. A mechanism for splitting of pairs into separate single faults is proposed. This allows sideways movement and ordering of single boundaries. An image of the dislocation structure at the termination of a Ga4TiO8 pair inside an ordered structure Ga4Ti25O56 provides the first evidence that the CSP are extrinsic, involving insertion of extra metal atoms, rather than intrinsic, involving oxygen removal from sites within the crystal. It is shown that the Burgers vector of the dislocation terminating the defect pairs is [0 ½ 0]. This resolves the apparent discrepancy between the displacement vector determinations of Gibb and Anderson (1972), who found [0 ½ 0] for isolated faults, and the present authors who found [½ ¼ ½] for single boundaries in ordered Ga4Tim-4O2m-2. Atomic mechanisms for the growth of Ga4TiO8 defect-pairs and the core structure of the partial dislocation are discussed. A mechanism is proposed whereby ion-exchange 4Ga3+⇄3Ti4+ occurs at the growing tip without the necessity for long-range diffusion of oxygen or oxygen vacancies.