High resolution microscopy study of [001] symmetric tilt boundary with a tilt angle of 66° in rutile-type TiO ₂ bicrystal

The grain boundary structure in a rutile-type TiO ₂ bicrystal with a [001] symmetric tilt boundary having a tilt angle of 66° was investigated. The tilt angle of this boundary deviates by 1.4° from an exact ∑13a to ∑17a relation. High-resolution transmission electron microscopy (HRTEM) has revealed that the grain boundary was free from any secondary phases, and the two adjoining crystals contacted each other perfectly at an atomic scale. The boundary showed almost straight feature without any step structures though a part of the boundary formed facet structures consisting of low index planes such as {310} and {110}. In addition, the weak-beam dark field observation revealed that the contrasts due to strong strain fields existed on the grain boundary plane with a spacing of 7.6 nm. As the result of atomic structural analysis using HRTEM, the strain field was found to result from a distorted ∑13a unit structure. This distorted unit structure was similar to ∑17a structure. Namely, the boundary consisted of a periodical array of major ∑13a unit structures and minor ∑17a-like unit structures. The deviation angle from the exact ∑13a relation in this boundary was not accommodated by DSC dislocations but by introducing different structural units.