Twin-corner disclinations in YBa 2 CU 3 O 7-?

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Abstract
Twins form on (110) and (110) planes in yttrium barium copper oxide. The twins usually lie on only one of these possible planes in any single grain, but occasionally both twin systems are observed within a grain. When this occurs, the terminations of one set of twins on the coherent twin planes of the other take a characteristic form, with the twins tapering down nearly to zero thickness. We have obtained high-resolution transmission electron microscopy images of these twin terminations, which clearly indicate that the terminating twins embody both possible twin variants, with coherent interfaces on two nearly perpendicular planes. The corners where these interfaces meet are well described as wedge disclinations of small rotation angle. We discuss the shapes of the terminating twins in terms of the interactions between pairs of adjacent disclinations. The shapes of the terminating twins are explained through application of the disclination model.