FACETING, DISLOCATION NETWORK STRUCTURE, AND VARIOUS SCALES OF HETEROGENEITY IN A YBA2CU3O7-GAMMA LOW-ANGLE-[001] TILT BOUNDARY

The grain boundary topography and grain boundary dislocation network s tructure of a 6 degrees [001] bicrystal of YBa2Cu3O7-delta were studie d using diffraction-contrast transmission electron microscopy (TEM). S aw-tooth-shaped arrays of facets composed of facets with lengths of a few tens of nanometers were observed in each of two widely separated s ections of the boundary. The facet planes were {110}, {310}, and {22(1 ) over bar}. Further subfaceting of the (130) facets into a smaller-sc ale (a few nanometers) saw-tooth configuration of (010) and (110) face ts produced a hierarchy of facets in at least one boundary section. Th e dislocation content observed in each type of facet agreed well with Frank's formula. However, the dislocations within individual facets fr equently were inhomogeneously distributed, contrasting the picture of evenly spaced dislocations that is derived for boundaries of infinite extent, Certain types of dislocations repeatedly were grouped near the facet centers and ends. Well-separated partial dislocations frequentl y were observed near the facet midsections, but not near the facet jun ctions. Extended (similar to 30 nm) strain contrast was observed at al l of the facet junctions formed by facets with dimensions on the order of tens of nanometers. This long-range strain may be due to the finit e extent of the individual facets. These results all suggest that stru ctural inhomogeneities occur on various length scales ranging from mac roscopic to just a few nanometers. Such structural heterogeneity is co nsistent with the electrical heterogeneity that is indicated for many YBa2Cu3O7-delta grain boundaries.