A collection of published data on grain boundaries and planar defects in this oxide, together with data on boundaries in many-to-one epitaxial thin films, was analyzed in order to relate the critical current density across a boundary to its energy, geometrical form and nanoscopic structure. As there were many data which showed that not all high-angle boundaries had detrimental effects upon the critical current density, and exhibited weak-link behavior, the relevance of various types of boundary was emphasized with regard to their nanoscopic structure and to the trans-boundary critical currents. It was noted that not all high-angle boundaries were the same. Some high-angle but low-energy boundaries could sometimes support high critical currents, such as the  = 3 90 boundaries and  = 5,  = 17, off- = 13, 45 [001] tilt boundaries which were found in films grown by many-to-one epitaxy. The geometrical parameters could be useful in predicting to some extent the energetics of the boundaries and, therefore, together with a thermodynamic criterion, furnished useful criteria for explaining why some boundaries were clean and some were preferred sites for the precipitation of extraneous phases. Translational boundaries, stacking faults and twin boundaries had low energies, were atomically clean and were shown to support high critical current densities. It was concluded that a nanoscopically clean boundary was a necessary but not a sufficient condition for a high trans-boundary critical current density. In addition to the geometrical parameters, processing conditions and other extrinsic factors could also affect the trans-boundary critical current density. Examples were the 45 boundaries which resulted from bi-epitaxy and 90 boundaries at substrate etched steps; both of which were used as weak-link junctions.

S.W.Chan: Journal of the Physics and Chemistry of Solids, 1994, 55[12], 1415-32