SYSTEMATIC COPPER CONCENTRATION VARIATIONS ALONG GRAIN-BOUNDARIES IN BULK-SCALE YBA2CU3O7-DELTA BICRYSTALS

Local variations in the cation composition along grain boundaries in b ulk-scale YBa2Cu3O7-delta, bicrystals grown by the flux method were in vestigated by high-spatial-resolution energy-dispersive X-ray spectros copy (EDS) in a scanning transmission electron microscope (STEM). Comp osition profiles along 7 degrees [001] and 31 degrees [001] tilt bound aries reveal regularly-spaced regions that are rich in copper relative to both the grain interior and the neighboring sections of the grain boundary. The average spacing and range of spacings between adjacent c opper maxima are 75 nm and 60-100 nm, respectively, for the seven maxi ma observed in the 7 degrees bicrystal. Three maxima with a spacing of similar to 100 nm were detected in a shorter segment of the 31 degree s boundary. In both bicrystals, the spacing of the maxima equaled the wavelength of an approximately periodic strain field with dislocation array like character that was observed in the boundary. Long-wavelengt h strain fields of this type previously have been associated with saw- tooth-shaped facet structures in flux-grown YBa2Cu3O7-delta bicrystals . A saw-tooth facet structure was observed directly only in the 7 degr ees bicrystal, however. Less extensive investigations of additional bi crystals suggest that oscillatory cation nonstoichiometry is general t o [001] tilt boundaries in flux-grown YBa2Cu3O7-delta bicrystals and p ossibly to more general boundaries. Regular variations in boundary non stoichiometry are a possible origin for one scale of boundary subdivis ion and may contribute to the heterogeneity of the electromagnetic pro perties that is believed to occur on a range of different length scale s in YBa2Cu3O7-delta. (C) 1997 Elsevier Science B.V.