INVESTIGATION BY ELECTRON-DIFFRACTION MICROSCOPY OF (RE)BA2CU3O6- INTERPRETATION OF UNEXPECTED SUPERSTRUCTURES AND INFLUENCE OF SECONDARY PHASES(XSY POLYCRYSTALLINE CERAMIC COMPOUNDS )

An original synthesis route has been followed to prepare ceramic YBa2C u3O6+xSy materials. The samples have been studied by X-ray, high resol ution optically polarized microscopy, transmission electron microscopy and X-ray microanalysis. A granular layer of a sulphur containing pha se is observed at the crust of the pellet and leads to a set of broken rings on electron diffraction micrographs; the pellet itself is cover ed by hexagonal CuS micrograins. Although it is not clear how much sul phur is exactly entering the 1-2-3 phase, its presence does influence the behaviour of the oxygen deficient material. Instead of the Ortho I I superstructure, a weak Ortho IV superstructure is observed. Moreover most of the grains are untwinned. Thermodynamic data show an instabil ity region for sulphur reactivity in the oxygen range 6.55 to 6.75.