Structure determination by electron diffraction and HREM of the incommensurate modulated phase Ba CuO

We report the structure determination, by electron diffraction and high resolution electron microscopy, of two phases, Ba0.70CuO2 and Ba0.67CuO2, that characterise the Ba xCuO2 0.67FxF0.70. system where 13 K superconductivity has been recently found. Both phases show modulated structures that can be described in terms of composite crystals, resulting from the interpenetration of mutually incommensurate sublattices consisting of Barium atoms and infinite chains of edge sharing CuO4 planar units. The average Ba0.70CuO2 structure was solved by application of direct methods to electron diffraction data. Starting from this results and on the basis of diffraction patterns and high resolution images, the related Ba0.67CuO2 structure has been described in terms of interpenetration of three mutually incommensurate sublattices associated with twinning. Owing to the extreme complexity of this structure, only a qualitative model has been developed, which is, however, able to describe all the peculiar features of this unusual and interesting structure