Structure determination by electron diffraction and HREM of the incommensurate modulated phase BaxCuO2 (0.67 <= x <= 0.70)

We report the structure determination, by electron diffraction and high res olution electron microscopy, of two phases, Ba0.70CuO2 and Ba0.67CuO2, that characterise the BaxCuO2 (0.67 less than or equal to x less than or equal to 0.70) system where 13 K superconductivity has been recently found. Both phases show modulated structures that can be described in terms of composit e 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 applicati on of direct methods to electron diffraction data. Starting from this resul ts and on the basis of diffraction patterns and high resolution images, the related Ba0.67CuO2 structure has been described in terms of interpenetrati on of three mutually incommensurate sublattices associated with twinning, O wing to the extreme complexity of this structure, only a qualitative model has been developed, which is, however, able to describe all the peculiar fe atures of this unusual and interesting structure. (C) 1999 Elsevier Science B.V. All rights reserved.