• Misfit layer compound;
  • Structural investigation;
  • Superconductors;
  • Commensurate structures.

The structure of the misfit layer compound [Biinline imageSrOinline image]inline image[CoOinline image]inline image, bismuth strontium cobaltite, was determined by single-crystal X-ray diffraction using the five-dimensional superspace-group formalism. This composite crystal, of monoclinic symmetry, is composed of two subsystems exhibiting incommensurate periodicities along b, the binary axis direction. The first composite part [Biinline imageSrOinline image] displays an intrinsic modulation of planar monoclinic type characterized by the wavevector inline image. The second composite part [CoOinline image] shows two different centered lattice variants. The structure of the misfit layer crystal can be described as an alternation along c of distorted rock-salt-type slabs, formed from [BiO] and [SrO] layers (first subsystem), and of [CoOinline image] layers (second subsystem) displaying a distorted CdIinline image-type structure. Two main structural results are obtained. First, as a consequence of the intrinsic modulation, disordered zones, characterized by Bi vacancies, are regularly distributed in the [BiO] layers. Second, strong chemical bonds are implied between the strontium atoms of the first subsystem and the oxygen atoms of the second one.