Contribution of powder diffraction for structure refinements of aperiodic misfit cobalt oxides

Two families of misfit cobalt oxides have been reinvestigated using neutron and synchrotron powder diffraction data and Rietveld structure refinements in the four-dimensional formalism for aperiodic structures. In the first system [Ca2CoO3][CoO2](1.62), the structure-specific features of two complex polytypic phases previously described from single-crystal diffraction data have been confirmed for the main term of the series and are described with the C2/m(1,delta,0) s0 superspace symmetry (R-p = 0.026, R-wp = 0.031, R-B = 0.014). The high-resolution synchrotron diffraction experiments have been interpreted with a triclinic distortion of the structure, probably related to an intrinsic modulation of the [Ca2CoO3] sublattice in relation with Co and O split atomic sites. In the second system Ca-0.82(Cu0.65Co0.35O2), a new occupation modulation of the metallic site is shown with the Fmmm(alpha11) 0ss superspace symmetry (R-p = 0.031, R-wp = 0.039, R-B = 0.027) and is interpreted in relation with the modulated interatomic distances of the square coordination of the cationic site. In both cases, recently developed tools for structure refinements of aperiodic systems using powder diffraction data ( in the JANA2000 package) have proved their efficiency, in particular as specific complements to previous single-crystal structure refinements.