School of Chemistry, The University of Sydney
○Chris D Ling Karina Aivazian
Misfit layered cobaltites have attracted considerable recent attention due to their promising thermoelectric  and colossal magnetoresistive (CMR) properties . Their defining structural feature is the stacking of alternating layer types along the c axis: a hexagonal CdI2-type layer of edge-sharing CoO6 octahedra (subsystem 1); and 3 or 4 tetragonal NaCl-type metal oxide layers (subsystem 2). A mismatch between these two layers along the b axis (and also along the a axis in some compounds) means that the structures of these oxides must be described as an incommensurately modulated composite.
One of the most intensively studied misfit layered cobaltites is [Ca2CoO3][CoO2]q, commonly known by its approximate stoichiometry Ca3Co4O9. The so-called misfit parameter q = b1/b2 ~ 1.62 is defined in terms of the b axes of subsystems 1 and 2. It was recently shown by chemical analysis and thermogravimetry that "Ca3Co4O9" contains a significant number of oxygen anion vacancies in concentrations that depend strongly on synthetic conditions [3, 4]. Here we report the results of a synchrotron XRD and neutron powder diffraction study aimed at determining the precise location of these oxygen vacancies, and in particular, within which subsystem they principally reside. This has a direct bearing on the oxidation states of cobalt in each layer, and hence on the physical and magnetic properties of the material.
S. Li, R. Funahashi, I. Matsubara, et al., Chemistry of Materials 12 (2000) 2424.
A. C. Masset, C. Michel, A. Maignan, et al., Physical Review B 62 (2000) 166.
M. Karppinen, H. Fjellvåg, T. Konno, et al., Chemistry of Materials 5 (2004) 2790.
Y. Morita, J. Poulsen, K. Sakai, et al., Journal of Solid State Chemistry 177 (2004) 3149.