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 [1] and colossal magnetoresistive (CMR) properties [2]. 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.

[1]S. Li, R. Funahashi, I. Matsubara, et al., Chemistry of Materials 12 (2000) 2424.
[2]A. C. Masset, C. Michel, A. Maignan, et al., Physical Review B 62 (2000) 166.
[3]M. Karppinen, H. Fjellvåg, T. Konno, et al., Chemistry of Materials 5 (2004) 2790.
[4]Y. Morita, J. Poulsen, K. Sakai, et al., Journal of Solid State Chemistry 177 (2004) 3149.