Disordered misfit [Ca₂CoO₃][CoO₂]_1.62 structure revisited via a new intrinsic modulation

The structure of the thermoelectric lamellar misfit cobalt oxide [Ca₂CoO₃][CoO₂]_1.62 was refined again using single-crystal X-ray diffraction data. A new commensurate intrinsic modulation was observed involving a modulation vector orthogonal to the misfit direction (⅔,0,−⅓). The five-dimensional superspace group is C2/m(1δ0)(α0γ)gm and the structure was solved using a commensurate approximation. A new model is given involving an occupation modulation of the split sites of the [CoO] layer. This [CoO] layer can be described by triple chains running along b. The residual disorder along b can then be explained by the assumption of a local ordering with two types of clusters: CoO₂ and Co₅O₄. A powder neutron diffraction experiment confirmed the ordering evidenced by the single-crystal X-ray diffraction study, but was not sufficient by itself to deal with this double modulated scheme. The new intrinsic modulation is destroyed by partial metal substitutions in the [CoO] layer. The structural modifications of this layer directly influence the physical properties which are related to the electronic structure of the [CoO₂] layers.