Local structure of thermoelectric Ca₃Co₄O₉

We have combined temperature-dependent local structural measurements with first-principles density-functional calculations to develop a three-dimensional local structure model of the misfit system [Ca₂CoO₃] [CoO₂]_1.61 (referred to as Ca₃Co ₄O₉) which has a rocksalt structure stacked incommensurately on a hexagonal CoO₂ lattice. The local structural measurements reveal a low coordination of Co(2)-O bonds in the rocksalt layer with large static structural disorder. The temperature dependence of the Co(1)-Co(1) bond correlations in the CoO₂ layer is found to be normal above ∼75 K and has very small static disorder. An anomalous enhancement in the Co(1)-Co(1) correlations occurs at the onset of long-range magnetic order. Density-functional computations suggest that the reduction in the coordination of Co(2) is due to the formation of chains of Co (2) O_x in the a-b plane linked to the Ca-O layers by c -axis Co(2)-O bonds. The reduced dimensionality introduced by the chainlike structure in the rocksalt layer and high atomic order in the CoO₂ layer may enable low thermal conductivity and high electrical conductivity in the respective layers.