Local structure of thermoelectric Ca3Co4O9

We have combined temperature-dependent local structural measurements with first-principles densityfunctional calculations to develop a three-dimensional local structure model of the misfit system Ca2CoO3 CoO2 1.61 referred to as Ca3Co4O9 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 Ox 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.