  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
Ca3Co4O9) which has a rock salt structure stacked incommensurately on a
hexagonal CoO2 lattice. The local structural measurements reveal a low
coordination of Co(2)-O bonds in the rock salt layer with large static
structural disorder. The temperature dependence of the Co(1)-Co(1) bond
correlations in the CoO2 layer are found to be normal above ~75K and with a
very small static disorder component. 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 of 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 chain-like structure in the rock salt layer and high atomic
order in the C
