Electronic structure of the Ca3Co4O9 compound from ab initio local interactions

We used fully correlated ab initio calculations to determine the effective parameters of Hubbard and t-J models for the thermoelectric misfit compound Ca3Co4O9. As for the NaxCoO2 family, the Fermi level orbitals are the a(1g) orbitals of the cobalt atoms; the e'(g) being always lower in energy by more than 240 meV. The electron correlation is found very large U/t similar to 26 as well as the parameters fluctuations as a function of the structural modulation. The main consequences are a partial a(1g) electrons localization and a fluctuation of the in-plane magnetic exchange from antiferromagnetic to ferromagnetic. The behavior of the Seebeck coefficient and the figure of merit are discussed in view of the ab initio results, as well as the 496 K phase transition.