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We investigate the electronic state of a CoO_2 plane in the layered cobaltoxides Na_xCoO_2 using the 11 band d-p model on a two-dimensional triangularlattice, where the tight-binding parameters are determined so as to fit the LDAband structure. Effects of the Coulomb interaction at a Co site: the intra- andinter-orbital direct terms U and U', the exchange coupling J and thepair-transfer J', are treated within the Hartree-Fock approximation. We alsoconsider the effect of the Na order at x=0.5, where Na ions formone-dimensional chains, by taking into account of an effective one-dimensionalpotential Delta epsilon_{d}. It is found that the one-dimensional Na orderenhances the Fermi surface nesting and antiferromagnetism is caused which issuppressed due to the frustration effect in the case without the Na order. %Itis found that the Na order enhances the Fermi surface nesting resulting in ametallic antiferromagnetism which is suppressed due to the frustration effectin the case without the Na order. Furthermore, we consider the effect of theCoulomb interaction between the nearest-neighbor Co sites V and find that acoexistence of the magnetic, charge and orbital ordered state takes place forV>V_c where the system becomes insulator.

Antiferromagnetic, Charge and Orbital Ordered States of Na0.5CoO2 Based on the Two-Dimensional Triangular Lattice d-p Model