Electronic states and observed anisotropic band structure of Ca2Co2O5 type layered compound: A comprehensive study

The electronic states, band structures, bonding properties together with the electron conduction of the Ca2Co2O5 type layered compound are studied by the pseudopotential as well as the plane wave function method. The results show that the spin electrons of Ca2Co2O5 form five bands and the bands are anisotropic. The valence bands far from Fermi level are heavier and the conduction bands are lighter. The spin up band has an energy gap of 2 eV above Fermi level and an energy gap of 0.1 eV below Fermi level; the spin down band has two energy gaps of 0.3 eV and 0.8 eV above Fermi level. The carriers tend to conduct within the CoO2 layer allowing for its narrower gaps under reasonably larger electrical field. The density of states of the spin up electrons near Fermi level is apparently larger than that of the spin down electrons; the ability to supply carriers from CaCoO layer is stronger than that of the CoO(2 )layer. The total densities of states of the CoO2 layer and the CaCoO layer are mainly composed by Co d and O p electrons. The p state electrons within CaCoO layer and Co d electrons within CoO(2 )layer as well as the transitions between them take main part in conduction process. The interaction between Ca and O is weak and they exhibit ionic-like bonding. The interaction between Co and O is much strong and they exhibit covalent bonding, and the covalent bonding between Co and O within the CoO2 layer is even stronger.