Doping effects on the electronic and structural properties of CoO2: An LSDA + U study

A systematic local spin density approximation (LSDA)+U study of doping effects on the electronic and structural properties of single layer CoO2 is presented. Undoped CoO2 is a charge transfer insulator within LSDA+U and a metal with a high density of states (DOS) at the Fermi level within LSDA. (CoO2)1.0-, on the other hand, is a band insulator with a gap of 2.2 eV. Systems with fractional doping are metals if no charge orderings are present. Due to the strong interaction between the doped electron and other correlated Co d electrons, the calculated electronic structure of (CoO2)x- depends sensitively on the doping level. Zone center optical phonon energies are calculated under the frozen phonon approximation and are in good agreement with measured values. Softening of Eg phonon at doping x ∼ 0.25 seems to indicate a strong electron-phonon coupling in this system. Possible intermediate spin states of Co ions, Na ordering, as well as magnetic and charge orderings in this system are also discussed