  A general spin-state model and a qualitative physical picture have been
proposed for a class of lately synthesized layered cobalt oxides (LCOs) by
means of density functional calculations. As the plane corrugation of the
cobalt-oxygen layer decreases, the LCOs evolve from a high-spin (HS)
superexchange-coupled antiferromagnetic (AFM) insulator to an almost-HS
AFM/ferromagnetic (FM) competing system where the FM coupling is mediated via
the p-d exchange by an increasing amount of delocalized pd\sigma holes having
mainly the planar O 2p character. It is tentatively suggested that the
delocalized holes more than 0.3 per CoO_{2} basal square are likely necessary
for the insulator-metal and/or AFM-FM transitions in the corrugation-weakened
LCOs. A phase control may be realized in LCOs by varying the plane corrugation
(thus modifying the hole concentration) through an ionic-size change of the
neighboring layers on both sides of the cobalt-oxygen layer. In addition, a few
experiments are suggested for a check of the present model and picture.
