ABSTRACT
Cobalt oxides have attracted considerable attention because of their
peculiar and rich physical/chemical properties such as a spin-
state crossover, huge thermoelectric power, high energy density
as Li ion batteries, and superconductivity, partly resulting from
the simultaneous existence of strong electron-electron interaction
within the Co ion and a competing hopping energy between Co
3d and O 2p states, and a small difference in the total energies for spin states. Here, the concept of the valence and spin state
of Co ions plays a key role and is introduced in Section 2.2.
Their rich variety is generally believed to be associated with the
internal degree of freedom of the electrons. In Section 2.3, I would
like to focus attention on the spin-state crossover, in which the
way of occupying orbitals for the spins is changed, whereas the
number of electrons remains constant. Furthermore, typical spin-
state crossover systems such as RCoO3 (R: the rare-earth element), [Ca2CoO3][CoO2]1.62, and Ba2Co9O14 are also presented. The spin-
state crossover could lead to a change in the density of states near
the Fermi level, resulting in unconventional transport properties.
Themetal-insulator transition serves as an example, which is shown
