  We sought the origin of the metal-insulator transition in Sr2-xLaxCoO4, using
electron-correlation corrected density functional calculations. Our results
show that Sr2CoO4 is in an intermediate-spin (IS) state and a strong Co4+ 3d-O
2p hybridization is responsible for its ferromagnetic metallicity. Upon La
doping, however, a spin-state transition occurs in Sr1.5La0.5CoO4: IS-Co4+ x 2
+ 1e --> LS-Co4+ + HS-Co3+ (LS: low spin; HS: high spin). Then the spin-state
transition suppresses an electron hopping via a spin-blockade and gives rise to
the insulating behavior of Sr1.5La0.5CoO4. A corresponding superexchange
accounts for its ferromagnetism. Thus, spin state could provide a way to tune
materials properties.
