Metallic versus insulating behavior in the A-site ordered perovskite oxides ACu3Co4O12 (A = Ca and Y) controlled by Mott and Zhang-Rice physics

We report on a photoemssion study of A-site ordered perovskite oxides ACu(3)Co(4)O(12) (A=Ca and Y) which are metallic for A=Ca and insulating for A=Y. If the Cu valence is +2 as stabilized by the square planar coordination, the Co valence for A=Ca is formally +4 and the system could be a Mott insulator while that for A=Y should be +3.75 and the system is a doped Mott insulator. The photoemission study reveals that the Cu valence for A=Ca and Y is formally +3 with the d(9)L configuration (L: an O 2p hole) which corresponds to the Zhang-Rice singlet state. In addition, it is found that the Co valence for A=Y is formally +3 with the low-spin d(6) configuration consistent with its insulating behavior.