Metallic versus insulating behavior in the A -site ordered perovskite oxides A Cu3 Co4 O12 (A=Ca and Y) controlled by Mott and Zhang-Rice physics

We report on a photoemssion study of A -site ordered perovskite oxides A Cu3 Co4 O12 (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 d9 L configuration (L: an O2p 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 d6 configuration consistent with its insulating behavior.