Valence fluctuations and correlated metallic states in A -site ordered perovskite oxides ACu₃V₄O₁₂ ( A=Na, Ca, and Y)

We have studied the electronic structure of A -site ordered perovskite oxides ACu₃V₄O₁₂ (A=Na, Ca, and Y) using photoemission spectroscopy. V and Cu valence fluctuations are clearly observed in the V2p and the Cu2p core-level spectra. In going from A=Na to Ca to Y, electrons doped by the A -site substitution are mainly captured by the Cu site which has a mixture of +2 (d⁹) and +3 (d⁹ L, L is an O2p hole) and are partly distributed into the V site. The photoemission spectra near the Fermi level show a systematic change induced by the A -site substitution. The electronic structure of ACu₃V₄O₁₂ is very different from that in Ca_1-xSr_xVO₃ due to the existence of the Cu 3d electrons, and the unusual correlation effect in ACu ₃V₄O₁₂ can be related to the valence fluctuations observed in the core-level spectra.