Oxygen hole state in A-site ordered perovskite ACu₃Ru ₄O₁₂ (A = Na, Ca, and La) probed by resonant X-ray emission spectroscopy

In contrast to ordinary perovskites ABO₃, A-site ordered perovskites AA′₃B₄O₁₂ include transition metals in both A′ and B sites. The A′-O, B-O, and A′-B interactions in A-site ordered perovskites give rise to various physical properties. In this study, we report the O 1s and Cu 2p x-ray absorption spectroscopy (XAS) and resonant x-ray emission spectroscopy (RXES) study on the A-site ordered perovskite ACu₃Ru₄O₁₂ (A = Na, Ca, and La). Hartree-Fock calculation and cluster model calculation were carried out to analyze O 1s and Cu 2p XAS spectra. The O 1s RXES spectra indicate that the number of O 2p holes increases in going from La to Ca to Na. In addition, the Cu 2p RXES spectra reveal that the charge-transfer (CT) energy decreases in going from La to Ca to Na. The decrease of CT is consistent with our calculations and suggest an increase of the O 2p holes in the ground state. The substitution of the A site ion changes not only the carrier concentration in the Ru 4d and Cu 3d bands but also the O 2p to Cu 3d CT energy.