A Sequential Electron Doping for Quadruple Perovskite Oxides A Cu 3 Co 4 O 12 ( A = Ca, Y, Ce).

A novel quadruple perovskite oxide CeCu ₃ Co ₄ O ₁₂ has been synthesized in high-pressure and high-temperature conditions of 12 GPa and 1273 K. Rietveld refinement of the synchrotron X-ray powder diffraction pattern reveals that this oxide crystallizes in a cubic quadruple perovskite structure with the 1:3-type ordering of Ce and Cu ions at the A -site. X-ray absorption spectroscopy analysis demonstrates the valence-state transitions in the A Cu ₃ Co ₄ O ₁₂ series ( A = Ca, Y, Ce) from Ca ²⁺ Cu ³⁺ ₃ Co ^3.25+ ₄ O ₁₂ to Y ³⁺ Cu ³⁺ ₃ Co ³⁺ ₄ O ₁₂ to Ce ⁴⁺ Cu ^2.67+ ₃ Co ³⁺ ₄ O ₁₂ , where the electrons are doped in the order from B -site (Co ^3.25+ → Co ³⁺ ) to A '-site (Cu ³⁺ → Cu ^2.67+ ). This electron-doping sequence is in stark contrast to the typical B -site electron doping for simple AB O ₃ -type perovskite and quadruple perovskites CaCu ₃ B ₄ O ₁₂ ( B = V, Cr, Mn), further differing from the monotonical A '-site electron doping for Na _{1- x} La _x Mn ₃ Ti ₄ O ₁₂ and A '- and B -site electron doping for A Mn ₃ V ₄ O ₁₂ ( A = Na, Ca, La). The differences in the electron-doping sequences are interpreted by rigid-band models, proposing a wide variety of electronic states for the complex transition-metal oxides containing the multiple valence-variable ions.