A Sequential Electron Doping for Quadruple Perovskite Oxides ACu3Co4O12 (A = Ca, Y, Ce)

A novel quadruple perovskite oxide CeCu3Co4O12 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 ACu(3)Co(4)O(12) series (A = Ca, Y, Ce) from Ca2+Cu33+Co43.25+O12 to Y3+Cu33+Co43+O12 to Ce4+Cu32.67+Co43+O12, where the electrons are doped in the order from B-site (Co3.25+ -> Co3+) to A'-site (Cu3+ -> Cu2.67+). This electron-doping sequence is in stark contrast to the typical B-site electron doping for simple ABO(3)-type perovskite and quadruple perovskites CaCu3B4O12 (B = V, Cr, Mn), further differing from the monotonical A'-site electron doping for Na1-xLaxMn3Ti4O12 and A'- and B-site electron doping for AMn(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.