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

A novel quadruple perovskite oxide CeCuCoO 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 -site. X-ray absorption spectroscopy analysis demonstrates the valence-state transitions in the CuCoO series ( = Ca, Y, Ce) from CaCuCoO to YCuCoO to CeCuCoO, where the electrons are doped in the order from -site (Co → Co) to '-site (Cu → Cu). This electron-doping sequence is in stark contrast to the typical -site electron doping for simple O-type perovskite and quadruple perovskites CaCuO ( = V, Cr, Mn), further differing from the monotonical '-site electron doping for NaLaMnTiO and '- and -site electron doping for MnVO ( = 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.