Inverse Charge Transfer in the Quadruple Perovskite CaCu3Fe4O12

Structural and spectroscopic analyses revealed that the quadruple perovskite CaCu3Fe4O12 undergoes an inverse electron charge transfer in which valence electrons move from B-site Fe to A'-site Cu ions (similar to 3Cu(similar to 2.4+) + 4Fe(similar to 3.65+) -> + 4Fe(similar to 3.8+)) simultaneously with a charge disproportionation transition (4Fe(similar to 3.8+) -> similar to 2.4Fe(3+) + similar to 1.6Fe(5+)), on cooling below 210 K. The direction of the charge transfer for CaCu3Fe4O12 is opposite to those reported for other perovskite oxides such as BiNiO3 and ACu(3)Fe(4)O(12) (A = Sr2+ or the large trivalent rare-arth metal ions), in which the electrons move from A/A'-site to B-site ions. This finding sheds a light on a new aspect in intermetallic phenomena for complex transition metal compounds.