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High-pressure synthesis method can generate novel materials containing unusual high valence ions. CaCu3Fe4O12 and SrCu3Fe4O12 perovskites, with unusual high valence Fe4+ ions, demonstrate anomalous properties such as charge disproportionation, and intersite charge transfer. CaCu3Fe4O12 shows a charge disproportionation of 2Fe4+→Fe3++Fe5+ type whereas SrCu3Fe4O12 shows a crossover-like intersite charge transfer between Cu and Fe between 170 and 270 K. This gradual transition results in a giant negative thermal expansion with a linear thermal expansion coefficient of -2.26×10-5 K-1, followed by the charge disproportionation at a ratio of Fe3+:Fe5+≒4:1. The differences in the structural and electronic properties between CaCu3Fe4O12 and SrCu3Fe4O12 are attributed to the bond strains induced by A-site cations. [high-pressure synthesis, perovskites, transition metal oxides, unusual high valence ions, negative thermal expansion, charge transfer, charge disproportionation, Kawai-type cell]

High-Pressure Synthesis of Novel Transition Metal Oxides Containing Unusual High Valence Ions