High-pressure synthesis method can generate novel materials containing unusual high valence ions. CaCu
3Fe
4O
12 and SrCu
3Fe
4O
12 perovskites, with unusual high valence Fe
4+ ions, demonstrate anomalous properties such as charge disproportionation, and intersite charge transfer. CaCu
3Fe
4O
12 shows a charge disproportionation of 2Fe
4+→Fe
3++Fe
5+ type whereas SrCu
3Fe
4O
12 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 Fe
3+:Fe
5+≒4:1. The differences in the structural and electronic properties between CaCu
3Fe
4O
12 and SrCu
3Fe
4O
12 are attributed to the bond strains induced by A-site cations.
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