異常高原子価イオンを含む新奇遷移金属酸化物の高圧合成

抄録

  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.