異常高原子価鉄ペロブスカイトにおける電荷不均化・サイト間電荷移動と負の熱膨張
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概要
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ACu3Fe4O12 (A = Ca and Sr) perovskites containing unusual high valence Fe4+ ions demonstrate charge disproportionation, charge ordering, and intersite charge transfer. CaCu3Fe4O12 perovskite shows charge disproportionation of 2Fe4+→Fe3++Fe5+ type, leading to the charge ordering of Fe3+ and Fe5+ in a rocksalt manner. SrCu3Fe4O12 perovskite exhibits crossover-like intersite charge transfer between Cu and Fe in the temperature range of 170∼270 K, resulting in a giant negative thermal expansion with a liner 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 is attributed to the bond strains induced by A-site cation.