Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan.
Sci Rep. 2012;2:449. doi: 10.1038/srep00449. Epub 2012 Jun 11.
Unusual high-valence states of iron are stabilized in a few oxides. A-site-ordered perovskite-structure oxides contain such iron cations and exhibit distinct electronic behaviors at low temperatures, e.g. charge disproportionation (4Fe⁴⁺ → 2Fe³⁺ + 2Fe⁵⁺) in CaCu₃Fe₄O₁₂ and intersite charge transfer (3Cu²⁺ + 4Fe³·⁷⁵⁺ → 3Cu³⁺ + 4Fe³⁺) in LaCu₃Fe₄O₁₂. Here we report the synthesis of solid solutions of CaCu₃Fe₄O₁₂ and LaCu₃Fe₄O₁₂ and explain how the instabilities of their unusual valence states of iron are relieved. Although these behaviors look completely different from each other in simple ionic models, they can both be explained by the localization of ligand holes, which are produced by the strong hybridization of iron d and oxygen p orbitals in oxides. The localization behavior in the charge disproportionation of CaCu₃Fe₄O₁₂ is regarded as charge ordering of the ligand holes, and that in the intersite charge transfer of LaCu₃Fe₄O₁₂ is regarded as a Mott transition of the ligand holes.
一些氧化物中稳定存在铁的异常高化合价态。A 位有序钙钛矿结构氧化物包含这种铁阳离子，并在低温下表现出独特的电子行为，例如 CaCu₃Fe₄O₁₂中的电荷歧化（4Fe⁴⁺ → 2Fe³⁺ + 2Fe⁵⁺）和 LaCu₃Fe₄O₁₂中的位间电荷转移（3Cu²⁺ + 4Fe³·⁷⁵⁺ → 3Cu³⁺ + 4Fe³⁺）。在这里，我们报告了 CaCu₃Fe₄O₁₂和 LaCu₃Fe₄O₁₂固溶体的合成，并解释了它们异常铁价态的不稳定性是如何缓解的。尽管这些行为在简单的离子模型中看起来完全不同，但它们都可以通过配体空穴的局域化来解释，这是由氧化物中铁 d 和氧 p 轨道的强杂化产生的。CaCu₃Fe₄O₁₂中电荷歧化的局域化行为被视为配体空穴的电荷有序化，而 LaCu₃Fe₄O₁₂中位间电荷转移的局域化行为则被视为配体空穴的莫特转变。
