（Temperature-induced A–B intersite charge transfer in an A-site-ordered LaCu3Fe4O12 perovskite ）
Y. W. Long, N. Hayashi, T. Saito, M. Azuma, S. Muranaka Y. Shimakawa
Changes of valence states in transition-metal oxides often cause significant changes in their structural and physical properties . Chemical doping is the conventional way of modulating these valence states. In ABO3 perovskite and/or perovskite-like oxides, chemical doping at the A site can introduce holes or electrons at the B site, giving rise to exotic physical properties like hightransition-temperature superconductivity and colossal magnetoresistance. When valence-variable transition metals at two different atomic sites are involved simultaneously, we expect to be able to induce charge transfer—and, hence, valence changes— by using a small external stimulus rather than by introducing a doping element. Materials showing this type of charge transfer are very rare, however, and such externally induced valence changes have been observed only under extreme conditions like high pressure. Here we report unusual temperature-induced valence changes at the A and B sites in the A-site-ordered double perovskite LaCu3Fe4O12; the underlying intersite charge transfer is accompanied by considerable changes in the material's structural, magnetic and transport properties. When cooled, the compound shows a first-order, reversible transition at 393K from LaCu(2+) 3 Fe(3.75+)4O12 with Fe(3.75+) ions at the B site to LaCu(3+)3 Fe(3+)4O12 with rare Cu(3+) ions at the A site. Intersite charge transfer between the A-site Cu and B-site Fe ions leads to paramagnetism-to-antiferromagnetism and metal-to-insulator isostructural phase transitions. What is more interesting in relation to technological applications is that this above-roomtemperature transition is associated with a large negative thermal expansion.
龙有文是本课题组 2002 级的博士生（于 2007 年毕业），现在日本京都大学做博士后。该工作相关内容在京大网页上发布，朝日新闻、每日新闻、产经新闻、京都新闻等多家报社也相应报道，这个结果具有良好的应用潜力。 Cu(3+) 离子和 Fe(3.75+) 离子都是反常高价离子，但高压合成有利于它们在 LaCu3Fe4O12 这种 钙钛矿结构中存在。特别是包含 Cu(3+) 离子的稳定化合物是异常罕见的，但是它作为基态离子 存在于该钛矿 LaCu3Fe4O12 中。而高价的 Fe(3.75+) 离子则通过 Cu-Fe 间的电荷转移变为 Fe(3+) 离子，从而解决其不稳定性（转变温度为 393K ）。并且，伴随电荷转移，将发生顺磁金属－反铁磁绝缘体的转变。