Abstract Crystal structures and electronic transformations of quadruple iron perovskite solid solution Ca1−xSrxCu3Fe4O12 (x = 0.2, 0.4, 0.6, and 0.8) have been investigated by synchrotrons X-ray powder diffraction, Mössbauer spectroscopy, and magnetization measurements. For x = 0.2, a charge disproportionation transition (2Fe4+ → Fe3+ + Fe5+) occur simultaneously with electron charge transfers from Fe to Cu below ∼200 K, as well as CaCu3Fe4O12. In contrast, negative thermal expansions derived from continuous electron charge transfers from Cu and Fe are observed for x = 0.6 and 0.8 at low temperatures below room temperature, as in SrCu3Fe4O12, followed by charge disproportionation transitions. A two-phase coexistence is observed at low temperature below ∼200 K for x = 0.4, indicating that the phase boundary locates in the vicinity of this composition. We have discovered that the Fe–O bond lengths are closely related to their covalency which were estimated from Mössbauer isomer shift parameters. The Fe–O bond covalency plays a crucial role in the types of electronic phase transitions for the Ca1−xSrxCu3Fe4O12 and R3+Cu3Fe4O12 (R: trivalent rare earth metal ions, Y, La–Lu) systems, where the two different low-temperature electronic phases are separated by a common isomer shift value of ∼0.17 mm s−1.

中文翻译：

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四重铁钙钛矿 Ca 1−x Sr x Cu 3 Fe 4 O 12 的结构和电子转换

摘要 通过同步加速器 X 射线粉末衍射、穆斯堡尔光谱和磁化测量研究了四重铁钙钛矿固溶体 Ca1−xSrxCu3Fe4O12 (x = 0.2、0.4、0.6 和 0.8) 的晶体结构和电子转换。对于 x = 0.2，电荷歧化转变 (2Fe4+ → Fe3+ + Fe5+) 与电子电荷从 Fe 到低于 200 K 的 Cu 以及 CaCu3Fe4O12 的转移同时发生。相比之下，在低于室温的低温下，对于 x = 0.6 和 0.8，观察到源自 Cu 和 Fe 的连续电子电荷转移的负热膨胀，如 SrCu3Fe4O12，然后是电荷歧化转变。对于 x = 0.4，在低于 ~200 K 的低温下观察到两相共存，表明相界位于该成分附近。我们发现 Fe-O 键长与它们的共价密切相关，共价是根据穆斯堡尔异构体位移参数估计的。Fe-O 键共价在 Ca1−xSrxCu3Fe4O12 和 R3+Cu3Fe4O12（R：三价稀土金属离子，Y，La-Lu）系统的电子相变类型中起着至关重要的作用，其中两种不同的低温电子相被常见的异构体位移值 ∼0.17 mm s-1 分开。