Structural and electronic transformations in quadruple iron perovskite Ca1−xSrxCu3Fe4O12

Abstract Crystal structures and electronic transformations of quadruple iron perovskite solid solution Ca 1− x Sr x Cu 3 Fe 4 O 12 ( x  = 0.2, 0.4, 0.6, and 0.8) have been investigated by synchrotrons X-ray powder diffraction, Mossbauer spectroscopy, and magnetization measurements. For x  = 0.2, a charge disproportionation transition (2Fe 4+  → Fe 3+  + Fe 5+ ) occur simultaneously with electron charge transfers from Fe to Cu below ∼200 K, as well as CaCu 3 Fe 4 O 12 . 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 SrCu 3 Fe 4 O 12 , 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 Mossbauer isomer shift parameters. The Fe O bond covalency plays a crucial role in the types of electronic phase transitions for the Ca 1− x Sr x Cu 3 Fe 4 O 12 and R 3+ Cu 3 Fe 4 O 12 ( 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 .