Structural and electronic transformations in quadruple iron perovskite Ca1−xSrxCu3Fe4O12

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 FeO bond lengths are closely related to their covalency which were estimated from Mössbauer isomer shift parameters. The FeO 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.