Structural and electronic transformations in quadruple iron perovskite Ca_1−xSr_xCu₃Fe₄O₁₂

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.