Structural and physical studies of oxide ionic-conductive brownmillerite single crystals

Abstract

Large single crystals of brownmillerites in the Sr2(Fe,Co,Al)2O5 and Ca2(Fe,Co,Al)2O5 compositional triangles were grown using the floating-zone (FZ) method. A commensurate modulation with Pbma symmetry was identified in Sr2FeCoO5 for the first time and confirmed in Sr2Fe2O5 and Ca2FeCoO5, while Ca2FeAlO5 and Ca2(Fe0.95Zn0.05)2O5 adopted different unmodulated structures at room temperature, in agreement with previous reports. The Pbma phases persisted to very high temperatures (≤750 °C) in all cases, indicating negligible thermal disordering of the chain orientations. Electron diffraction images of Sr2FeCoO5 also revealed that the dominant commensurate modulation vector determined by diffraction from large sample volumes differed from the incommensurate vectors identified in individual crystallites of a powdered sample (as found previously for Sr2Fe2O5), highlighting the insufficiency of powder diffraction alone as a reliable tool to distinguish between different brownmillerite structure variants. The magnetic behaviours of Sr2FeCoO5 and Ca2Fe2O5 were characterised by magnetometry and magnetic structure refinements against neutron diffraction data. Detailed structure refinements and physical property measurements of Ca2Fe2O5 showed that a proposed reorientation of the magnetic spins below 140 K does not occur and that its anomalous magnetic susceptibility is not caused by changes to the long-range ordered magnetic structure. Finally, inelastic neutron scattering (INS) data collected for single-crystalline Sr2Fe2O5 and Ca2Fe2O5 generally supported a proposed ionic conduction model in which the “apical” oxygen atoms between the BO4 tetrahedral and BO6 octahedral layers become delocalised at elevated temperatures. The results point to the possible improvement of brownmillerite ionic conductors by doping with larger cations to lengthen the b axis, which in turn should lengthen and weaken the critical B–O bonds.