Crystal structure of the mixed conductor Sr4Fe4Co2O13

The crystal structure of Sr4Fe4Co2O13 has been determined on the basis of high-resolution powder neutron diffraction data. The space group is Iba2, a=1103.19(16), b=1898.63(26), c=554.92(8) pm; R(F2)=0.117, Rwp=0.088, Rp=0.067. In the investigated multi-phase sample, the Sr4Fe4Co2O13 phase exists as a major phase together with oxygen deficient SrFeO3–x, in about equal mass fractions. Sr4Fe4Co2O13 is isostructural with Sr4Fe6O13 and adopts a variant of the perovskite type-structure where layers of corner-sharing FeO6 octahedra are separated by double layers of (Fe,Co)O4 and (Fe,Co)O5 coordination polyhedra. Condensed chains of (Fe,Co)O4 and (Fe,Co)O5 polyhedra run along [001]. Of the three non-equivalent Fe sites, the octahedral site is entirely occupied by Fe, the square pyramidal site by 61% Co and the trigonal pyramidal site by 52% Co. The refined chemical composition of the unit cell is Sr16Fe15.0Co9.0O51.84 . Possible structural reasons for the reported very high ion conductivity of Sr4Fe4Co2O13 are discussed.