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(F-2)= 0.117, R-wp = 0.088, R-p = 0.057. 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)O-4 and (Fe,Co)O-5 coordination polyhedra. Condensed chains of (Fe,Co)O-4 and (Fe,Co)O-5 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% Ca 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.