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 i s 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 iso structural 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 entire ly occupied by Fe, the square pyramidal site by 61% Co and the trigona l pyramidal site by 52% Ca The refined chemical composition of the uni t cell is Sr16Fe15.0Co9.0O51.84. Possible structural reasons for the r eported very high ion conductivity of Sr4Fe4Co2O13 are discussed.