Crystal structure of the mixed conductor Sr4Fe4Co2O13

Abstract

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, R_wp=0.088, R_p=0.067. In the investigated multi-phase sample, the Sr₄Fe₄Co₂O₁₃ phase exists as a major phase together with oxygen deficient SrFeO_3–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 Sr₁₆Fe_15.0Co_9.0O_51.84 . Possible structural reasons for the reported very high ion conductivity of Sr₄Fe₄Co₂O₁₃ are discussed.