Evolution of oxygen-vacancy ordered crystal structures in the perovskite series SrnFenO3n-1 (n=2, 4, 8, and infinity), and the relationship to electronic and magnetic properties

Over the oxygen composition range 2.5 less than or equal to x less than or equal to 3.0, the SrFeOx system exists as four distinct compounds with the nominal composition SrnFenO3n-1 (n = 2, 4, 8, and infinity). The end member SrFeO3 (n = infinity) possesses a simple cubic perovskite crystal structur e, whereas the oxygen-deficient (n = 2, 4, and 8) members each adopt a diff erent vacancy-ordered perovskite crystal structure. Using time-of-flight ne utron powder diffraction, we show that previously proposed structures for t he Sr4Fe4O11 (n = 4) and Sr8Fe8O23 (n = 8) compounds are incorrect. We dete rmine the correct crystal structures for Sr4Fe4O11 (orthorhombic, space gro up Cmmm, a = 10.974(1) Angstrom, b = 7.702(1) Angstrom, and c = 5.473(1) An gstrom) and Sr8Fe8O23 (tetragonal, space group I4/mmm, a = 10.929(1) Angstr om and c = 7.698(1) Angstrom) through comparisons of the goodness of fit fo r Rietveld refinements of candidate models and bond-length distributions fo r each model. Using the correct crystal structures, we are able to assign v alence states to the Fe crystallographic sites and to achieve consistency w ith published Mossbauer results for the same compounds. (C) 2000 Academic P ress.