Evolution of oxygen-vacancy ordered crystal structures in the perovskite series SrnFenO3n-1 (n=2, 4, 8, and ∞), 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 structure, whereas the oxygen-deficient (n = 2, 4, and 8) members each adopt a different vacancy-ordered perovskite crystal structure. Using time-of-flight neutron powder diffraction, we show that previously proposed structures for the Sr4Fe4O11 (n = 4) and Sr8Fe8O23 (n = 8) compounds are incorrect. We determine the correct crystal structures for Sr4Fe4O11 (orthorhombic, space group Cmmm, a = 10.974(1) Angstrom, b = 7.702(1) Angstrom, and c = 5.473(1) Angstrom) and Sr8Fe8O23 (tetragonal, space group I4/mmm, a = 10.929(1) Angstrom and c = 7.698(1) Angstrom) through comparisons of the goodness of fit for Rietveld refinements of candidate models and bond-length distributions for each model. Using the correct crystal structures, we are able to assign valence states to the Fe crystallographic sites and to achieve consistency with published Mossbauer results for the same compounds. (C) 2000 Academic Press.