title: Crystallographic and magnetic structure of the perovskite-type compound BaFeO2.5: unrivaled complexity in oxygen vacancy ordering.
abstract: We report here on the characterization of the vacancy-ordered perovskite-type structure of BaFeO2.5 by means of combined Rietveld analysis of powder X-ray and neutron diffraction data. The compound crystallizes in the monoclinic space group P2(1)/c [a = 6.9753(1) Å, b = 11.7281(2) Å, c = 23.4507(4) Å, β = 98.813(1)°, and Z = 28] containing seven crystallographically different iron atoms. The coordination scheme is determined to be Ba7(FeO4/2)1(FeO3/2O1/1)3(FeO5/2)2(FeO6/2)1 = Ba7Fe([6])1Fe([5])2Fe([4])4O17.5 and is in agreement with the (57)Fe Mössbauer spectra and density functional theory based calculations. To our knowledge, the structure of BaFeO2.5 is the most complicated perovskite-type superstructure reported so far (largest primitive cell, number of ABX2.5 units per unit cell, and number of different crystallographic sites). The magnetic structure was determined from the powder neutron diffraction data and can be understood in terms of "G-type" antiferromagnetic ordering between connected iron-containing polyhedra, in agreement with field-sweep and zero-field-cooled/field-cooled measurements.
authors.affiliation: Joint Research Laboratory Nanomaterials and §Fachgebiet Materialmodellierung, Technische Universität Darmstadt , Jovanka-Bontschits-Straße 2, 64287 Darmstadt, Germany.
authors.forename: Oliver
authors.initials: O
authors.lastname: Clemens
authors.forename: Melanie
authors.initials: M
authors.lastname: Gröting
authors.forename: Ralf
authors.initials: R
authors.lastname: Witte
authors.forename: J Manuel
authors.initials: JM
authors.lastname: Perez-Mato
authors.forename: Christoph
authors.initials: C
authors.lastname: Loho
authors.forename: Frank J
authors.initials: FJ
authors.lastname: Berry
authors.forename: Robert
authors.initials: R
authors.lastname: Kruk
authors.forename: Kevin S
authors.initials: KS
authors.lastname: Knight
authors.forename: Adrian J
authors.initials: AJ
authors.lastname: Wright
authors.forename: Horst
authors.initials: H
authors.lastname: Hahn
authors.forename: Peter R
authors.initials: PR
authors.lastname: Slater