Crystallographic and Magnetic Structure of the Perovskite-Type Compound BaFeO_2.5: Unrivaled Complexity in Oxygen Vacancy Ordering

We report here on the characterization of the vacancy-ordered perovskite-type structure of BaFeO_2.5 by means of combined Rietveld analysis of powder X-ray and neutron diffraction data. The compound crystallizes in the monoclinic space group <i>P</i>2₁/<i>c</i> [<i>a</i> = 6.9753(1) Å, <i>b</i> = 11.7281(2) Å, <i>c</i> = 23.4507(4) Å, β = 98.813(1)°, and <i>Z</i> = 28] containing seven crystallographically different iron atoms. The coordination scheme is determined to be Ba₇(FeO_4/2)₁(FeO_3/2O_1/1)₃(FeO_5/2)₂(FeO_6/2)₁ = Ba₇Fe^[6]₁Fe^[5]₂Fe^[4]₄O_17.5 and is in agreement with the ⁵⁷Fe Mössbauer spectra and density functional theory based calculations. To our knowledge, the structure of BaFeO_2.5 is the most complicated perovskite-type superstructure reported so far (largest primitive cell, number of ABX_2.5 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