Crystallographic and Magnetic Structure of the Perovskite-Type Compound BaFeO2.5: Unrivaled Complexity in Oxygen Vacancy Ordering

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 P21/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 57Fe 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.

本研究报道了BaFeO2.5的空位有序钙钛矿型结构表征工作，采用粉末X射线衍射与中子衍射数据联合里特维尔德精修法（Rietveld analysis）完成。该化合物以单斜晶系空间群P2₁/c结晶，晶胞参数为a=6.9753(1) Å、b=11.7281(2) Å、c=23.4507(4) Å，β=98.813(1)°，单胞内化学式单位数Z=28，晶胞中包含7个晶体学上独立的铁原子位点。经确定，其配位模式为Ba7(FeO4/2)1(FeO3/2O1/1)3(FeO5/2)2(FeO6/2)1 = Ba7Fe[6]1Fe[5]2Fe[4]4O17.5，该结果与57Fe穆斯堡尔（Mössbauer）谱及基于密度泛函理论（density functional theory）的计算结果一致。据我们所知，BaFeO2.5的结构是迄今为止报道的最为复杂的钙钛矿型超结构，其原始晶胞尺寸最大、单胞内ABX2.5型结构单元数目最多，且独立晶体学位点数目最多。磁结构通过粉末中子衍射数据得以解析，其可通过相连含铁多面体间的"G-type"反铁磁有序加以阐释，该结论与场扫及零场冷却/场冷却测量结果相符。