High Dilution of Anionic Vacancies in Sr_0.8Ba_0.2Fe(O,F)_∼2.5

The (Ba,Sr)­FeO3−δ system is known for its strong tendency for oxygen and vacancies to order into several forms including fully ordered pseudobrownmillerites, hexagonal perovskites with segregation of the vacancies in particular anionic layers and low deficient (pseudo)­cubic compounds (generally δ < 0.27, Fe3/4+). We show for the first time, using a simple chemical process, the easy access to a large amount of vacancies (δ ≈ 0.5, Fe3+) within the room-temperature stable tetragonal (pseudocubic) Sr0.8Ba0.2FeF∼0.1(O,F)∼2.5. The drastic effect of the incorporation of a minor amount of fluoride passes through the repartition of local O/F/□ constraints shifting the tolerance factor into the pseudocubic range for highly deficient compounds. It is stable up to 670 K, where an irreversible reoxidation process occurs, leading to the cubic-form. The comparison with the cubic oxide Sr0.8Ba0.2FeO∼2.7 shows the increase of the resistivity (3D-VRH model) by two decades due to the almost single valent Fe3+ of the oxofluoride. In addition, the G-type magnetic ordering shows relatively weak moment for Fe3+ cations (MFe ≈ 2.64(1) μB at room temperature) attributed to incoherent magnetic components expected from local disorder in such anionic-deficient compounds.(Ba,Sr)FeO₃−δ体系因晶格氧与氧空位极易有序化而闻名，可形成多种结构，包括完全有序的假褐锰矿（pseudobrownmillerites）相、空位在特定阴离子层中偏析的六方钙钛矿（hexagonal perovskites）相，以及低缺位（伪）立方相（通常δ<0.27，Fe价态为+3/+4）。本研究首次通过简单化学合成方法，在室温稳定的四方（伪立方）相Sr₀.₈Ba₀.₂FeF₀.₁(O,F)₂.₅中实现了大量氧空位的便捷制备（δ≈0.5，Fe为+3价）。少量氟离子的掺入通过调控局域O/F/空位的约束分布，将高缺位体系的容忍因子调控至伪立方区间，产生了显著调控效果。该相在670 K以下保持稳定，当温度升至670 K时会发生不可逆再氧化过程，转变为立方相。与立方氧化物Sr₀.₈Ba₀.₂FeO₂.₇的对比研究表明，由于该氧氟化物中Fe几乎仅以+3价存在，其电阻率（基于三维变程跳跃模型（3D-VRH））提升了两个数量级。此外，该体系的G型磁有序结构中，Fe³+阳离子的磁矩相对较弱（室温下MFe≈2.64(1) μB），这可归因于这类阴离子缺位化合物中局域无序所引发的非相干磁分量。