Anion Order-to-Disorder Transition in Layered Iron Oxyfluoride Sr2FeO3F Single Crystals

Controlling the distribution of mixed anions around a metal center is a long-standing subject in solid state chemistry. We successfully obtained single crystals of an iron-based layered perovskite compound, Sr2FeO3F, by utilizing a high-pressure and high-temperature technique. The phase prepared at 1300 °C and 3 GPa crystallized in tetragonal space group P4/nmm with O/F atoms at the apical sites being ordered. However, a temperature of 1800 °C and a pressure of 6 GPa resulted in partial O/F site disordering. The degree of anion disordering, which was 5%, showed that the anion-ordered arrangement was quite robust, in sharp contrast to that of Sr2BO3F (B = Co or Ni) with the fully disordered state. 57Fe Mössbauer spectroscopy measurements revealed no large difference in Néel temperatures between the two phases, but the phase prepared under the latter condition exhibited a quasi-continuous distribution of hyperfine fields caused by O/F site disordering. We discuss the mechanism of the anion order-to-disorder transition observed in related oxyfluoride perovskites.调控金属中心周围混合阴离子的空间分布，是固态化学领域经久不衰的研究课题。本研究通过高压高温合成技术，成功制备出铁基层状钙钛矿化合物Sr₂FeO₃F的单晶体。在1300℃、3 GPa条件下制备的物相，结晶为四方晶系，空间群为P4/nmm，其轴向位点的O/F原子呈现有序排布。但在1800℃、6 GPa条件下制备的物相，则出现了部分O/F位点无序的情况；该物相的阴离子无序度为5%，表明阴离子有序排布结构具有极强的稳定性，这与完全无序的Sr₂BO₃F（B=Co或Ni）形成鲜明对比。57Fe穆斯堡尔谱学（Mössbauer spectroscopy）测试结果表明，两种物相的奈耳温度（Néel temperature）并无显著差异；但后者因O/F位点无序，呈现出超精细场的准连续分布。我们针对相关氟氧化物钙钛矿中观测到的阴离子有序-无序转变机制展开了讨论。