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

Abstract

Controlling the distribution of mixed\nanions around a metal center\nis a long-standing subject in solid state chemistry. We successfully\nobtained single crystals of an iron-based layered perovskite compound,\nSr₂FeO₃F, by utilizing a high-pressure and high-temperature\ntechnique. The phase prepared at 1300 °C and 3 GPa crystallized\nin tetragonal space group <i>P</i>4/<i>nmm</i> with O/F atoms at the apical sites being ordered. However, a temperature\nof 1800 °C and a pressure of 6 GPa resulted in partial O/F site\ndisordering. The degree of anion disordering, which was 5%, showed\nthat the anion-ordered arrangement was quite robust, in sharp contrast\nto that of Sr₂BO₃F (B = Co or Ni) with the fully\ndisordered state. ⁵⁷Fe Mössbauer spectroscopy measurements\nrevealed no large difference in Néel temperatures between the\ntwo phases, but the phase prepared under the latter condition exhibited\na quasi-continuous distribution of hyperfine fields caused by O/F\nsite disordering. We discuss the mechanism of the anion order-to-disorder\ntransition observed in related oxyfluoride perovskites.