Evolution of magnetic properties in F-doped brownmillerite Ca2Fe2O5

Brownmillerite structure is derived from an ordered arrangement of oxygen vacancies within a perovskite structure, where alternating stacked layers of tetrahedral and octahedral units are formed with oxygen vacancy channels aligned in the tetrahedral layer. Naturally it becomes an important proposition to attempt anion engineering with such structures. Accordingly, we have synthesized oxyfluoride of brownmillerite Ca2Fe2O5 by low temperature fluorination technique with poly(vinylidene fluoride) as a fluorinating agent. The global as well as local structural changes and the magnetic properties upon fluorination were investigated. Extended X-Ray Absorbtion Fine Structure Spectroscopy (EXAFS) confirmed substitution of oxygen by fluorine at the tetrahedral sites, thereby changing the oxidation state of Fe which in turn tunes the magnetic properties converting the parent G-type antiferromagnet structure to a spin glass-like one at low temperature.