Fluorination of epitaxial oxides: Creating ferrite and nickelate oxyfluoride films

In ABO$_3$ perovskites, the physical properties are directly coupled to the nominal valence state of the B-site cation. In epitaxial thin films, the dominant strategy to control B-site valence is through the selection of a di- or trivalent cation on the A-site. However, this approach is limited, particularly when electron doping on the B-site is desired. Here we report a simple method for realizing oxyfluoride films, where the substitution of F for O is expected to reduce the B-site valence, providing a new means to tune electronic, optical and magnetic properties in thin films. Fluorination is achieved by spin coating an oxygen deficient film with poly(vinylidene fluoride). The film/polymer bilayer is then annealed, promoting the diffusion of F into the film. We have used this method to synthesize SrFeO$_{3-\delta}$F$_\delta$ and LaNiO$_{3-\delta}$F$_\delta$ ($\delta$ ? 0.5) films, as confirmed by x-ray photoemission spectroscopy and x-ray absorption spectroscopy.