Fabrication of Fluorite-Type Fluoride Ba _0.5Bi _0.5F _2.5 Thin Films by Fluorination of Perovskite BaBiO ₃ Precursors with Poly(vinylidene fluoride)

Abstract

Metal fluorides are gathering significant interest for use in many applications, such as optical glasses, chemical sensors, and solid electrolytes using fluoride ion batteries, due to their high transparency over a wide wavelength range (ultraviolet to infrared) and fast fluoride ion conductivity. Here, we present a topotactic route for synthesizing thin films of fluorite-type Ba _0.5Bi _0.5F _2.5 (BBF), a promising fluoride ion conductor, from perovskite-type BaBiO ₃ (BBO) precursor films by fluorination using poly(vinylidene fluoride). The fluorination reaction fully converted BBO to BBF without stopping at the oxyfluoride stage. The BBF films obtained at relatively low reaction temperatures (150–200 °C) showed Ba/Bi cation ordering in the [001] direction, indicating that the cation framework of perovskite BBO was maintained during the fluorination reaction. Meanwhile, increasing the fluorination temperature led to mixtures of cations, resulting in random distribution of Ba and Bi. This demonstrates that the degree of cation ordering in BBF can be controlled by adjusting the fluorination temperature.