Selective fluorination of perovskite iron oxide/ruthenium oxide heterostructures via a topotactic reaction.

We demonstrated selective fluorination of a SrFeO2.5 layer in a SrFeO2.5/SrRuO3 epitaxial bilayer film via a topotactic reaction with polyvinylidene difluoride while maintaining the epitaxial relationship. Physical property measurements of the SrFeO2F/SrRuO3 heterostructure confirmed that the SrRuO3 layer could work as a good bottom electrode.

Fabrication of Fluorite-Type Fluoride Ba0.5Bi0.5F2.5 Thin Films by Fluorination of Perovskite BaBiO3 Precursors with Poly(vinylidene fluoride).

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 Ba0.5Bi0.5F2.5 (BBF), a promising fluoride ion conductor, from perovskite-type BaBiO3 (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.

Reversible Changes in Resistance of Perovskite Nickelate NdNiO3 Thin Films Induced by Fluorine Substitution.

Perovskite nickel oxides are of fundamental as well as technological interest because they show large resistance modulation associated with phase transition as a function of the temperature and chemical composition. Here, the effects of fluorine doping in perovskite nickelate NdNiO3 epitaxial thin films are investigated through a low-temperature reaction with polyvinylidene fluoride as the fluorine source. The fluorine content in the fluorinated NdNiO3-xFx films is controlled with precision by varying the reaction time. The fully fluorinated film (x ≈ 1) is highly insulating and has a bandgap of 2.1 eV, in contrast to NdNiO3, which exhibits metallic transport properties. Hard X-ray photoelectron and soft X-ray absorption spectroscopies reveal the suppression of the density of states at the Fermi level as well as the reduction of nickel ions (valence state changes from +3 to +2) after fluorination, suggesting that the strong Coulombic repulsion in the Ni 3d orbitals associated with the fluorine substitution drives the metal-to-insulator transition. In addition, the resistivity of the fluorinated films recovers to the original value for NdNiO3 after annealing in an oxygen atmosphere. By application of the reversible fluorination process to transition-metal oxides, the search for resistance-switching materials could be accelerated.