Reactive solid phase epitaxy of layered aurivillius-type oxyfluorides Bi2TiO4F2 using polyvinylidene fluoride

Aurivillius-type oxyfluorides are promising ferroelectric and photocatalytic materials. However, their thin films have yet to be fabricated because of the difficulty of synthesis when using both conventional high-temperature gas-phase processes and low-temperature topotactic methods. Here, we present reactive solid phase epitaxy of a layered Aurivillius-type oxyfluoride Bi2TiO4F2 from room-temperature fabricated Bi2TiOx using polyvinylidene fluoride (PVDF) as a fluorine source. Bi2TiO4F2 epitaxial films are obtained by reacting a room-temperature fabricated precursor with PVDF at 330 °C under an Ar flow. However, crystallization does not proceed through PVDF treatment in air, indicating that a reduced atmosphere is crucial to removing oxide ions from the precursor and incorporating fluoride ions. The Bi2TiO4F2 film shows a peak at 240 K in the dielectric constant-versus-temperature curve, which originates from the tilting of Ti(O,F)6 octahedra. This peak temperature is lower than that of the bulk (284 K), suggesting that the local structural distortion is suppressed because of the epitaxial strain from the substrate. Reactive solid phase epitaxy using PVDF as described in this paper should provide a new means of synthesizing transition-metal oxyfluorides in the epitaxial thin-film form.

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.

Synthesis and characterisation of fluorinated epitaxial films of BaFeO2F: tailoring magnetic anisotropyviaa lowering of tetragonal distortion

In this article, we report on the synthesis and characterisation of fluorinated epitaxial films of BaFeO₂F via low-temperature fluorination of thin films of BaFeO_2.5+d grown by pulsed laser deposition. Diffraction measurements show that fluoride incorporation only results in a contraction of the film perpendicular to the film surface, where clamping by the substrate is prohibitive for strong in-plane changes. The fluorinated films were found to be homogenous regarding the fluorine content over the whole film thickness, and can be considered as single crystal equivalents to the bulk phase BaFeO₂F. Surprisingly, fluorination resulted in the change of the tetragonal distortion to a nearly cubic symmetry, which results in a lowering of anisotropic orientation of the magnetic moments of the antiferromagnetically ordered compound, confirmed by Mössbauer spectroscopy and magnetic studies.

Fluorination of epitaxially grown thin films of BaFeO_2.5 to BaFeO₂F results in increased magnetic anisotropy.

Synthesis of new Ln4(Al2O6F2)O2 (Ln = Sm, Eu, Gd) phases with a cuspidine-related structure

The first fluorination of the cuspidine-related phases of Ln₄(Al₂O₇□)O₂ (where Ln = Sm, Eu, Gd) is reported. A low-temperature reaction with poly(vinyl-idene difluoride) lead to the fluorine being substituted in place of oxygen and inserted into the vacant position between the dialuminate groups. X-ray photoelectron spectroscopy shows the presence of the F 1s photoelectron together with an increase in Al 2p and rare-earth 4d binding energies supporting F incorporation. Energy-dispersive X-ray spectroscopy analyses are consistent with the formula Ln₄(Al₂O₆F₂)O₂, confirming that substitution of one oxygen by two fluoride atoms has been achieved. Rietveld refinements show an expansion in the cell upon fluorination and confirm that the incorporation of fluoride in the Ln₄(Al₂O₇□)O₂ structure results in changes in Al coordination from four to five. Thus, the isolated tetrahedral dialuminate Al₂O₇ groups are converted to chains of distorted square-based pyramids. These structural results are also discussed based on Raman spectra.

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 Ba_0.5Bi_0.5F_2.5 (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.

Anion Doping of Ferromagnetic Thin Films of La0.74Sr0.26MnO3-δ via Topochemical Fluorination

Chemical doping via insertion of ions into the lattice of a host material is a key strategy to flexibly manipulate functionalities of materials. In this work, we present a novel case study on the topotactic insertion of fluoride ions into oxygen-deficient ferromagnetic thin films of La_0.74Sr_0.26MnO_3−δ (LSMO) epitaxially grown onto single-crystal SrTiO₃ (STO) substrates. The effect of fluorination on the film structure, composition, and magnetic properties is compared with the case of oxygen-deficient and fully-oxidized LSMO films. Although incorporation of F⁻ anions does not significantly alter the volume of the LSMO unit cell, a strong impact on the magnetic characteristics, including a remarkable suppression of Curie temperature and saturation magnetization accompanied by an increase in magnetic coercivity, was found. The change in magnetic properties can be ascribed to the disruption of the ferromagnetic exchange interactions along Mn-anion-Mn chains driven by F⁻ doping into the LSMO lattice. Our results indicate that F⁻ doping is a powerful means to effectively modify the magnetic functional properties of perovskite manganites.