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.Keywords: electronic structures; oxyfluorides; pulsed-laser depositions; resistance modulations; topotactic reactions;

We investigated the polyvinylidene fluoride (PVDF)-mediated topotactic fluorination of two perovskite ruthenate thin films with different crystallographic structures, non-layered perovskite SrRuO3 and layered perovskite Sr2RuO4. While the former ruthenate did not react with PVDF, the latter was fluorinated to form Sr2RuO3F2 with a structure that exhibited a largely expanded c-axis. These results indicate that fluorine ions are preferentially inserted into the SrO rocksalt blocks in perovskite ruthenates, and that the oxygen atoms in the precursor oxide were partially removed upon fluorination. Finally, we noted that the Sr2RuO3F2 film was insulating, with a resistivity of 4.1 × 10 Ω cm at 300 K. It should be noted that this resistivity is five orders of magnitude higher than that of the metallic Sr2RuO4 film (6.7 × 10−4 Ω cm).

A new phase of oxyhydride NdNiOxHy with a defect-fluorite structure was obtained by a soft chemical reaction of NdNiO3 epitaxial thin films on a substrate of SrTiO3 (100) with CaH2. The epitaxial relationship of this phase relative to SrTiO3 could be controlled by changing the reaction temperature. At 240 °C, NdNiOxHy grew with a [001] orientation, forming a thin layer of infinite-layer NdNiO2 at the interface between the NdNiOxHy and the substrate. Meanwhile, a high-temperature reaction at 400 °C formed [110]-oriented NdNiOxHy without NdNiO2.

Since the discovery of oxyfluoride cuprate superconductors, many efforts have been made to search for new transition-metal oxyfluoride compounds. Recently, the topotactic fluorination reaction using polyvinylidene fluoride (PVDF) has gained attention because of the low-temperature synthesis of oxyfluorides. In this study, we report the fabrication of SrCoOxFy epitaxial thin films via topotactic fluorination of SrCoO2.5 precursor films with PVDF. X-ray diffraction analysis showed that the SrCoOxFy film, with an anion-vacant perovskite structure, was obtained by fluorination at 150 °C and that the in-plane lattice constant was completely dependent on the substrate. Energy dispersive X-ray spectrometry revealed that the chemical composition of the fluorinated film was SrCoO1.9±0.4F0.5±0.1 and X-ray photoemission spectroscopy showed that the Co ions had a mixed valence state of 2+ and 3+. This valence state was smaller than that in the SrCoO2.5 precursor film, indicating that PVDF acted as a reductive fluorinating agent for the SrCoO2.5 film. Moreover, the SrCoO1.9±0.4F0.5±0.1 film did not exhibit ferromagnetism even at 10 K, suggesting the presence of an antiferromagnetic interaction between the Co ions.

We report herein the topotactic fluorination of SrFeO3−δ thin films (δ ∼ 0, 0.5, 1) with polyvinylidene fluoride (PVDF). SrFeO3−xFx epitaxial thin films were obtained by fluorination at 150–270 °C, which is substantially lower than the reaction temperature for polycrystalline bulk samples prepared with PVDF. The fluorine content (x) of the film was widely varied by controlling the PVDF-treatment temperature and/or the amount of oxygen vacancies in the precursor film. The higher reactivity of the SrFeO2 and SrFeO2.5 thin films can be reasonably explained by a fluorine-diffusion mechanism via oxygen vacancies.

We investigated the electronic states of a single-crystal SrFeO2 epitaxial thin film in the valence-band and conduction-band regions using synchrotron-radiation X-ray photoemission and absorption spectroscopies. Fe 2p–3d resonant photoemission measurements revealed that the Fe 3d states have higher densities of states at binding energies of 3–5 eV and 5–8.5 eV in the valence-band region. The O K-edge X-ray absorption spectrum exhibited three peaks in the Fe 3d-derived conduction band hybridized with O 2p states; these can be assigned to Fe 3dxy, 3dxz + 3dyz, and 3dx2–y2. In addition, the indirect bandgap value of the SrFeO2 film was determined to be 1.3 eV by transmission and absorption spectroscopies.Highlights► Electronic states of infinite-layer SrFeO2 films have been experimentally observed. ► Fe 3d states have higher densities of states in the valence-band region. ► Three peaks derived from Fe 3d states were observed in the conduction-band region. ► Indirect bandgap value was determined to be 1.3 eV.

We report herein the topotactic fluorination of SrFeO3−δ thin films (δ ∼ 0, 0.5, 1) with polyvinylidene fluoride (PVDF). SrFeO3−xFx epitaxial thin films were obtained by fluorination at 150–270 °C, which is substantially lower than the reaction temperature for polycrystalline bulk samples prepared with PVDF. The fluorine content (x) of the film was widely varied by controlling the PVDF-treatment temperature and/or the amount of oxygen vacancies in the precursor film. The higher reactivity of the SrFeO2 and SrFeO2.5 thin films can be reasonably explained by a fluorine-diffusion mechanism via oxygen vacancies.

A new phase of oxyhydride NdNiOxHy with a defect-fluorite structure was obtained by a soft chemical reaction of NdNiO3 epitaxial thin films on a substrate of SrTiO3 (100) with CaH2. The epitaxial relationship of this phase relative to SrTiO3 could be controlled by changing the reaction temperature. At 240 °C, NdNiOxHy grew with a [001] orientation, forming a thin layer of infinite-layer NdNiO2 at the interface between the NdNiOxHy and the substrate. Meanwhile, a high-temperature reaction at 400 °C formed [110]-oriented NdNiOxHy without NdNiO2.