Strain-induced creation and switching of anion vacancy layers in perovskite oxynitrides

Using strain to control oxynitride properties. 京都大学プレスリリース. 2020-12-01.原子空孔の配列を制御する新手法の発見. 京都大学プレスリリース. 2020-12-02.Perovskite oxides can host various anion-vacancy orders, which greatly change their properties, but the order pattern is still difficult to manipulate. Separately, lattice strain between thin film oxides and a substrate induces improved functions and novel states of matter, while little attention has been paid to changes in chemical composition. Here we combine these two aspects to achieve strain-induced creation and switching of anion-vacancy patterns in perovskite films. Epitaxial SrVO3 films are topochemically converted to anion-deficient oxynitrides by ammonia treatment, where the direction or periodicity of defect planes is altered depending on the substrate employed, unlike the known change in crystal orientation. First-principles calculations verified its biaxial strain effect. Like oxide heterostructures, the oxynitride has a superlattice of insulating and metallic blocks. Given the abundance of perovskite families, this study provides new opportunities to design superlattices by chemically modifying simple perovskite oxides with tunable anion-vacancy patterns through epitaxial lattice strain

Improvement of electric insulation in dielectric layered perovskite nickelate films via fluorination

Layered perovskite nickelates have recently emerged as materials with colossal dielectric permittivity. However, they exhibit relatively high values of loss tangent (tan delta) owing to insufficient electric insulation; thus, lowering of tan delta is crucial for their use in practical applications. Herein, we demonstrate that fluorine doping is an effective way to improve the electrical insulation. Epitaxial thin films of La3/2Sr1/2NiOxFy were prepared via low-temperature topotactic fluorination of oxide precursors. The fluorine content (y) was controllable over a wide range of 0.4-3. The film with y similar to 0.4 exhibited 10(4) times lower leakage current than the precursor oxide film, leading to a low tan delta of 0.02-0.03 at 1-10 kHz. First-principles calculations showed that high electric insulation is a consequence of suppressed hopping of holes in the Ni 3d orbitals owing to random distortion of Ni-O-Ni and Ni-F-Ni bonds. Fluorine doping can provide large and random bond distortions, unlike conventional cation doping. In addition, the dielectric constant of the film with y similar to 0.4 was maintained at a high value of 9.4 x 10(2) at 1 kHz, which can be rationalized by assuming that holes were located at the Ni 3d orbital in less-tilted octahedrons

Ionic order and magnetic properties of double-perovskite GdBaCo2O5.5 films on SrTiO3 substrates

Double-perovskite GdBaCo2O5.5 epitaxial films were fabricated on SrTiO3(001) substrates. X-ray diffraction and scanning transmission electron microscopy measurements of a 37 nm thick film confirmed the ionic order of Gd and Ba along the out-of-plane direction. The Gd/Ba order was absent within the region 1 nm from the film/ substrate interface owing to the migration of Sr ions from the substrate. Unlike the 37 nm thick film, a thin film with a thickness of 10 nm did not show a distinct antiferromagnetic to ferromagnetic (FM) transition, whereas large magnetization was observed at low temperature. This was proposed to be because the cation-disordered phase near the interface was FM, even at low temperatures. ??2022 The Ceramic Society of Japan. All rights reserved

Fluorination and reduction of CaCrO3 by topochemical methods

Topochemical reactions between CaCrO3 and polyvinylidene difluoride yield the new fluorinated phase CaCrO2.5F0.5, which was characterized by powder synchrotron X-ray diffraction, X-ray photoemission spectroscopy, and magnetic susceptibility measurements. The reaction proceeds via reduced oxide intermediates, CaCrO2.67 and CaCrO2.5, in which CrO6 octahedral and CrO4 tetrahedral layers are stacked in a different manner along the c axis of CaCrO3. These two intermediate phases can be selectively synthesized by the carbothermal reduction with g-C3N4. Both CaCrO3 and CaCrO2.5F0.5 adopt the same orthorhombic space group, Pbnm; however, the fluorinated phase has decreased Cr-O-Cr bond angles as compared to the parent compound in both the ab plane and along the c-direction, which indicates an increased orthorhombic distortion due to the fluorination. While the oxygen vacancies are ordered in both intermediate phases, CaCrO2.67 and CaCrO2.5, a site preference for fluorine in the oxyfluoride phase cannot be confirmed. CaCrO3 and CaCrO2.5F0.5 undergo antiferromagnetic phase transitions involving spin canting, where the fluorination causes the transition temperature to increase from 90 K to 110 K, as a result of the competition between the increased octahedral tilting and the enhancement of superexchange interactions involving Cr3+ ions in the CaCrO2.5F0.5 structure