Anion ordering enables fast H¯ conduction at low temperatures

H¯イオンの低温高速伝導を実現. 京都大学プレスリリース. 2021-06-03.Bringing order to hydrogen energy devices. 京都大学プレスリリース. 2021-06-03.The introduction of chemical disorder by substitutional chemistry into ionic conductors is the most commonly used strategy to stabilize high-symmetric phases while maintaining ionic conductivity at lower temperatures. In recent years, hydride materials have received much attention owing to their potential for new energy applications, but there remains room for development in ionic conductivity below 300°C. Here, we show that layered anion-ordered Ba2−δH3−2δX (X = Cl, Br, and I) exhibit a remarkable conductivity, reaching 1 mS cm⁻¹ at 200°C, with low activation barriers allowing H⁻ conduction even at room temperature. In contrast to structurally related BaH2 (i.e., Ba2H4), the layered anion order in Ba2−δH3−2δX, along with Schottky defects, likely suppresses a structural transition, rather than the traditional chemical disorder, while retaining a highly symmetric hexagonal lattice. This discovery could open a new direction in electrochemical use of hydrogen in synthetic processes and energy devices

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

トポケミカルおよび高圧合成法を用いた遷移金属複合アニオン酸化物の合成

京都大学0048新制・課程博士博士(工学)甲第20763号工博第4415号新制||工||1686(附属図書館)京都大学大学院工学研究科物質エネルギー化学専攻(主査)教授 陰山 洋, 教授 阿部 竜, 教授 江口 浩一学位規則第4条第1項該当Doctor of Philosophy (Engineering)Kyoto UniversityDGA

Direct synthesis of barium titanium oxyhydride promising for electrode material with hydrogen permeability

Barium titanium oxyhydride BaTiO3–xHx is a promising functional material that exhibits H–/e– mixed conduction and catalytic activity. Here we firstly report a direct synthesis of BaTiO3–xHx by mechanochemical method. The prepared polycrystalline sample was able to work as electrodes exhibiting hydrogen pearmeability

Theoretical band structure of the superconducting antiperovskite Sr₃₋ₓ SnO

In order to investigate the position of the strontium deficiency in superconductive Sr₃₋ₓ SnO, we synthesized and measured X-ray-diffraction patterns of Sr₃₋ₓ SnO (x ~ 0.5). Because no clear peaks originating from superstructures were observed, strontium deficiency is most likely to be randomly distributed. We also performed first-principles band-structure calculations on Sr₃₋ₓ SnO (x = 0, 0.5) using two methods: full-potential linearized-augmented plane-wave plus local orbitals method and the Korringa-Kohn-Rostoker Green function method combined with the coherent potential approximation. We revealed that the Fermi energy of Sr₃₋ₓ SnO in case of x ~ 0.5 is about 0.8 eV below the original Fermi energy of the stoichiometric Sr₃SnO, where the mixing of the valence p and conduction d orbitals are considered to be small

Hypervalent Bismuthides La₃MBi₅ (M = Ti, Zr, Hf) and Related Antimonides: Absence of Superconductivity

We successfully synthesized the ternary bismuthides La₃MBi₅ (M = Ti, Zr, Hf). These compounds crystallize in the hexagonal Hf₅Sn₃Cu-anti type structure (space group: <i>P</i>6₃/<i>mcm</i>) consisting of face-sharing MBi₆ octahedral chains and hypervalent Bi linear chains, both separated by La atoms. Magnetic susceptibility and electrical resistivity measurements revealed that all of the compounds, including the solid solution La₃Ti­(Bi_1–<i>x</i>Sb_<i>x</i>)₅, exhibit a Pauli paramagnetic behavior without any trace of superconductivity down to 1.85 K, as opposed to a recently reported 4 K superconductivity in La₃TiSb₅. The absence of superconductivity is supported by first-principles band calculations of La₃TiBi₅ and La₃TiSb₅ that demonstrate similar electronic structures with three-dimensional Fermi surfaces

Single Crystal Growth of Sillén–Aurivillius Perovskite Oxyhalides Bi₄NbO₈X (X = Cl, Br)

Sillén–Aurivillius perovskite Bi₄NbO₈X (X = Cl, Br) is a promising photocatalyst for water splitting under visible light, as well as a potential ferroelectric material. In this work, we investigate the crystal growth conditions by mainly varying soak temperature, soak time and cooling rate. Under the optimal conditions, we successfully obtained yellow platelet single crystals with an in-plane distance of several hundred microns. As opposed to conventional crystal growth, a moderate cooling is essential to suppress an evaporation of the Bi–O–Cl species from a melt zone. The single crystals of Bi4NbO8Br were also grown using a similar condition. We suggest that the knowledge obtained in this study can be generally applied to other Sillén–Aurivillius phases and related oxyhalide

Promoted Hydride/Oxide Exchange in SrTiO₃ by Introduction of Anion Vacancy via Aliovalent Cation Substitution

We investigated topochemical anion exchange reactions for a Sc^III-substituted SrTi^IVO₃ perovskite, Sr­(Ti_1–<i>y</i>Sc_<i>y</i>)­O_{3–<i>y</i>/2}□_<i>y</i>/2 (<i>y</i> ≤ 0.1), using CaH₂. It was found that the initial introduction of a small amount of anion vacancies (<i>y</i>/2) is crucial to enhance the anion (H^–/O^2–) exchangeability. For example, hydride reduction of Sr­(Ti_0.95Sc_0.05)­O_2.975 yielded the oxyhydride SrTi_0.95Sc_0.05O_2.56H_0.41 in which the hydride concentration is increased by 33% with respect to pristine SrTiO₃ (leading to SrTiO_2.76H_0.24). This observation highlights the importance of anion vacancies to improve anion (H^–/O^2–) diffusion, which is a well-known strategy for improving oxide anion conductivity, and suggests that such a vacancy-assisted reaction could be applied to other anion exchange reactions (e.g., F^–/O^2– and N^3–/O^2–) to extend the solubility range