Yoshiyuki Inaguma

We synthesized a perovskite-type RbNbO3 at 1173 K and 4 GPa from non-perovskite RbNbO3 and investigated its crystal structure and properties towards ferroelectric material design. Single-crystal X-ray diffraction analysis revealed an orthorhombic cell in the perovskite-type structure (space group Amm2, no. 38) with a = 3.9937(2) Å, b = 5.8217(3) Å, and c = 5.8647(2) Å. This non-centrosymmetric space group is the same as the ferroelectric BaTiO3 and KNbO3 but with enhanced distortion. Structural transition from orthorhombic to two successive tetragonal phases (Tetra1 at 493 K, Tetra2 at 573 K) was observed, maintaining the perovskite framework before reverting to the triclinic ambient phase at 693 K, with no structural changes between 4 and 300 K. The first transition is similar to that of KNbO3, whereas the second to Tetra2, marked by c-axis elongation and a significant cp/ap ratio jump (from 1.07 to 1.43), is unique. This distortion suggests a transition similar to that of PbVO3, where an octahedron's oxygen separates along the c-axis, forming a pyramid. Ab initio calculations simulating negative pressure like thermal expansion predicted this phase transition (cp/ap = 1.47 at -1.2 GPa), aligning with experimental findings. Thermal analysis revealed two endothermic peaks, with the second transition entailing a greater enthalpy change and volume alteration. Strong second harmonic generation signals were observed across Ortho, Tetra1, and Tetra2 phases, similar to BaTiO3 and KNbO3. Permittivity increased during the first transition, although the second transition's effects were limited by thermal expansion-induced bulk sample collapse. Perovskite-type RbNbO3 emerges as a promising ferroelectric material.

We attempted to synthesize complex metal fluorides via reaction between metal and solid-state fluorine sources and succeeded in preparing trirutile-type Li2MoF6 using LiF, the metal Mo, and CuF2. We also found a new phase of Li2MoF6 that is isostructural with trigonal Li2ZrF6 via a combination of solid-state fluorine sources and high-pressure synthesis. The reaction occurs exothermically and involves conversion and addition associated with redox reaction, and CuF2 then functions as both an oxidizing agent and fluorine source. Because the overall reaction proceeds stoichiometrically, the required amount of fluorine can be controlled by the amount of solid-state fluorine agents. The synthesis route was also applicable for the preparation of other known fluorides, Li2MF6 (M = Ti, Zr, and Nb) and β-Li3MF6 (M = Ti and V). The synthetic route using a solid-state fluorine source is suitable for the exploration of novel inorganic complex metal fluorides.

<p>The PbFeO₂F serves as a bifunctional material for a water-oxidation photoanode workable under a wide range of visible light and a water-oxidation electrocatalyst operatable at a relatively low overpotential.</p>

<p>New Sillén–Aurivillius layered oxychlorides with triple-, quadruple- and quintuple-perovskite layers (<italic>n</italic> = 3−5) are successfully synthesized and their photocatalytic activities are enhanced with increasing the number of the perovskite layers.</p>

<p>A mechanochemical synthesis was developed to generate a highly active electrocatalyst featuring catalytic cobalt sites embedded within an electron-withdrawing fluorophosphate host.</p>

<p>A synthetic approach involving the HSAB principle provided the perovskite-type oxyfluoride AgTiO₂F.</p>