Hidetaka Kasai

Design and Synthesis of Hydroxide Ion–Conductive Metal–Organic Frameworks Based on Salt Inclusion

We demonstrate a metal-organic framework (MOF) design for the inclusion of hydroxide ions. Salt inclusion method was applied to an alkaline-stable ZIF-8 (ZIF = zeolitic imidazolate framework) to introduce alkylammonium hydroxides as ionic carriers. We found that tetrabutylammonium salts are immobilized inside the pores by a hydrophobic interaction between the alkyl groups of the salt and the framework, which significantly increases the hydrophilicity of ZIF-8. Furthermore, ZIF-8 including the salt exhibited a capacity for OH(-) ion exchange, implying that freely exchangeable OH(-) ions are present in the MOF. ZIF-8 containing OH(-) ions showed an ionic conductivity of 2.3 × 10(-8) S cm(-1) at 25 °C, which is 4 orders of magnitude higher than that of the blank ZIF-8. This is the first example of an MOF-based hydroxide ion conductor.

Unusual Strong Incommensurate Modulation in a Tungsten-Bronze-Type Relaxor PbBiNb5O15.

Pb- or Bi-based perovskite oxides have been widely studied and used because of their large ferroelectric polarization features induced by stereochemically active 6s(2) lone pair electrons. It is intriguing whether this effect could exist in other related systems. Herein, we designed and synthesized a mixed Pb and Bi A site polar compound, PbBiNb5O15, with the TTB framework. The as-synthesized material turns out to be a relaxor with weak macroscopic ferroelectricity but adopts strong local polarizations. Whats more, unusual five orders of incommensurate satellite reflections with strong intensities were observed under the electron diffraction, suggesting that the modulation is highly developed with large amplitudes. The structural modulation was solved with a (3 + 1)D superspace group using high-resolution synchrotron radiation combined with anomalous dispersion X-ray diffraction technique to distinguish Pb from Bi. We show that the strong modulation mainly originates from lone-pair driven Pb(2+)-Bi(3+) ordering in the large pentagonal caves, which can suppress the local polarization in x-y plane in long ranges. Moreover, the as-synthesized ceramics display strong relaxor ferroelectric feature with transition temperature near room temperature and moderate dielectric properties, which could be functionalized to be electromechanical device materials.

Synthesis and catalytic application of PVP-coated Ru nanoparticles embedded in a porous metal–organic framework

A hybrid catalyst consisting of polymer-coated Ru nanoparticles (Ru-PVP, PVP: poly(N-vinyl-2-pyrrolidone)) embedded in a porous metal-organic framework of ZIF-8 (Ru-PVP@ZIF-8) was synthesized by the crystallization of ZIF-8 in a methanol solution of Ru-PVP. The structural properties of Ru-PVP@ZIF-8 were examined by N2 gas adsorption, infrared spectra, and X-ray powder diffraction measurements. We successfully identified the most appropriate pretreatment conditions for surface activation of the Ru nanoparticles in the catalyst. The pretreated Ru-PVP@ZIF-8 was applied for a CO oxidation reaction with H2 gas feeds. Ru-PVP@ZIF-8 was found to exhibit higher catalytic activities and higher CO2 selectivity than those observed on a carbon-supported Ru-PVP (Ru-PVP/C), implying that the pores of the ZIF-8 provide a more suitable environment for the reaction with O2 and CO gases.

X-ray electron density investigation of chemical bonding in van der Waals materials

Van der Waals (vdW) solids have attracted great attention ever since the discovery of graphene, with the essential feature being the weak chemical bonding across the vdW gap. The nature of these weak interactions is decisive for many extraordinary properties, but it is a strong challenge for current theory to accurately model long-range electron correlations. Here we use synchrotron X-ray diffraction data to precisely determine the electron density in the archetypal vdW solid, TiS2, and compare the results with density functional theory calculations. Quantitative agreement is observed for the chemical bonding description in the covalent TiS2 slabs, but significant differences are identified for the interactions across the gap, with experiment revealing more electron deformation than theory. The present data provide an experimental benchmark for testing theoretical models of weak chemical bonding.Electron density in TiS2 is determined by synchrotron X-ray diffraction, which reveals significant differences between experimental data and theory for interlayer van der Waals interactions.

Metal-Semiconductor transition concomitant with a structural transformation in tetrahedrite Cu12Sb4S13

The tetrahedrite Cu12Sb4S13 undergoes a metal–semiconductor transition (MST) at TMST = 85 K, whose mechanism remains elusive. Our Cu 2p X-ray photoemission spectroscopy study revealed the monovalent state of Cu ions occupying the two sites in this compound. This fact excludes the possibilities of previously proposed antiferromagnetic order and Jahn–Teller instability inherent in a divalent Cu system. A synchrotron X-ray diffraction study has revealed that the body-centered cubic cell of Cu12Sb4S13 transforms into a body-centered 2a × 2a × 2c tetragonal supercell below TMST, where the cell volume per formula unit expands by 0.25%. We have further studied pressure effects on the MST as well as the effects of the substitution of As for Sb. The application of pressure above 1 GPa completely inhibits the MST and leads to a metallic state, suggesting that the low-temperature structure with a larger volume becomes unstable under pressure. The As substitution also reduces the volume and suppresses the MST but the...

Carrier concentration dependence of structural disorder in thermoelectric Sn1−xTe

The crystal structure of SnTe is investigated from 20 to 800 K in two samples with different carrier concentrations by single-crystal and powder synchrotron X-ray diffraction, coupled with maximum entropy analysis.

Spatial distribution of electrons near the Fermi level in the metallic LaB6 through accurate X-ray charge density study

Charge densities of iso-structural metal hexaborides, a transparent metal LaB6 and a semiconductor BaB6, have been determined using the d > 0.22 Å ultra-high resolution synchrotron radiation X-ray diffraction data by a multipole refinement and a maximum entropy method (MEM). The quality of the experimental charge densities was evaluated by comparison with theoretical charge densities. The strong inter-octahedral and relatively weak intra-octahedral boron-boron bonds were observed in the charge densities. A difference of valence charge densities between LaB6 and BaB6 was calculated to reveal a small difference between isostructural metal and semiconductor. The weak electron lobes distributed around the inter B6 octahedral bond were observed in the difference density. We found the electron lobes are the conductive π-electrons in LaB6 from the comparison with the theoretical valence charge density. We successfully observed a spatial distribution of electrons near the Fermi level from the X-ray charge density study of the series of iso-structural solids.

Enhanced thermoelectric performance and high-temperature thermal stability of p-type Ag-doped β-Zn4Sb3

Here we report that the thermoelectric properties of bulk β-Zn4Sb3 can be improved in the range 300–575 K by Ag doping at the Zn sites. Proper Ag doping leads to decreased electrical resistivity and increased Seebeck coefficient, thus resulting in a large improvement in power factor. The figure of merit, zT, has an obvious enhancement due to Ag doping although the thermal conductivity is slightly increased. (Zn0.9925Ag0.0075)4Sb3 exhibits a promising zT of ∼1.2 at 575 K, which is superior to most previously reported p-type doped Zn4Sb3 materials. Furthermore, the high-temperature thermal stability is studied in detail. The (Zn0.9925Ag0.0075)4Sb3 bulk sample does not decompose even when the temperature is elevated to 793 K in vacuum. When the bulk sample is heated to 573 K in air, (Zn0.9925Ag0.0075)4Sb3 is also stable, unlike undoped Zn4Sb3 where Zn whiskers come out of the surface. In-house in situ powder X-ray diffraction (PXRD) and multi-temperature synchrotron PXRD (up to 793 K) reveal that the undoped Zn4Sb3 powder sample starts decomposing into ZnSb at 473 K if exposed to the air and it is fully decomposed into ZnSb, ZnO, and Sb after cooling down from 793 to 300 K. However, there is ∼24 wt% Zn4Sb3 preserved in the (Zn0.995Ag0.005)4Sb3 powder sample after the same heat treatment, while only ∼6 wt% Zn4Sb3 remains in (Zn0.99Ag0.01)4Sb3. The above result indicates that proper Ag doping leads to enhanced high-temperature thermal stability in β-Zn4Sb3. This work thereby suggests Ag-doped Zn4Sb3 bulk material as a promising candidate for thermoelectric applications in terms of enhanced performance as well as improved high-temperature thermal stability.

High-Temperature Crystal Structure and Chemical Bonding in Thermoelectric Germanium Selenide (GeSe)

The discovery of the ultra-high thermoelectric figure of merit of 2.6 in SnSe has drawn attention to other lead-free IV-VI orthorhombic semiconductors. GeSe has been predicted to possess thermoelectric performances comparable to SnSe. Here, a complete structural study is reported of GeSe with temperature by means of high-resolution synchrotron powder X-ray diffraction. In the orthorhombic phase, the evolution of the bond distances with temperature is shown to deviate significantly with respect to SnSe. Analysis of the chemical bonding within the Quantum Theory of Atoms in Molecules shows that GeSe is ionic with van der Waals interlayer interactions. The signature of the N shell lone pair of Ge is also evident from both the electron density Laplacian and the ELF topologies.

Multiple powder diffraction data for an accurate charge density study using synchrotron radiation x-ray

In recent years multiple synchrotron radiation (SR) powder x-ray diffraction profiles have been successfully applied to advanced structural studies such as an accurate charge density study and a structure determination from powder diffraction. The results have been presented with several examples. Abilities and future prospects have been discussed using state of the art powder diffraction data.