Hiroshi Fujihisa

Distinct Responses to Mechanical Grinding and Hydrostatic Pressure in Luminescent Chromism of Tetrathiazolylthiophene

Luminescent mechanochromism has been intensively studied in the past few years. However, the difference in the anisotropic grinding and the isotropic compression is not clearly distinguished in many cases, in spite of the importance of this discrimination for the application of such mechanochromic materials. We now report the distinct luminescent responses of a new organic fluorophore, tetrathiazolylthiophene, to these stresses. The multichromism is achieved over the entire visible region using the single fluorophore. The different mechanisms of a blue shift by grinding crystals and of a red shift under hydrostatic pressure are fully investigated, which includes a high-pressure single-crystal X-ray diffraction analysis. The anisotropic and isotropic modes of mechanical loading suppress and enhance the excimer formation, respectively, in the 3D hydrogen-bond network.

Superconductivity in Novel BiS2-Based Layered Superconductor LaO1-xFxBiS2

Layered superconductors have provided some interesting fields in condensed matter physics owing to the low dimensionality of their electronic states. For example, the high-Tc (high transition temperature) cuprates and the Fe-based superconductors possess a layered crystal structure composed of a stacking of spacer (blocking) layers and conduction (superconducting) layers, CuO2 planes or Fe-Anion layers. The spacer layers provide carriers to the conduction layers and induce exotic superconductivity. Recently, we have reported superconductivity in the novel BiS2-based layered compound Bi4O4S3. It was found that superconductivity of Bi4O4S3 originates from the BiS2 layers. The crystal structure is composed of a stacking of BiS2 superconducting layers and the spacer layers, which resembles those of high-Tc cuprate and the Fe-based superconductors. Here we report a discovery of a new type of BiS2-based layered superconductor LaO1-xFxBiS2, with a Tc as high as 10.6 K.

Pressure-Induced Enhancement of Superconductivity and Structural Transition in BiS2-Layered LaO1¹xFxBiS2

BiS2-based LaO1−xFxBiS2 (x = 0.5) becomes superconductive at Tc = 2.5 K. Electrical resistivity and magnetization measurements are performed under pressure to determine the pressure dependence of the superconducting transition temperature Tc up to 18 GPa. We observe that Tc abruptly increases from 2.5 to 10.7 K at a pressure of 0.7 GPa. According to high-pressure X-ray diffraction measurements of LaO1−xFxBiS2 (x = 0 and 0.5), a structural phase transition from tetragonal to monoclinic also occurs at ∼0.8 GPa. We consider that the pressure-induced enhancement of superconductivity for LaO1−xFxBiS2 is caused by the structural phase transition.

A new layered iron arsenide superconductor: (Ca,Pr)FeAs2.

A new iron-based superconductor, (Ca,Pr)FeAs2, was discovered. Plate-like crystals of the new phase were obtained, and its crystal structure was investigated by single-crystal X-ray diffraction analysis. The structure was identified as the monoclinic system with space group P2₁/m, composed of two Ca(Pr) planes, Fe2As2 layers, and As2 zigzag chain layers. Plate-like crystals of the new phase showed superconductivity, with a T(c) of ~20 K in both magnetization and resistivity measurements.

New-Structure-Type Fe-Based Superconductors: CaAFe4As4 (A = K, Rb, Cs) and SrAFe4As4 (A = Rb, Cs)

Fe-based superconductors have attracted research interest because of their rich structural variety, which is due to their layered crystal structures. Here we report the new-structure-type Fe-based superconductors CaAFe4As4 (A = K, Rb, Cs) and SrAFe4As4 (A = Rb, Cs), which can be regarded as hybrid phases between AeFe2As2 (Ae = Ca, Sr) and AFe2As2. Unlike solid solutions such as (Ba(1-x)K(x))Fe2As2 and (Sr(1-x)Na(x))Fe2As2, Ae and A do not occupy crystallographically equivalent sites because of the large differences between their ionic radii. Rather, the Ae and A layers are inserted alternately between the Fe2As2 layers in the c-axis direction in AeAFe4As4 (AeA1144). The ordering of the Ae and A layers causes a change in the space group from I4/mmm to P4/mmm, which is clearly apparent in powder X-ray diffraction patterns. AeA1144 is the first known structure of this type among not only Fe-based superconductors but also other materials. AeA1144 is formed as a line compound, and therefore, each AeA1144 has its own superconducting transition temperature of approximately 31-36 K.

HYDROGEN-BOND SYMMETRIZATION AND MOLECULAR DISSOCIATION IN HYDROGEN HALIDS

Abstract Hydrogen chloride is a simple diatomic molecule forming a planar zig-zag chain of molecules connected by hydrogen bonds in the solid phase. Raman spectra were measured for solid HCl to 60 GPa at room temperature. The molecular stretching frequency falls toward zero at about 51 GPa, where the molecular vibrational peaks disappear and the lattice peaks remain. The spectral changes are very similar to those observed for HBr at about 42 GPa and interpreted as hydrogen bond symmetrization. Molecular dissociation into diatomic halogen molecules, which has been observed for HBr, does not occur in HCl.

Superconductivity in Fe-Based Compound EuAFe4As4 (A = Rb and Cs)

We report the discovery of a novel Fe-based superconductor EuAFe4As4 (A = Rb, Cs) and describe its superconducting properties. EuAFe4As4 has a tetragonal unit cell with a P4/mmm (No. 123) space group, indicating that this material is an 1144-type compound. The magnetic susceptibility and electrical resistivity indicate superconducting transitions at approximately 36 and 35 K for EuRbFe4As4 and EuCsFe4As4, respectively. Moreover, an anomalous magnetic transition appears at approximately 15 K, suggesting the coexistence of superconductivity and a magnetic ordered state formed by the Eu2+ ions. The determined upper critical magnetic fields and coherence lengths are approximately 920 kOe and 1.8 nm for EuRbFe4As4 and 875 kOe and 1.9 nm for EuCsFe4As4, respectively.

distribution of butane in the host water cage of structure II clathrate hydrates.

To understand host-guest interactions of hydrocarbon clathrate hydrates, we investigated the crystal structure of simple and binary clathrate hydrates including butane (n-C4 H10 or iso-C4 H10 ) as the guest. Powder X-ray diffraction (PXRD) analysis using the information on the conformation of C4 H10 molecules obtained by molecular dynamics (MD) simulations was performed. It was shown that the guest n-C4 H10 molecule tends to change to the gauche conformation within host water cages. Any distortion of the large 5(12) 6(4) cage and empty 5(12) cage for the simple iso-C4 H10 hydrate was not detected, and it was revealed that dynamic disorder of iso-C4 H10 and gauche-nC4 H10 were spherically extended within the large 5(12) 6(4) cages. It was indicated that structural isomers of hydrocarbon molecules with different van der Waals diameters are enclathrated within water cages in the same way owing to conformational change and dynamic disorder of the molecules. Furthermore, these results show that the method reported herein is applicable to structure analysis of other host-guest materials including guest molecules that could change molecular conformations.

Crystal Structure of the High-Pressure γ Phase of Mercury: A Novel Monoclinic Distortion of the Close-Packed Structure

The crystal structure of the high-pressure γ phase of solid mercury, stable between 12 and 37 GPa, has been determined by powder x-ray diffraction experiments. The structure is monoclinic C 2/ m with six atoms in the unit cell. A mercury atom is coordinated by 10 to 11 atoms, which are distributed at distances up to 10% longer than the first nearest one. The structure is intimately related to α (simple rhombohedral), β (body-centered-tetragonal) and δ (hexagonal-close-packed) phases of mercury, where the stacking of pseudohexagonal layers is systematically modified. This novel monoclinic distortion of the close-packed structure serves as a new class of dense structures for metallic elements.

Crystal structure of anhydrous 5-aminotetrazole and its high-pressure behavior

Anhydrous 5-aminotetrazole (5-amino-1H-tetrazole, 5-ATZ) is an energetic material that produces a large amount of nitrogen gas by thermal decomposition and is often used as a gas generator agent. Although it is used widely as an air bag inflator, its crystal structure has not been determined yet. Thus, we performed a powder X-ray diffraction experiment, a Rietveld analysis, and the density functional theory calculation to investigate its structure and determined it to be an orthorhombic P212121 with a 1H-form molecule. We also investigated the high-pressure behavior and found that the orthorhombic phase is stable up to at least 11.6 GPa. Furthermore, the quantum molecular dynamics calculations at high-temperature and high-pressure of 5-ATZ were carried out and predicted the phase change including molecular decomposition.