Eisuke Magome

In-plane chemical pressure essential for superconductivity in BiCh2-based (Ch: S, Se) layered structure.

BiCh2-based compounds (Ch: S, Se) are a new series of layered superconductors, and the mechanisms for the emergence of superconductivity in these materials have not yet been elucidated. In this study, we investigate the relationship between crystal structure and superconducting properties of the BiCh2-based superconductor family, specifically, optimally doped Ce1−xNdxO0.5F0.5BiS2 and LaO0.5F0.5Bi(S1−ySey)2. We use powder synchrotron X-ray diffraction to determine the crystal structures. We show that the structure parameter essential for the emergence of bulk superconductivity in both systems is the in-plane chemical pressure, rather than Bi-Ch bond lengths or in-plane Ch-Bi-Ch bond angle. Furthermore, we show that the superconducting transition temperature for all REO0.5F0.5BiCh2 superconductors can be determined from the in-plane chemical pressure.

Noncentrosymmetric Structure of LuFeO3 in Metastable State

The crystal structure of metastable LuFeO3 synthesized by containerless processing has been revealed to be a non-centrosymmetric structure (space group P63cm) by analyzing the high-energy synchrotron-radiation powder-diffraction data using the maximum entropy method (MEM)/Rietveld method. The structural characteristics are found in the FeO5 trigonal bipyramid distorted and tilted from the c-axis, which is cause by the hybridization of atomic orbitals between the O atom constituent of the polyhedron and the neighboring Lu atom. The spontaneous polarization expected from the polar structure is estimated at about 5 µC/cm2.

Superconducting Double Perovskite Bismuth Oxide Prepared by a Low‐Temperature Hydrothermal Reaction

Perovskite-type structures (ABO3) have received significant attention because of their crystallographic aspects and physical properties, but there has been no clear evidence of a superconductor with a double-perovskite-type structure, whose different elements occupy A and/or B sites in ordered ways. In this report, hydrothermal synthesis at 220 °C produced a new superconductor with an A-site-ordered double perovskite structure, (Na(0.25)K(0.45))(Ba(1.00))3(Bi(1.00))4O12, with a maximum T(c) of about 27 K.

Electric field induced lattice strain in pseudocubic Bi(Mg1/2Ti1/2)O3-modified BaTiO3-BiFeO3 piezoelectric ceramics

Contributions to the piezoelectric response in pseudocubic 0.3BaTiO3-0.1Bi(Mg1/2Ti1/2)O3-0.6BiFeO3 ceramics were investigated by synchrotron X-ray diffraction under electric fields. All of the lattice strain determined from the 110, 111, and 200 pseudocubic diffraction peaks showed similar lattice strain hysteresis that was comparable to the bulk butterfly-like strain curve. It was suggested that the hysteresis of the lattice strain and the lack of anisotropy were related to the complex domain structure and the phase boundary composition.

Compositional and temperature evolution of crystal structure of new thermoelectric compound LaOBiS2−xSex

We examined the crystal structure of the new thermoelectric material LaOBiS2-xSex, whose thermoelectric performance is enhanced by Se substitution, by using powder synchrotron X-ray diffraction and Rietveld refinement. The emergence of metallic conductivity and enhancement of the thermoelectric power factor of LaOBiS2-xSex can be explained with the higher in-plane chemical pressure caused by the increase of Se concentration at the in-plane Ch1 site (Ch = S, Se). High-temperature X-ray diffraction measurements for optimally substituted LaOBiSSe revealed anomalously large atomic displacement parameters (Uiso) for Bi and Ch atoms in the BiCh2 conduction layers. The anisotropic analysis of the atomic displacement parameters (U11 and U33) for the in-plane Bi and Ch1 sites suggested that Bi atoms exhibit large atomic displacement along the c-axis direction above 300 K, which could be the origin of the low thermal conductivity in LaOBiSSe. The large Bi vibration along the c-axis direction could be related to in-plane rattling, which is a new strategy for attaining low thermal conductivity and phonon-glass-electron-crystal states.

Crystal Structure of BaTiO3?KNbO3 Nanocomposite Ceramics: Relationship between Dielectric Property and Structure of Heteroepitaxial Interface

High-energy synchrotron radiation powder diffraction experiments have been carried out to investigate the crystal structure of solvothermally synthesized KNbO3 (KN)/BaTiO3 (BT) nanocomposite ceramics in which a ceramic grain consists of a BT nanoparticle thinly coated with KN crystals through the heteroepitaxial interface. Rietveld analysis reveals that the ceramic grain has the core/multishell structure consisting of a BT core and distorted BT and KN multishells. BT is gradually distorted in the large region to form the interface with KN from the tetragonal structure at the core toward the cubic structure at the boundary between BT and KN. The variations of the volume of the distorted interface region of BT and the dielectric property of the ceramics show similar trends to the variation of KN/BT molar ratio, which suggests that the electrically soft interface of BT nanoparticles governs the dielectric properties of the ceramics.

Hydrothermal Synthesis, Structure, and Superconductivity of Simple Cubic Perovskite (Ba0.62K0.38)(Bi0.92Mg0.08)O3 with Tc ∼ 30 K

We have synthesized a new superconducting perovskite bismuth oxide by a facile hydrothermal route at 220 °C. The choice of starting materials, their mixing ratios, and the hydrothermal reaction temperature was crucial for obtaining products with superior superconducting properties. The structure of the powder sample was investigated using laboratory X-ray diffraction, high-resolution synchrotron X-ray diffraction (SXRD) data, and electron diffraction (ED) patterns [transmission electron microscopy (TEM) analysis]. The refinement of SXRD data confirmed a simple perovskite-type structure with a cubic cell of a = 4.27864(2) Å [space group Pm3̅m (No. 221)]. Elemental analysis detected magnesium in the final products, and a refinement based on SXRD and inductively coupled plasma data yielded an ideal undistorted simple cubic perovskite-type structure, with the chemical composition (Ba0.62K0.38)(Bi0.92Mg0.08)O3. ED patterns also confirmed the simple cubic perovskite structure; the cube-shaped microstructures and compositional homogeneity on the nanoscale were verified by scanning electron microscopy and TEM analyses, respectively. The fabricated compound exhibited a large shielding volume fraction of about 98% with a maximum Tcmag of ∼30 K, which was supported by the measured bismuth valence as well. Its electrical resistivity dropped at ∼21 K, and zero resistivity was observed below 7 K. The compound underwent thermal decomposition above 400 °C. Finally, the calculated band structure showed a metallic behavior for this hydrothermally synthesized bismuth oxide.

Structural Difference in Superconductive and Nonsuperconductive Bi-S Planes within Bi4O4Bi2S4 Blocks.

The relationship between the structure and superconductivity of Bi4O4S3 powders synthesized by heating under ambient and high pressures was investigated using synchrotron X-ray diffraction, Raman spectroscopy, and transmission electron microscopy (TEM) observation. The Bi4O4S3 powders synthesized under ambient pressure exhibited a strong superconductivity (diamagnetic) signal and zero resistivity below ∼4.5 K, while the Bi4O4S3 powder synthesized by the high-pressure method exhibited a low-intensity signal down to 2 K. Further annealing of the latter Bi4O4S3 powder under ambient pressure led to the development of a strong signal and zero resistivity. The crystal structures of all Bi4O4S3 phases consisted of Bi4O4Bi2S4 blocks including a Bi-S layer and anion(s) sandwiched between Bi4O4Bi2S4 blocks, but minor structural differences were detected. A comparison of the structures of the superconductive and nonsuperconductive Bi4O4S3 samples suggested that the superconductive Bi4O4S3 phases had slightly smaller lattice parameters. The average structures of the superconductive Bi4O4S3 phases were characterized by a slightly shorter and less bent Bi-S plane. Raman spectroscopy detected vibration of the S-O bonds, which can be attributed to sandwiched anion(s) such as SO4(2-). TEM observation showed stacking faults in the superconductive Bi4O4S3 phases, which indicated local fluctuation of the average structures. The observed superconductivity of Bi4O4S3 was discussed based on impurity phases, enhanced hybridization of the px and py orbitals of the Bi-S plane within Bi4O4Bi2S4 blocks, local fluctuation of the average structures, compositional deviation related to suspicious anion(s) sandwiched between Bi4O4Bi2S4 blocks, and the possibility of suppression of the charge-density-wave state by enriched carrier concentrations.

Role of structure gradient region on dielectric properties in Ba(Zr,Ti)O3–KNbO3 nanocomposite ceramics

Crystal structures of KNbO3 (KN)/BaZrxTi1−xO3 [BZT, (0.1 ≤ x ≤ 1)] nanocomposite ceramics have been investigated using synchrotron radiation X-ray powder diffraction, where BZT nanoparticles thinly coated with KN crystals through the heteroepitaxial interface are sintered. The Rietveld analysis based on the multicomponent model reveals that the ceramic grain has the core/multishell structure consisting of a BZT core, distorted BZT and KN multishells. The variations of the volume ratio of the distorted BZT shell region corresponding to the structure gradient interface region and the dielectric property of the ceramics show similar trends as a function of x. From these results, we propose that the structure gradient region is electrically soft, and provides a crucial contribution to the dielectric properties of the nanocomposite ceramics.

Bonding preference of carbon, nitrogen, and oxygen in niobium-based rock-salt structures.

Carbon, nitrogen, and oxygen are essential components in solid-state materials. However, understanding their preference on the bonding to metals has not been straightforward. Here, niobium carbide, nitride, and oxide with simple rock-salt-based structures were analyzed by first-principles calculations and synchrotron X-ray diffraction. We found that an increase in the atomic number from carbon to oxygen formed fewer and shorter bonds to metals with better hybridization of atomic orbitals. This can provide a simple guiding principle for understanding the bonding and designing carbides, nitrides, oxides, and mixed-anion compounds.