Daisuke Urushihara

Direct observation of the ferroelectric polarization in the layered perovskite Bi4Ti3O12

We investigated the crystal structure and ferroelectric domains of Bi4Ti3O12 (BTO) by means of transmission electron microscopy (TEM) and single-crystal X-ray diffractometry. From the extinction rule, we determined that the space group in the ferroelectric phase of BTO is P1a1 rather than B2cb and B1a1 which have been proposed previously. We successfully refined the crystal structure based on the space group P1a1. The 180° and 90° ferroelectric domain structures were observed by the [001]-zone dark-field TEM imaging. In the 180° domain structure, we determined that one component of the polarization vector is parallel to the a-axis. An annular bright-field scanning transmission electron microscopy (ABF-STEM) was performed for the direct observation of the crystal structures. The ABF-STEM images displayed the contrasts with respect to every atomic position in spite of the highly distorted structure of BTO. We could evaluate the tilting and distortion of the [TiO6] octahedra relatively. Therefore, we directl...

Electron density distribution and crystal structure of Ca 1- x/2 AlSi(N 3- x O x ):Eu 2+ ( x ∼ 0.11)

Crystal structure of Ca 1- x/2 AlSi(N 3- x O x ):Eu 2+ ( x ∼ 0.11) has been characterized using an X-ray powder diffractometer and a transmission electron microscope equipped with an energy dispersive X-ray analyzer (EDX) and an electron energy loss spectrometer (EELS). The title compound is orthorhombic with space group Cmc 2 1 , Z = 4, unit-cell dimensions a = 0.979780(7) nm, b = 0.565197(4) nm, c = 0.506356(3) nm, and V = 0.280404(3) nm 3 . The atom ratio Al:Si was determined to be 1:1 by EDX, and the presence of O atoms in the crystal structure was confirmed by EELS. The x -value and the atomic coordinates of the final structural model were determined by the Rietveld method. The maximum-entropy methods-based pattern fitting (MPF) method was used to confirm the validity of the structural model, in which conventional structure bias caused by assuming intensity partitioning was minimized. The reliability indices calculated from MPF are R wp = 9.18%, S = 1.17, R p = 6.77%, R B = 1.91%, and R F = 0.86%. Atomic arrangements of the final structural model are in an excellent agreement with the three dimensional electron-density distributions determined by MPF.

Discovery of the High-Pressure Phase of Ba3W2O9 and Determination of Its Crystal Structure

A new polymorphism of Ba3W2O9 is discovered with the use of a high-pressure synthesis technique and its crystal structure is determined by single-crystal X-ray diffraction and transmission electron microscopy. The crystal structure was isostructural with that of Ba3Re2O9, having a hexagonal unit cell of R3̅m symmetry with a = 0.574060(10) nm and c = 2.08256(4) nm. The high-pressure (HP) phase is obtained from a transformation of an ambient-pressure (AP) phase of the compound, which has the Cs3Tl2Cl9-type structure. The most notable change in the transformation is the connection of WO6 octahedra. The HP phase has corner-sharing octahedra, which form a bilayer structure, while the AP phase has face-sharing octahedra of isolated [W2O9] dimers. This type of the structural phase transition is unreported although it is possibly that a sequence of high-pressure structural transformations occurs for similar chemical compositions. The HP phase has W ions in WO6 octahedra with an unusual off-center displacement; although the displacement is slightly relaxed compared with that of the AP phase. The off-center displacement suggests strong hybridization between the W 5d orbitals and O 2p orbitals.

Crystallographic features related to a van der Waals coupling in the layered chalcogenide FePS3

We investigated the crystallographic structure of FePS3 with a layered structure using transmission electron microscopy and powder X-ray diffraction. We found that FePS3 forms a rotational twin structure with the common axis along the c*-axis. The high-resolution transmission electron microscopy images revealed that the twin boundaries were positioned at the van der Waals gaps between the layers. The narrow bands of dark contrast were observed in the bright-field transmission electron microscopy images below the antiferromagnetic transition temperature, TN ≈ 120 K. Low-temperature X-ray diffraction showed a lattice distortion; the a- and b-axes shortened and lengthened, respectively, as the temperature decreased below TN. We propose that the narrow bands of dark contrast observed in the bright-field transmission electron microscopy images are caused by the directional lattice distortion with respect to each micro-twin variant in the antiferromagnetic phase.

Improved thermoelectric property of B-doped Si/Ge multilayered quantum dot films prepared by RF magnetron sputtering

The use of nanostructured thermoelectric materials that can effectively reduce the lattice conductivity with minimal effects on electrical properties has been recognized as the most successful approach to decoupling three key parameters (S, σ, and κ) and reaching high a dimensionless figure of merit (ZT) values. Here, five-period multilayer films consisting of 10 nm B-doped Si, 1.1 nm B, and 13 nm B-doped Ge layers in each period were prepared on Si wafer substrates using a magnetron sputtering system. Nanocrystallites of 22 nm diameter were formed by post-annealing at 800 °C in a short time. The nanostructures were confirmed by X-ray diffraction analysis, Raman spectroscopy, and transmission electron microscopy. The maximum Seebeck coefficient of Si/Ge films is significantly increased to 850 µV/K at 200 °C with their electrical resistivity decreased to 1.3 × 10−5 Ωm, and the maximum power factor increased to 5.6 × 10−2 Wm−1K−2. The improved thermoelectric properties of Si/Ge nanostructured films are possibly attributable to the synergistic effects of interface scattering, interface barrier, and quantum dot localization.

High-pressure synthesis and crystal structure of the strontium tungstate Sr3W2O9

The strontium tungstate compound Sr3W2O9 was prepared by a high-pressure synthesis technique. The crystal structure was determined by single-crystal X-ray diffraction and transmission electron microscopy. The structure was found to be a hettotype structure of the high-pressure phase of Ba3W2O9, which has corner-sharing octahedra with a trigonal symmetry. Sr3W2O9 has a monoclinic unit cell of C2/c symmetry. One characteristic of the structure is the breaking of the threefold rotation symmetry existing in the high-pressure phase of Ba3W2O9. The substitution of Sr at the Ba site results in a significant shortening of the interlayer distances of the [AO3] layers (A = Ba, Sr) and causes a distortion in the crystal structure. In Sr3W2O9, there is an off-centre displacement of W6+ ions in the WO6 octahedra. Such a displacement is also observed in the high-pressure phase of Ba3W2O9.