Toshihiko Takama

Crystal Structure of High-Temperature Paraelectric Phase in Bi-Layered Perovskite Sr0.85Bi2.1Ta2O9

The crystal structure of Bi-layered perovskite Sr0.85Bi2.1Ta2O9 was determined at 750 K by X-ray Rietveld analysis. Sr0.85Bi2.1Ta2O9 with space group A21am at room temperature transforms to the paraelectric phase above 670 K, passing through a probable intermediate state at 510 K. Assuming two possible orthorhombic Fmmm and tetragonal I4/mmm space groups, analysis of the crystal structure of the paraelectric phase gave the final discrepancy factors Rwp of 11.3% and 9.9%, respectively. Examination of possible reflections and peak positions at high-angles proved that the true space group is I4/mmm for the high temperature paraelectric phase. The lattice constants are a=3.927 (1), c=25.142 (1) A at 750 K. The TaO6 octahedron is not perfect and is slightly distorted along the c-axis, having the dipole moment of 0.478 debye.

Growth of a (GaAs)n/(InAs)n Superlattice Semiconductor by Molecular Beam Epitaxy

Molecular beam epitaxial growth of (GaAs)n/(InAs)n superlattice semiconductors is reported. Reflection electron diffraction was used to monitor the growth process. Successful growth of (GaAs)n/(InAs)n (n=1, 2) on InP substrate was confirmed by X-ray diffraction.

Measurement of the structure factors of diamond

The absolute values of the structure factors of diamond are determined for nine low-order reflections by measuring the X-ray Pendellosung beats on the wavelength scale. Parallel-sided wafers of synthetic diamond single crystals are used for specimens. The deformation charge density and the Debye-Waller B factor are evaluated from the structure factors. The charge density of pile-up electrons is estimated to be 0.44(17) eA-3 at the midpoint between the nearest-neighbour atoms. The density is slightly smaller than that determined by the powder diffraction method. The obtained B factor, 0.142 (9) A2, is in good agreement with that evaluated to date from neutron diffraction measurements.

Ferroelectric Phase Transition in Layered Perovskite SrBi 2 Ta 2 O 9

Crystal structure of Bi-layered perovskite SrBi 2 Ta 2 O 9 was studied at 1000 K by X-ray diffraction. Rietveld analysis confirmed antiparallel arrangements of TaO 6 octahedra in the high-temperature paraelectric phase (tetragonal 14/mmm, R wp =7.37%). The TaO 6 octahedra are distorted along the c -axis. This crystal favors canted octahedral arrangements below T c (608 K). These result in net spontaneous polarization along the a -axis. We report the first observation of canted ferroelectricity or weak ferroelectricity in SrBi 2 Ta 2 O 9 .

Atomic Scattering Factors of Zn Determined from Measurement of X-Ray Pendellosung Intensity Beats Using White Radiation

The Pendellosung intensity beats of white radiations diffracted from Zn single crystals grown by a solidification method have been measured by the energy-dispersive diffraction technique using a solid-state detector. The atomic scattering factors of Zn were evaluated by measuring the positions of the intensity extrema and utilizing the dynamical diffraction theory for the eight low-order reflections. The absolute atomic scattering factors were found to be smaller than those calculated for free atoms by the relativistic Hartree-Fock method, especially for low-order reflections, as well as for the reflections from the basal plane. The result suggests that the outer electrons diffuse out, particularly in the direction of the C-axis, from the spherical electron clouds in Zn atoms in the crystals.

X-ray study of the short-range order structure in Cu–24.3 at.% Mn alloy

Abstract The diffuse X-ray scattering from a single crystal of Cu–24.3 at.% Mn aged within the fcc phase at 770 K was measured at room temperature. Diffuse peaks were found at 2 n 1 +0.2, 2 n 2 +0.2, 0 and equivalent positions ( n 1 and n 2 are integers) in reciprocal space. The Warren–Cowley short-range order parameters were derived from the separated short-range order scattering and used to generate the corresponding local atomic arrangements on a computer. The generated arrangements indicated that Mn atoms form clusters. A set of effective pair interaction energies obtained using an inverse Monte Carlo method showed that the interaction between the first nearest neighbors mainly affects the clustering of like atoms.

Li-Induced ferroelectricity and its structural phase transition in ZnO

Abstract Ferroelectricity in semiconducting ZnO by Li-substitution was studied. A ferroelectric phase transition occurs at 470 K in Zn0.91Li0.09O. The revised Tc – x phase diagram was given. The Rietveld analysis of X-ray powder pattern showed a small displacement of Zn of 0.002 A along the polar c-axis in Zn0.91Li0.09O.

In Situ X-Ray Measurement of Order-Disorder Transition Temperature of Fe-Rich Fe-Pt Alloys

The composition dependence of order-disorder transition temperature Tc of Fe1-cPtc alloys (c=0.22, 0.25, 0.28, 0.33, 0.345 and 0.36) was investigated by measuring the X-ray integrated intensity of powder diffraction at high temperature. The temperatures Tc between the L12-type ordered structure and disordered fcc structure were significantly different from those reported thus far. The maximum Tc of the L12 structure was not at the composition of 25 at% Pt, but at about 33 at% Pt. It was found that the L12 and the L10 ordered phases coexisted at around 34.5 at% Pt. A new phase diagram was proposed based on the measurements.

Novel ferroelectricity in polar semiconductor ZnO by Li-substitution

Ferroelectricity in Li-substituted II-VI semiconductor ZnO ceramics was studied. A ferroelectric phase transition temperature T/sub c/ is 370 K in hot-pressed ceramics synthesized by the spark plasma sintering method, although the previous cold-pressed ceramics showed a relative low T/sub c/ (330 K). The Rietveld analysis of X-ray powder diffraction pattern showed R/sub WP/=7.4%, u=0.3821 (3) for Zn (Li) and x=0.09 at 293 K in Zn/sub 1-x/Li/sub x/O hot-pressed ceramics. The ferroelectricity in Zn/sub 1-x/Li/sub x/O results from small structural distortion (of order 10/sup -3/ /spl Aring/) induced along the polar c-axis by substitutional Li atoms, instead of the host Zn atoms in the wurtzite structure.

Effects of H2O Absorption on Electrical Property and Crystal Structure in YBa2Cu3Ox

Polycrystals of YBa2Cu3Ox (x=6.09, 6.24, 6.4, 6.52, 6.71 and 6.95) were annealed in air. The specimens with x6.4 were transformed from the superconducting to normal conducting phase at annealing temperatures between 100 and 200°C. The powder X-ray diffraction profiles revealed that the original orthorhombic phase disappears and a new phase with a long lattice spacing grows at the above temperatures. Annealing tests carried out in several kinds of atmospheres clarified that the changes in resistivity and in crystal structure are induced by the absorption of H2O molecules in air by means of a diffusion process.