Yukio Noda

First results from the crystal structure analysis beamline at SPring-8.

Results from the first measurements at the crystal structure analysis beamline (BL02B1) at SPring-8 are presented. The capabilities of this beamline are also discussed.

Ferroelectric Behaviors in Semiconductive Cd1-xZnxTe Crystals

XAFS studies as well as dielectric and X-ray diffraction studies of II-VI semiconductive ternary alloys Cd1-xZnxTe with x=0.0, 0.05, 0.1, 0.2, 0.3, 0.7 and 1.0 were carried out in the temperature range of 10K to 600K. Ferroelectric phase transitions occurred in the mixed crystals with 0.0<x<1.0. Note that in CdTe (x=0.0) and ZnTe (x=1.0) the phase transition does not occur. An anomaly of the structural phase transition was found in the X-ray diffraction studies, however the local structure determined from the XAFS studies was independent of temperature down to 20K.

Neutron powder diffraction study of intercalation compound FexTiS2

The crystal structures of the intercalation compounds Fe x TiS 2 (x = 1/3, 1/4), which show a behavior characteristic of a cluster-glass phase at low temperature, were refined by means of Rietveld analysis of neutron powder-diffraction data. When x = 1/3, the space group is P31c and a √3a x √3a x 2c superlattice is formed. When x = 1/4, the space group becomes C2/m and a 2√3a x 2a x 2c superlattice is formed. For both concentrations, the occupancy at the Fe sites is less than 1.0.

Neutron incoherent scattering of structural glass NH4IKI mixed crystal

The structural glass behavior of the mixed crystal (NH 4 ) 0.5 K 0.5 I was investigated by means of neutron incoherent scattering as well as single-crystal X-ray diffraction. The profiles of elastic scattering S(Q,O;T) change near the glass transition temperature T g (about 80 K). The difference in elastic intensity between T = 150 K and 6.5 K shows a broad peak around 2.7 A -1 . The inelastic scattering intensity S(Q,ω;T) exhibits very broad quasielastic behavior extending up to 20 meV, and it suddenly decreases at T g on cooling. Diffuse scattering was observed by X-ray scattering with an image plate, and a new extinction rule was found for the intensity distribution. These results strongly suggest that the NH 4 ions start to freeze at T g and that only anti-ferroelectric short-range correlations of the NH 4 and iodine ions exist at low temperature.

Glass transition in K1−x(NH4)xI mixed crystals

X-ray diffraction studies were performed on K1−x(NH4)xI mixed crystals for the range of ammonium ion concentration 0.23⩽ x ⩽ 0.68. Systematically we observed the anomalous expansion of lattice constants and the appearance of diffuse scattering around the Brillouin-zone boundary of an FCC lattice (X point) below the glass transition temperature Tg. Neutron incoherent inelastic scattering experiments were also performed to elucidate the freezing of NH4+ ion motion. Experimental results suggest that a low-frequency lattice vibration of iodine atoms which couples with reorientational freezing of NH4+ ion sis relevant for the glass transition.

Neutron and synchrotron radiation studies of the phase transition of h-BaTiO3

Abstract Structures of the low temperature phases of a hexagonal type BaTiO3 were studied by neutron powder diffraction. In addition, high resolution powder diffraction experiments by using SPring-8 synchrotron radiation was carried out in order to determine the symmetry of phase II and III. The unit cell and the space group of phase II was confirmed to be the orthorhombic and C2221. The symmetry in phase III was newly determined as monoclinic with the deformation of the γ-angle by synchrotron radiation. The most possible symmetry in phase III is C1121, or equivalently P1121 as a crystallographic conventional unit cell. The shift patterns of atoms in phase II and phase III were determined.

Polarized X-ray Absorption Fine Structure of High-Tc Superconductor; La2-xSrxCuO4 Single Crystals

Polarized XAFS studies of high-Tc superconductive La2-xSrxCuO4 with x=0.0, 0.09,0.1 and 0.14 were carried out over the temperature range of 15K to room temperature by the transmission method. The c-plate single crystal was used and the polarization vector of the SOR beam was perpendicular to the c-axis and the a-axis in the tetragonal system. At room temperature, polarized XANES spectrum was slightly different from the non-doped La2CuO4-y reported by Oyanagi et al.. A bond length between the copper atom and the oxygen atom was drastically changed below 60K.

Anomalous LO-phonon broadening at the phase transition in KD2PO4

Abstract Anomalous broadening of the peak at E = 28 meV in KH2PO4 was recently observed by incoherent neutron scattering experiments upon the ferroelectric phase transition. On the other hand, coherent neutron scattering experiments with pulse neutron source revealed the existence of an optical phonon mode around 30 meV in KD2PO4, but the nature of the optical mode was still unclear. In order to investigate the character of this particular phonon mode, coherent neutron scattering experiment was performed at JRR-3M of JAERI. We found that the character of the mode at about 30 meV was mainly that of LO-phonon with slight TO-character, and the width of the peak showed a remarkable change as a function of temperature. This phonon mode seems to couple strongly the motion of the hydrogen atom.