Akiji Yamamoto

Assignment of the Powder X-Ray Diffraction Pattern of Superconductor Bi2(Sr, Ca)3-xCu2Oy

The powder X-ray diffraction pattern of superconductor Bi2(Sr, Ca)3-xCu2Oy was assigned taking into account the satellite reflections arising from an incommensurate modulated structure. All reflections are indexable with h a*+k b*+l c*+m k, provided that the wave vector k of the modulated structure is b*/4.8. The result shows that many satellites can be observed and some satellite intensities are strong.

Structure factor of modulated crystal structures

A general structure-factor formula for n-dimensionally modulated structures (n = 1, 2, ...) has been derived to have both density (or substitutional) and displacive modulations. This structure-factor formula covers any modulated structure described by a (3 +n)-dimensional space group. Future papers show the applications of this formula to one- and three-dimensional modulations.

The structure of a decagonal Al72Ni20Co8 quasicrystal.

The structure of a decagonal Al72Ni20Co8 quasicrystal with space group P10(5)/mmc has been determined on the basis of a single-crystal X-ray data set using the five-dimensional description. The best-fit model structure based on a cluster model having lower symmetry than the decagonal symmetry with 103 parameters gives wR = 0.045 and R = 0.063 for 449 reflections. The structure is well described by the hexagon, boat and star tiling with an edge length of 6.36 A and is very consistent with recent high-resolution electron-microscopy images. The refined structure is compared with previously discussed model structures including cluster-based models having 20 A tenfold symmetric clusters.

A new large radius imaging plate camera for high-resolution and high-throughput synchrotron x-ray powder diffraction by multiexposure method

A new large radius imaging plate diffraction camera for high-resolution and high-throughput synchrotron x-ray powder diffraction by means of multiple exposures has been developed for an insertion device beamline of SPring-8, Japan. The new imaging plate camera consists of a large radius cylindrical shape imaging plate cassette that is 400 mm in length and 954.9 mm in cylinder radius. The cassette is designed to be mounted on the 2 theta arm of the diffractometer of BL15XU in SPring-8. One imaging plate covers 24 degrees and several times of exposure changing the 2 theta-setting angle is necessary to obtain whole powder diffraction data up to a high angle region. One pixel of the imaging plate corresponds to 0.003 degrees in 2 theta when the readout pixel size is 50 microm squares. Separately collected data are translated to 2 theta-intensity format and are connected by comparing the peak and background intensity included in the overlapped area. The exposure time is less than 120 s for most samples and the readout time is about 3 min; thus, the total measurement time for one powder diffraction pattern is less than 20 min. The measurement time is the same order as the continuous 2 theta-scanning method of the third generation synchrotron powder diffractometer. The angular resolution of the new imaging plate camera was evaluated by comparing the full width at half maximum of the 111 reflection of NBS-Si. The observed angular resolution is not so high as a powder diffractometer with a Si or a Ge analyzer monochromator in the third generation synchrotron facility but higher than a powder diffractometer with a Ge analyzer monochromator at a bending magnet beamline of the second generation synchrotron. The Rietveld analysis of NBS-CeO2 was successfully carried out with the data taken by the new imaging plate camera.

The stability of the phases in the Ln2O3FeOFe2O3 systems which are stable at elevated temperatures (Ln: Lanthanide elements and Y)

Abstract Thermochemical data on LnFeO3, Ln3Fe5O12, and (LnFeO3)nFeO (n = 1 and 2) which are stable in the Ln2O3FeOFe2O3 systems at elevated temperatures are reevaluated and interpreted on the basis of a simple point charge ionic model (Ln: lanthanide elements and Y). As the radius of the Ln ion in each compound decreases, the LnFeO3 compound (distorted perovskite type) becomes less stable, the Ln3Fe5O12 compound (garnet type) is the most stable for Ln = Tb, and the (LnFeO3)nFeO compounds (layered structures) become more stable. The stability of each phase is strongly dependent upon the fitting of Ln ion in A-site position for the perovskite structure, in the dodecahedral site position for the garnet structure, and in the octahedral site positions for the layered structure compounds, respectively. It is qualitatively useful to estimate the Madelung energies of a series of isostructural phases on the basis of a simple point charge ionic model, especially as a guide for understanding a series of new phase diagrams which describe the isostructural phases.

Modulated structure of the NC-type (N = 5.5) pyrrhotite, Fe1−xS

The modulated structure of the NC-type pyrrhotite Fe1 - xS with x = 0.09 has been determined. The analysis is based on a four-dimensional space group, Wpna21qq1, and an anharmonic modulation model with 53 positional, two thermal, and eight occupational parameters including up to fourth-order harmonics, a = 11.952, b = 6.892, c = 5.744 A. The final R for 404 observed reflections is 0.097. The modulation wave for occupation probability of Fe shows a strong anharmonicity and has a low value in a narrow range corresponding to successive Fe layers perpendicular to the c axis of the fundamental NiAs-type structure. From this and the four-dimensional symmetry, it is concluded that Fe vacancies are distributed spirally along the c axis. The relationship between the usual superstructure model and this four-dimensional description is discussed.

Ln2Cu2O5 compounds [Ln: Lu, Yb, Tm, Ho, Er, Dy, Tb, and Y]

In the process of establishing the phase diagram of the Yb/sub 2/O/sub 3/-Fe/sub 2/O/sub 3/-CuO system in air at 1000/sup 0/C, Kimizuka and Takayama prepared a new compound, Yb/sub 2/Cu/sub 2/O/sub 5/, and reported its d-spacings and relative intensities of powder X-ray diffraction. Uhlig and Hahn synthesized the Cu/sub 2/Y/sub 2/O/sub 5/ compound at 1000/sup 0/C in air and reported its crystal data (space group: P/sub 2/, a = 13.60 (angstrom), b = 3.27 (angstrom), c = 13.60 (angstrom), and ..beta.. = 133.15/sup 0/, z = 4, Dx = 5.79). In the present note, we report crystal data of Yb/sub 2/Cu/sub 2/O/sub 5/ and its isostructural compounds, Ln/sub 2/Cu/sub 2/O/sub 5/ (Ln:Lu, Yb, Tm, Ho, Er, Dy, Tb, and Y), and their synthesis conditions.

Rietveld analysis of the composite crystal in superconducting Bi2+xSr2−xCuO6+y

Abstract The modulated crystals of Bi-based superconducting oxides are well described as composite crystals [Walker and Que, Phys. Rev. B 45 (1992) 14]. The structure of the superconducting oxide Bi 2+ x Sr 2− x CuO 6+ y was analyzed for the first time as composite crystal by the Rietveld method with the combined use of X-ray and TOF neutron diffraction data. It consists of two interpenetrating one-dimensionally modulated structures with the superspace groups N B2/b 1 1 and P I2/m 1 s . The analysis revealed large atomic displacements from average positions in the Bi-O, Sr-O, Cu-O layers. The oxygen arrangement in the Bi-O layer is similar to this in Bi 2+ x (Sr, Ca) 3− x Cu 2 O 8+ y .

Structure refinement of the incommensurate composite crystal Sr1.145TiS3 through the Rietveld analysis process

The structure of the incommensurate composite crystal of the trigonal sulfide Sr 1.145 TiS 3 , M r =244.42, has been analyzed on the basis of the four-dimensional superspace group P R3m 1s using powder X-ray diffraction data and the Rietveld analysis process. The unit cell and other crystal data are a 1 =a 2 =11.5108 (2), a 3 =2.99094 (5) A, a=[000.57227 (2)], V=343.19 A 3 , D x =3.548 Mg m -3 and Z=3. R wp was 0.104 on an incommensurate modulated structure model with 47 structural parameters. The crystal is composed of columns of face-shared polyhedra, (TiS 6/2 )∞, and rows of Sr, and both are parallel to the c axis

Noncentrosymmetric Structure of a Decagonal Al70Mn17Pd13 Quasicrystal

A noncentrosymmetric structure of decagonal Al 70 Mn 17 Pd 13 with the space group P10 5 mc was determined for the first time on the basis of a single-crystal X-ray data set. This analysis gave a considerable improvement over the corresponding centrosymmetric model [Weber & Yamamoto (1997). Philos. Mag. A76, 85-106]. For 1428 independent reflections, R w = 0.119 (R = 0.167) and a reasonable chemical composition of Al 69.1 Mn 18.5 Pd 12.4 was obtained by the introduction of the shift from an ideal atom position. The structure loses the inversion center mainly by the breakdown of the mirror plane perpendicular to the tenfold axis which is caused by the asymmetric chemical arrangement.