Peter D'Antonio

Determination of atomic positions using electron nanodiffraction patterns from overlapping regions: Si[110]

Abstract A procedure for determining atomic positions is described that utilizes electron nanodiffraction patterns from overlapping regions. The technique is applied using experimental data collected for Si[110] with a beam 4 A in diameter. Autocorrelation functions are calculated for each beam position and correlated with theoretical functions. The positions of the Si atoms are established with an accuracy of ±0.2 A, and images with 1 A resolution are computed for individual beam positions. It is also demonstrated how the technique may be used to determine, with similar resolution, the structure of noncrystalline regions adjacent to crystalline regions.

Structural Ordering in Amorphous TbFe2 and YFe2

Total neutron scattering data were collected on sputtered YFe2 at 298u2005K and TbFe2 at 423u2005K with a wavelength of 0.7u2005A. The TbFe2 data were collected above the magnetic ordering temperature of 383u2005K. In addition, the elastic neutron scattering of TbFe2 was measured with the use of a pyrolytic graphite analyzer at a wavelength of 1.5u2005A, and its total X-ray scattering was measured with Mo radiation and a silicon-lithium drifted detector. Experimental radial distribution functions, with statistical error limits, were calculated. Errors due to an incorrect background, scaling of the data and termination effects were minimized. The scale and shape of the experimental background and the coordination numbers, internuclear distances and disorder parameters, for the first six coordination spheres, have been determined. The contribution of paramagnetic inelastic scattering from TbFe2 to the total neutron scattering is quite appreciable. The shape of the background scattering, which goes through a maximum, is indicative of residual coherence and suggests short-range magnetic ordering where neighboring atom spins are aligned. These effects are not observed in YFe2, nor in the elastic TbFe2 data. The metallic glasses have a structural topology which is quite different from that found in their crystalline analogues. The transition-metal substructure, consisting of corner-sharing tetrahedra, is the only aspect of the crystalline topology preserved in the amorphous phase. The structural parameters suggest a tendency of the rare-earth atoms to cluster, thereby decreasing the number of Fe nearest neighbors relative to the crystalline structure.

Image reconstruction using electron microdiffraction patterns from overlapping regions

Abstract An image reconstruction procedure is described that utilizes electron microdiffraction patterns from overlapping regions. The autocorrelation function is calculated for an array of beam positions surrounding the sample area of interest, and the beam positions that maximize each autocorrelation vector are identified. Such maxima occur with the beam centered midway between the related atoms, and, thus, each maximum serves to identify the positions of two atoms. Atomic resolution images are constructed from microdiffraction patterns calculated for crystalline silicon viewed along the [110] direction, and for a thin slice from a model of amorphous silica (SiO 2 ). Comparisons are made with dark-field images calculated for annular detector data.

Morphology of surface crystallites on amorphous Fe87Zr7B5Cu1

Surface crystallites a few tens of nanometres in breadth have been observed on the as-melt-spun surface of an alloy of compositionFe87Zr7B6Cu1. These crystallites are preferentially oriented, and exhibit a (2 0 0) texture. Heat treatments at temperatures in the interval 623–773 °C caused the surface grains to coarsen and new, randomly oriented crystals to form. Compositional inhomogeneities were found to contribute to the formation of the surface crystallites. Results suggest that surface energy effects are more important during the nucleation of crystallites in the liquid than they are below the glass transition temperature.

Diffraction evidence for distorted graphite-like ribbons in an activated carbon of very large surface area

Abstract A carbon prepared from petroleum coke which possesses a very large active surface area (3500 m 2 /gm) has been examined with X-ray diffraction techniques. Simultaneous analysis of the low and high angle scattering suggests that the material is composed of distorted ribbons of graphite-like layers. On the average, each ribbon contains only one or a very few graphite-like layers, and these layers are distorted with an average radius of curvature of ~ 25A. This very open structure results in a large surface area and gives rise to extensive small angle scattering.

Simulation and interpretation of STEM side-band holograms

Abstract Simulations of side-band holograms produced with a STEM equipped with a coherent, point electron source and a beam splitter are presented and interpreted. Holograms are calculated for various electron voltages, beam separations and thicknesses of GaAs [110]. The results illustrate features in a hologram associated with a stable, well-adjusted microscope. The simulations also provide data on the dynamic range and resolution necessary for recording the patterns and indicate techniques for removing from the reconstructed images interferences from unscattered beams.

Utilization of STEM nanodiffraction data

Abstract A scanning transmission electron microscope (STEM) designed to simultaneously record all of the diffraction data at each beam position is necessary to fully utilize the imaging potential of a STEM. However, analysis of this large quantity of data requires considerable computation that should be undertaken only with the assurance that the data were collected under near optimum conditions from the desired region of the sample. This paper develops equations for the total elastic scattering produced with a small, coherent electron beam incident on a thin sample. These equations are used as the basis for simulations which indicate that high-angle annular-dark-field (ADF) images may be used to indicate the quality of the diffraction data. When the microscope is adjusted such that the portion of the data falling within a high-angle annulus forms a real-time high-resolution image with 2.5A˚resolution or better, the simultaneously recorded nanodiffraction data are suitable for the reconstruction of images possessing 1A˚resolution or better. While the resolution of the ADF image is determined by the electron beam size, the coherent nanodiffraction data contain structural information for each beam position to the resolution limit of the diffraction pattern.

Crystal structure of (E)-5-hydroxypyrrolizidin-3-one and (Z)-5-thioketalpyrrolizidin-3-one

The crystal structures of (E)-5-hydroxypyrrolizidin-3-one (2) and (Z)-5-thioketalpyrrolizidin-3-one (3) have been determined by single-crystal x-ray diffraction techniques and refined by full-matrix least squares. Molecule2 crystallizes in the monoclinic space groupP21/a(No. 14) witha=7.887(3) Å,b=9.788(5) Å,c=9.316(4) Å,β=100.6(1), andZ=4. Molecule 3 crystallizes in the monoclinic space groupP21/c witha=12.947(14) Å,b=10.553(14) Å,c=9.629(14) Å,β=101.4(1), andZ=4. The calculated density for both molecules is 1.33 g cm−1. FinalR-factors were 5.1% for2 and 8.59% for3. The x-ray results showed that a change in configuration occurred in the reaction going from2 to3.