Photoelectron diffraction is by now a powerful technique for studying surface structures, with special capabilities for resolving chemical and magnetic states of atoms and deriving direct structural information from both forward scattering and backscattering. Fitting experiment to theory can lead to structural accuracies in the 0.03 Å range. Holographic inversions of such diffraction data also show considerable promise for deriving local three-dimensional structures around a given emitter with accuracies of 0.2-0.3 Å. Resolving the photoelectron spin in some way and using circularly polarized radiation for excitation provide added dimensions for the study of magnetic systems and chiral experimental geometries. Synchrotron radiation with the highest brightness and energy resolution, as well as variable polarization, is crucial to the full exploitation of these techniques. X-ray fluorescence holography also has promise for structural studies, but will require intense excitation sources and multichannel detection to be feasible.

We report the use of photoelectron holography for the study of the atomic geometry of the (111) surface of single crystal CoSi2. We employ as a reference wave the Co 3p core electron emitted with a kinetic energy of 695 eV by absorption of synchrotron radiation. Remarkably sharp images of the nearest Si neighbors of the Co emitter were found using a computer reconstruction algorithm which compensates for the anisotropies of both reference and object waves. Comparison with other reconstruction schemes such as the Helmholtz-Kirchoff (HK) algorithm and the scattered-wave included Fourier transform (SWIFT), demonstrates the necessity of compensating for the reference-wave anisotropy.

Platinum atoms near a (111) single-crystal face have been imaged using photoelectron holography. Electron angular intensity patterns were collected at equally spaced wavenumbers from 6Å−1 to 12Å−1. Images of atoms near expected atomic positions are obtained from single-wavenumber analyses over the range of the data set. Positions are detected further from the emitter than we have seen previously, and we have also adopted a three dimensional means of representing the data in order to help understand the results. Twin image suppression and artifact reduction in the holographically reconstructed data set are obtained when images at different wavenumbers are correctly phase-summed. We are assessing the capability of the technique for rendering true three-dimensional structural information for unknown systems.

Pre-transitional behavior prior to the onset of the cubic-to-tetragonal phase transformation in KnbO3 has been studied with synchrotron x-ray scattering at NSLS, Brookhaven National Lab. Measurements of the Thermal Diffuse Scattering (TDS), Grazing Incidence X-ray Scattering (GIXS), and precursor Bragg scattering have been performed at a series of temperatures on a single crystal sample in the cubic phase (Tc = 405 °C). The TDS was measured in the (001), (011) and (111) cubic planes. The GIXS was measured above, at and below the critical scattering angle of the sample. Based on localized soft mode theory, dynamic tetragonal fluctuations may be expected prior to the onset of the phase transition initiated by defects or surfaces. The GIXS data indicate that the tetragonal phase is apparent prior to the phase transition of the bulk - - consistent with the notion that nucleation originates from potent defect sites during a martensitic-like displacive transformation. The TDS intensity distribution is disk-shaped; arising from a localized, low energy transverse optic mode in the [100] directions extending out to the Brillouin zone boundary (reduced wave vector, q =0.5). Scans made in the (011) planes show TDS intensity extending out to a reduced wave vector of q ˜ 0.2. Preliminary analysis indicates that a low energy transverse optic mode extends in the [011] direction as well. The TDS intensity decreases as the transition temperature is approached from above. Comparison of the TDS results with inelastic neutron scattering data will be made.

A metastable (face centered) cubic form of indium has been identified in melt-spun Al-4.3at% indium using high resolution synchrotron radiation diffraction. The microstructure of the melt-spun ribbons consisted of particles of nominally pure indium dispersed in an aluminum matrix. These particles ranged in size from 10 nm to 1000 nm through the thickness of the ribbons. Only the smallest indium particles were identified as being cubic and these were observed in electron transparent regions of the as-spun ribbons. The cubic indium phase had the following orientation relationship with the matrix: [001]In′ // [001]Al and (100)In′ // (001)Al. The size distribution and average diameter of the cubic particles were obtained by quantitative analysis of the diffraction line profiles. These measurements were corroborated with transmission electron microscopy observations. The volume percent of the cubic indium phase was determined by matching simulations of the x-ray diffraction patterns with the diffracted intensities found experimentally. Approximately 10–20% of the indium is present in the cubic form and the remainder is the stable tetragonal phase.

Results for a study using anomalous scattering to increase the X-ray contrast between Fe and Cr in metallic multilayers are reported. This increased contrast is sufficient to provide reliable values for actual bilayer distances and to begin semi-quantitative analysis of peak intensities even in the presence of orientational texture.

Synchrotron radiation from the Cornell High Energy Synchrotron Source (CHESS) was used to probe the alignment kinetics of a class of thermotropic siloxane-based liquid crystals under the influence of an electric field. The high flux allows for real-time investigation of microstructural changes as a function of frequency and temperature. The packing behavior of a pentamethylcyclosiloxane ring with pendant biphenyl mesogens was thoroughly investigated. The director orientation parameter, Sd, typically increased with decreasing temperature and was not strongly affected by frequency. Electric-field induced changes in the director orientation were also monitored. Upon application of an electric field, the nematic system exhibited a tendency to pack in columns with short-range correlation of the mesogens. Column correlation lengths are examined as a function of temperature for two frequencies. At the low temperature end of the mesophase, a tendency for the mesogens to layer pack was observed.

This manuscript discusses two different powder diffraction methods which possess high resolution, low background and very high counting rates. Both methods utilize analyzer crystals in combination with position sensitive detectors. The first method uses a curved perfect crystal analyzer diffracting in the scattering plane to diffract a range of angles into the position sensitive detector. The second method is somewhat simpler, it uses a graphite analyzer crystal diffracting out of the scattering plane. Applications of these methods to anomalous scattering from both powder and amorphous materials are also discussed. One of the techniques should also be very useful for small angle scattering.

Magnetron sputtered amorphous FexGe100-x films have been examined with anomalous small-angle x-ray scattering (ASAXS) in an attempt to characterize composition fluctuations which have been previously reported in this system. Films grown under various deposition conditions have been studied, with the scattering vector both in and oblique to the plane of the films, to search for anisotropy. By manipulating the deposited power flux and rates of growth, films which have the same composition can be grown to different states of phase separation. The total correlation functions have been calculated from the oblique scattering experiments. The anisotropy can be successfully modeled as a close-packing of oriented prolate ellipsoidal particles, with the elongated axis along the direction of film growth. A method for using these measurements to determine the compositions of the phase-separating species has been developed and utilized. The results indicate phase separation into a-Ge and a-FeGe2 for the a-FexGe100-x (x<33) alloy.

A new experimental technique for the characterisation of the thermal-morphological properties of materials has been developed at Daresbury. Many thermal events, for example melting endotherms, are signals of changes in morphology covering size scales from the atomic to the microscopic, that is Å to μm. There are obvious advantages in collecting both the wide angle (1-20Å) and small angle (20-1000Å) patterns simultaneously to unambiguously characterise such thermal events. The new apparatus comprises a Linkam hot-stage capable of controlled heating and cooling mounted in a combined SAXS/WAXS camera. The camera is equipped with a multiwire quadrant detector (SAXS) located 3.5 m from the sample position and a curved knifeedge detector (WAXS) that covers 120° of arc at a radius of 0.4 m. SAXSIWAXS is possible with a time resolution of 0.1 seconds and heating/cooling rates up to 120 °C min-1.

Local atomic arrangements in Hg0.80Cd0. 20Te were investigated by measuring the diffuse x-ray scattering at two different energies near the HgLIII absorption edge to yield intensity due only to Hg-Hg, Hg-Te, and Hg-Cd pair interactions. The data were separated into short range order and displacement intensities. Simulation revealed ordered regions with 3:1 Hg-Cd near neighbor configurations. The Hg-Te length is contracted.

Al/Cu sputtered-sliced phase zone plates, designed to operate with a focal length of 0.18 m and 1.12 m at 8.05 keV, were tested at the Stanford Synchrotron Radiation Laboratory. The results of lens characterization and the implementation of a microscope are reported.

Finite element results are presented for the case of an elastically bent isotropic rectangular crystal with clamped boundary conditions. Results show that the anticlastic curvature can be eliminated in the center of the crystal provided the crystal length to width ratio fits a “golden aspect ratio” which is dependent on the Poisson coefficient ν. For ν=0.262 (appropriate for Si(111)), this ratio is approximately equal to 1.42.