The high-energy x-ray diffraction and scattering beamline at the Canadian Light Source.

The optical design for the high-energy x-ray diffraction and scattering beamline of the Brockhouse sector at the Canadian Light Source is described. The design is based on a single side-bounce silicon focusing monochromator that steers the central part of a high-field permanent magnet wiggler beam into the experimental station. Two different configurations are proposed: a higher energy resolution with vertical focusing and a lower energy resolution with horizontal and vertical focusing. The monochromator will have the possibility of mounting three crystals: one crystal optimized for 35 keV that focuses in the horizontal and vertical directions using reflection (1,1,1) and two other crystals both covering the energies above 40 keV: one with only vertical focusing and another one with horizontal and vertical focusing. The geometry of the last two monochromator crystals was optimized to use reflections (4,2,2) and (5,3,3) to cover the broad energy range from 40 to 95 keV.

Undulator beamline of the Brockhouse sector at the Canadian Light Source.

The Brockhouse project at the Canadian Light Source plans the construction of three beamlines, two wiggler beamlines, and one undulator beamline, that will be dedicated to x-ray diffraction and scattering. In this work, we will describe the undulator beamline main components and performance parameters, obtained from ray tracing using XOP-SHADOW codes. The undulator beamline will operate from 4.95 to 21 keV, using a 20 mm period hybrid undulator placed upstream of the wiggler in the same straight section. The beamline optics design was developed in cooperation with the Brazilian Synchrotron - LNLS. The beamline will have a double crystal monochromator with the options of Si(111) or Si(311) crystal pairs followed by two mirrors in the KB configuration to focus the beam at the sample position. The high brilliance of the undulator source will produce a very high flux of ~10(13) photons/s and high energy resolution into a small focus of 170 µm horizontal and 20-60 µm vertical, depending on the optical configuration and energy chosen. Two multi-axis goniometer experimental stations with area detectors and analyzers are foreseen to enable diffraction, resonant and inelastic scattering experiments, and SAXS/WAXS experiments with high resolution and time resolving capabilities.

Disentangling neighbors and extended range density oscillations in monatomic amorphous semiconductors.

High energy x-ray diffraction measurements of pure amorphous Ge were made and its radial distribution function (RDF) was determined at high resolution, revealing new information on the atomic structure of amorphous semiconductors. Fine structure in the second peak in the RDF provides evidence that a fraction of third neighbors are closer than some second neighbors; taking this into account leads to a narrow distribution of tetrahedral bond angles, (8.5 ± 0.1)°. A small peak which appears near 5 Å upon thermal annealing shows that some ordering in the dihedral bond-angle distribution takes place during structural relaxation. Extended range order is detected (in both a-Ge and a-Si) which persists to beyond 20 Å, and both the periodicity and its decay length increase upon thermal annealing. Previously, the effect of structural relaxation was only detected at intermediate range, involving reduced tetrahedral bond-angle distortions. These results enhance our understanding of the atomic order in continuous random networks and place significantly more stringent requirements on computer models intending to describe these networks, or their alternatives which attempt to describe the structure in terms of an arrangement of paracrystals.

Evolution of thermodynamic potentials in closed and open nanocrystalline systems: Ge-Si:Si(001) islands.

An open (closed) system, in which matter is (not) exchanged through surface diffusion, was realized via growth kinetics. Epitaxially grown Si-Ge:Si (001) islands were annealed in different environments affecting the diffusivity of Si adatoms selectively. The evolution of the driving forces for intermixing while approaching the equilibrium was inferred from Synchrotron x-ray measurements of composition and strain. For the open system, intermixing due to the Si inflow from the wetting layer (reservoir) caused a decrease in the Ge content, leading to a lowering of the elastic energy and an increase in the mixing entropy. In contrast, for the closed system, while keeping the average Ge composition constant, atom rearrangement within the islands led to an increase in both elastic and entropic contributions. The Gibbs free energy decreased in both cases, despite the different evolution paths for the composition profiles.

Specific heat of the dilute Ising magnet LiHoxY1-xF4.

We present specific heat data on three samples of the dilute Ising magnet LiHoxY1-xF4 with x=0.018, 0.045, and 0.080. Previous measurements of the ac susceptibility of an x=0.045 sample showed the Ho3+ moments to remain dynamic down to very low temperatures, and the specific heat was found to have unusually sharp features. In contrast, our measurements do not exhibit these sharp features in the specific heat and instead show a broad feature, for all three samples studied, which is qualitatively consistent with a spin glass state. Integrating C/T, however, reveals an increase in residual entropy with lower Ho concentration, consistent with recent Monte Carlo simulations showing a lack of spin glass transition for low x.

3D composition of epitaxial nanocrystals by anomalous X-ray diffraction: observation of a Si-rich core in Ge domes on Si(100).

Three-dimensional composition maps of nominally pure Ge domes grown on Si(001) at 600 degrees C were obtained from grazing incidence anomalous x-ray scattering data at the Ge K edge. The data were analyzed in terms of a stack of layers with laterally varying concentration. The results demonstrated that the domes contained a Si-rich core covered by a Ge-rich shell and were independently supported by selective etch experiments. The composition profile resulted from substrate Si alloying into the Ge during growth to partially relax the stress in and under the domes.

High-resolution X-ray diffraction beamline at the LNLS for the study of charge, orbital and magnetic structures.

A high-resolution X-ray diffraction beamline at the Brazilian Synchrotron Light Laboratory (LNLS) has been commissioned for the study of crystalline magnetic materials. The beamline optics is based on a Rh-coated vertical-focusing X-ray mirror and a sagittal-focusing double-crystal monochromator. The primary instrument is a six-circle diffractometer equipped with energy and polarization analysers and a closed-cycle He cryostat. The beamline source is a bending magnet of the 1.37 GeV storage ring of the LNLS, delivering approximately 4 x 10(10) photons s(-1) at 8 keV at the sample position. Resonant and non-resonant scattering are the main techniques used to study charge, orbital and magnetic structures. Examples of magnetic scattering in Ho and NiO single crystals, as well as orbital ordering in manganites thin films, are presented.

Absolute structure-factor measurements of an Al-Pd-Mn quasicrystal.

A number of X-ray reflections from an icosahedral quasicrystal Al-Pd-Mn have been measured with great accuracy on an absolute basis by making use of Bragg-case diffraction. Since the specimen had high crystal quality, the dynamical theory was used for analyzing the results and to extract structure factors from measured integrated intensities. Good agreement was found between theory and experiment for strong reflections. Anomalous transmission was found to be strong in the 'good' regions of the quasicrystalline specimen and it was measured on an absolute basis, but the small residual strains present in the specimen prevented an accurate comparison between theory and experiment. A detailed discussion is presented on the parameters that mostly affect anomalous transmission in relationship to the adopted structural model.

Enantiomorph determination using inverse reference-beam diffraction images.

It is shown that enantiomorph structures of a noncentrosymmetric crystal can be determined, in the absence of anomalous diffraction signals, by measuring two series of reference-beam oscillation diffraction patterns related by an inverse-beam geometry. The corresponding intensities of the Friedel pairs recorded on the two sets of images exhibit the characteristic three-beam interference effects that provide the unambiguous phase information. The experimental arrangement and the data-analysis procedure are demonstrated through an experimental example on tetragonal lysozyme.

Triplet-phase measurements using reference-beam X-ray diffraction.

Reference-beam diffraction (RBD) is a recently developed phase-sensitive X-ray diffraction technique that incorporates the principle of multiple-beam diffraction into the standard oscillating-crystal data-collection method [Shen (1998). Phys. Rev. Lett. 80, 3268-3271]. Using this technique, a large number of multiple-beam interference profiles can be recorded simultaneously on an area detector, from which a large number of triplet phases of Bragg reflections can be determined in a crystallography experiment. In this article, both the theoretical developments and the experimental procedures of the RBD technique are described in detail. Approximate theoretical approaches for RBD are outlined and simple analytical expressions are obtained that provide the basis for an automated data-analysis procedure that can be used to extract triplet phases from a large number of measured reference-beam diffraction profiles. Experimental examples are given for a variety of crystals including GaAs, tetragonal lysozyme and AlPdMn quasicrystal, using both image plates and a charge-coupled device (CCD) as the area detector. Possible uses of the measured phases for crystal structure determination are discussed as well as future prospects of the RBD technique.