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[en] 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 4x10¹⁰ 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

[en] Nearest- and higher-neighbor distances as well as bond length distributions (static and thermal) of the In_xGa_1-xAs (0≤x≤1) semiconductor alloys have been obtained from high-real-space resolution atomic pair distribution functions. Using this structural information, we modeled the local atomic displacements in In_xGa_1-xAs alloys. From a supercell model based on the Kirkwood potential, we obtained three-dimensional As and (In,Ga) ensemble average probability distributions. These clearly show that As atom displacements are highly directional and can be represented as a combination of <100> and <111> displacements. Examination of the Kirkwood model indicates that the standard deviation (σ) of the static disorder on the (In,Ga) sublattice is around 60% of the value on the As sublattice and the (In,Ga) atomic displacements are much more isotropic than those on the As sublattice. The single-crystal diffuse scattering calculated from the Kirkwood model shows that atomic displacements are most strongly correlated along <110> directions

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[en] Results of high-energy synchrotrons radiation experiments are presented demonstrating the advantages of the atomic Pair Distribution Function technique in determining the structure of materials with high resolution

[en] A continuous bent sagittally focusing monochromator has been designed and built. The monochromator is compatible with the present single-point bender apparatus designed for polygonal (ribbed) triangular sagittally focusing monochromators. This monochromator implements a new design concept taking advantage of a tapered rectangular wafer to allow for sagittal bending while simultaneously minimizing anticlastic bending. The monochromator was optimized to operate at x-ray energies in the range of 5 to 25 keV. The design was derived from finite element analysis using ANSYS. The monochromator performance was tested by means of an apparatus implementing an x-ray tube source and a double-crystal configuration. This method yields precise contour maps of the entire monochromator surface. Details of the monochromator design, test apparatus, and corresponding results will be presented. copyright 1996 American Institute of Physics

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[en] The authors have measured atomic pair distribution functions (PDF) of La_1-xCa_xMnO₃ using high energy x-ray diffraction. This approach yields accurate PDFs with very high real-space resolution. It also avoids potential pitfalls from the more usual neutron measurements that magnetic scattering is present in the measurement, that the neutron scattering length of manganese is negative leading to partial cancellation of PDF peaks, and that inelasticity effects might distort the resulting PDF. They have used this to address the following questions which do not have a satisfactory answer: (1) What are the amplitudes and natures of the local Jahn-Teller and polaronic distortions in the CMR region. (2) Is the ground-state of the ferromagnetic metallic phase delocalized or polaronic. (3) As one moves away from the ground-state, by raising temperature or decreasing doping, towards the metal insulator transition, how does the state of the material evolve?

[en] In the present work, an X-ray diffraction (XRD) and thermal study of calcium undecanoate is presented. The measured high-resolution XRD powder pattern of the synthesized salt at room temperature, using synchrotron radiation, showed that the salt is a mixture of monohydrated and anhydrous calcium undecanoate. Calcium undecanoate monohydrate proved to have a monoclinic cell with a symmetry described by the P2 1/a space group. The structure dehydrates at about 100 deg. C. After dehydration, the salt undergoes a phase transformation which results in a thermotropic mesophase. Further heating of the salt leads to decomposition and melting. Ketones are the probable products of decomposition at 400 deg. C