Papers by Keyword: Powder Diffraction

Abstract: Mechanical alloying has recently attracted considerable attention as researchers try to improve materials properties. The process can be performed at room temperature and homogeneous alloys can be produced. In this work Fe–28 wt. % Al; Fe–26 wt. % Al–2 wt. % Sn and Fe–26 wt. % Al–2 wt. % V alloys were synthesized by mechanical alloying to investigate the effects of tin and vanadium additions on the structural and microstructural properties of Nanocrystalline FeAl Alloy. Fe₇₂Al₂₈, Fe₇₂Al₂₆Sn₂ and Fe₇₂Al₂₆Sn₂ were ball milled for 30 h under argon atmosphere using a rotating speed of 200 rpm with 15 min pause time after every 15 min running time. The structural and microstructural properties of the ball milled powders were analyzed using X-ray diffraction (DRX) and Mössbauer spectroscopy techniques. The final powders are characterized by an average crystallite size of 10 nm for the Fe₇₂Al₂₈ alloy, 6 nm for the Fe₇₂Al₂₆Sn₂ alloy and 19 nm for the Fe₇₂Al₂₆V₂, accompanied by the introduction of a lattice strain of order of 1.55 %, 0.78 % and 0.80% respectively. The Mossbauer study of the Fe₇₂Al₂₆V₂ samples showed doublet with isomer shift IS= 0.17 mm/s and three magnetically split sextet.

Abstract: In this work, Ga₂S₃ crystals were obtained by vertical Bridgman method. The presence of cracks in the grown crystals was interpreted as a result of phase transition into monoclinic structure during cooling. This suggests the use of another approach for the growth of high quality samples, e.g. chemical transport method or melt-solution method. Maximal transparency range of 0.48-22.5 μm and at least 10 times higher damage threshold to that for GaSe render anisotropic Ga₂S₃ crystal among the most prospective crystals for nonlinear applications.

Abstract: Beamline I11 at Diamond began accepting users for high resolution powder diffraction experiments in Oct 2008. We present the design, key specifications, performance and the hardware of this new beamline which receives an intense and highly collimated x-ray beam generated by an in-vacuum undulator. With the simple optics (a double-crystal monochromator, harmonic rejection mirrors and slits), a high purity beam of low energy-bandpass X-rays optimised at 15 keV is delivered at the sample. The heavy duty diffraction instrument is designed to have the flexibility to house a variety of sample environments and holds two detection systems to collect high quality diffraction data, i.e. multi-analysing crystals (MAC) for high angular resolution experiments and a fast position sensitive detector (PSD) for time-resolved studies. A recent addition to the beamline capabilities is the installation of a specifically designed gas control system. This allows the in-situ dosing of a powder sample with gases such as hydrogen and carbon dioxide, at low (~10 mbar) and high pressures (<100 bar). In addition a low pressure capillary sample cell is described which is now available to users of the beamline.

Abstract: A series of yttrium-doped lithium trivanadates LiV_3-yY_yO₈ (y=0, 0.01, 0.03, 0.05, 0.1 and 0.2) were synthesized by a solid-state reaction. X-ray diffraction (XRD) tests show that a proper amount of yttrium doping in LiV₃O₈ can modify its structure. Charge-discharge tests show that doped samples with a proper amount of yttrium display good cycling stability compared with undoped sample. The cyclic voltammetry (CV) tests indicate that the proper amount of yttrium doping in LiV₃O₈ can stabilize its original structure. Yttrium doping is a convenient and effective method for improving the electrochemical performances of LiV₃O₈.

Abstract: Multifunctional ferromagnetic shape memory Heusler alloys are frequently characterized by structural modulation in martensitic phases. In particular, modulated martensitic phases, showing the higher magnetic field induced strain (MFIS) performance, are the most promising candidates for technological applications. Depending on the composition, as well as pressure and temperature conditions, this periodic structural distortion, consisting of shuffling of atomic layers along defined crystallographic directions, accompanies the martensitic transformation. Over the years, different Ni-Mn-Ga modulated martensitic structures have been observed and classified depending upon the periodicity of corresponding ideal nM superstructure (where n indicates the number of basic unit cells constituting the superlattices). On the other hand, it has been demonstrated that in most cases such structural modulation is incommensurate and the crystal structure has been fully determined by applying superspace formalism. The results, obtained by structure refinements on powder diffraction data, suggest a unified crystallographic description of the modulated martensitic structures, here presented, where every different “nM” periodicity can be straightforwardly represented.

Abstract: Recent advances in Line Profile Analysis of powder diffraction patterns must be paralleled by increasing attention to the quality and quantity of experimental data. The analysis of simulated data with different noise levels demonstrates the importance of statistical quality to reveal fine details of interest in the analysis of nanocrystalline materials, like the crystallite shape. It is also shown how synchrotron radiation diffraction can improve data quality with respect to laboratory measurements, both in terms of statistical quality and in terms of accessible information.

Abstract: EXPO2009 [1], the updated version of EXPO2004 [2], performs all the steps for the ab-initio crystal structure solution by powder diffraction data in a completely automatic way: unit cell determination, space group identification, decomposition of the experimental pattern into single integrated intensities, structure solution by traditional (Direct/Patterson) or direct space/hybrid approaches, structure refinement by Rietveld techniques [3]. New procedures have been introduced in EXPO2009 to strengthen its phasing ability, especially for organic compounds and for low resolution data, for which traditional approaches may fail.

Abstract: Molecular motion by refinement of TLS matrices from high resolution laboratory powder diffraction data: implementation in the program TOPAS and application to crystalline naphthalene

Abstract: While various software packages exist to study powder patterns, few are accessible to beginners and yet remain highly customisable. In this paper we will give guidelines for biologists interested in analysing powder patterns of proteins with Topas. Several topics will be discussed, from basic methods like indexing on a restricted list of spacegroups, to advanced use of command input les for pattern modelling and rigid body renement.

Abstract: The TOPAS macro language can be a powerful tool for increasing the capabilities of X-ray powder diffraction analysis. New corrections and constraints can be implemented without altering the program's code, allowing for experimentation with new ideas and approaches. Examples are given, exposing the power and flexibility of the macro language to help solving problems with a few lines of code. The use of simulated annealing for structure solution of an organic material from data exhibiting preferential orientation is one example. Another one is about extraction of useful structural information in Rietveld refinement of natural hydrotalcite-group minerals, a problematic case that would normally be regarded as over-parameterized for the data available.