We describe a novel cold neutron spectrometer under development at NIST optimized for wave vector resolved spectroscopy with incident energies between 2.1 meV and 20 meV and energy resolution from 0.05 meV (E i = 2.1 meV) to 3.0 meV (E i = 20 meV). By using a 1428 cm 2 double focusing PG (0 0 2) monochromator close to the National Institute of Standards and Technology (NIST) cold neutron source the instrument provides up to 5 × 10 8 neutrons cm −2 s −1 on a 8 cm 2 sample area. The measured performance is consistent with Monte Carlo simulations. The monochromating system, which includes radial collimators, three filters and a variable beam aperture, offers considerable flexibility in optimizing Q-resolution, energy resolution and intensity. The detector system will consist of an array of 20 channels which combined will subtend a solid angle of 0.2 sr. This is approximately a factor of 40 more than a conventional triple axis spectrometer. Each detector channel contains a vertically focusing double crystal analyzer system (DXAL) actuated by a single stepping motor. We find identical integrated reflectivity at approximately 10% coarser energy resolution for the 130 mosaic double bounce analyzer as compared to a conventional 25 analyzer at the same energy. The vertical focusing of the DXAL allows for smaller detectors for enhanced signal to noise with 8 • vertical acceptance. Options for post sample collimators and filters provide flexibility in the choice of scattered beam energy and wavevector resolution.
A basic aluminium chloride with high chlorine content (basicity 1.85) has been crystallized from a concentrated aqueous solution. Its crystal structure has been determined by X-ray structure analysis. The structure contains (Figure 1) with a Keggintype structure.[2] The latter structure is characterized by a particular resonance shift in its 27 Al-NMR spectrum, even in solutions of basic aluminium chlorides, [3] caused by the tetrahedrally coordinated, central aluminium, which is surrounded by 12 other octahedrally coordinated Al atoms. Various crystalline basic aluminium chlorides have been prepared from solutions by Walter-Levy and Breuil.[4] The crystalline phases were characterized by means of X-ray powder analysis and their thermal decomposition was investigated. The powder diagrams of some of these crystalline phases were unambiguously indexed and the lattice constants were determined. This is also the case for a cubic phase prepared at Dow Chemical Co.[5] However, as far as we are aware, results of crystal structure analyses have hitherto not been reported.In this paper, we report on a structure analysis of the most chlorine-rich of the basic aluminium chlorides described by Walter-Levy and Breuil.[4] [6] These authors obtained the salt as a crystalline precipitate that formed after several days following dilution and storage of concentrated basic aluminium chloride solutions. Suitable monocrystals for our structure analysis were obtained serendipitously, in the course of studies on the colloid structures of basic aluminium chlorides. Colloid solutions with an Al:Cl ratio in
In this work a method is developed that allows the computation of the single-crystal elastic constants for crystals of cubic symmetry from the diffraction elastic constants. The diffraction elastic constants can be obtained by measuring the hkl-dependent lattice strain response to an applied stress. Because of their hkl dependence they represent, partially, the anisotropic nature of the single-crystal elastic constants. The computation of the single-crystal elastic constants is carried out by a least-squares re®nement which ®ts the calculated diffraction elastic constants to the measured ones.
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