摘要:

A laboratory dispersive mode spectrometer, capable of operating in either the analysing crystal transmission mode or a reflection mode, is described. Extended X‐ray absorption fine structure (EXAFS) spectra of Re and ReO2, obtained in the transmission mode, compare favourably with those from a scanning spectrometer at a synchrotron source. Factors affecting resolution, intensity and background in this transmission mode are discussed. Experiments with asymmetric reflection geometries, which have shown both improved resolution for X‐ray absorption near edge structure (XANES) and reduced collection times, are reported. Methods of reducing backgrounds due to multiple Bragg reflections and Compton scattering are propo

ISSN:1600-5767    

DOI:10.1107/S0021889887008641

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

Quantitative phase analysis of multicomponent mixtures using X‐ray powder diffraction data has been approached with a modified version of the Rietveld computer program of Wiles&Young [J. Appl. Cryst.(1981),14, 149–151]. This new method does not require measurement of calibration data nor the use of an internal standard; however, the approximate crystal structure of each phase of interest in a mixture is necessary. The use of an internal standard will allow the determination of total amorphous phase content in a mixture. Analysis of synthetic mixtures yielded high‐precision results, with errors generally less than 1.0% absolute. Since this technique fits the complete diffraction pattern, it is less susceptible to primary extinction effects and minor amounts of preferred orientation. Additional benefits of this technique over traditional quantitative analysis methods include the determination of precise cell parameters and approximate chemical compositions, and the potential for the correction of preferred orientation and microabsorption ef

ISSN:1600-5767    

DOI:10.1107/S0021889887009415

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

An indirect method of determining the intensity of the incoherent X‐ray scattering component of the total scattering intensity from a disordered carbon material is presented. It utilizes the incoherent scattering from a crystalline carbon sample (standard) measured at a diffraction angle 2θfar from a Bragg interference maximum, and requires only knowledge of the linear absorption coefficientμand densityρof the material. The results obtained for angles 2θin the range 100–160° show that the direct and indirect determinations of the incoherent scattering are in excellent agreement. The indirect method takes a simple form when the disordered and crystalline carbon sample thicknesses,t, are optimized either for small‐angle scattering,μt= 1, or for wide‐range diffraction, exp [

ISSN:1600-5767    

DOI:10.1107/S0021889887009427

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

The pair functionP(r) determined from atomic models was analysed for models in the form of small atomic layers made up of six‐membered rings occurring in the amorphous structures of materials with crystalline structure of the hexagonal type. the effect of the properties of a single‐layer model onP(r) was determined, including its dimensions and shape, and for models from two layer types the influence of the combination and intermixing of their elements was also investigated. The effect onP(r) of the combination of such layers into turbostratic stacks was analysed for the initial stage of formation of such stacks. From the form ofP(r) one can determine both the dimensions of the layer and the existence of stacks of layers, and also the initial stage of the formation of sta

ISSN:1600-5767    

DOI:10.1107/S0021889887009890

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

From studies on six petrographic components of Polish coals by the radial distribution function method and the interatomic distances calculated from them the proportions of atoms withsp2andsp3hybridization and the extent of units withsp3‐type structure were determined. From these data and the results of the pair‐function analysis performed in paper I [Grigoriew (1988).J. Appl. Cryst.21, 97–101], the model interpretation of the structure of these materials was carried out by matching experimental and model pair functionsP(r). The model was defined as an average structural unit which scatters X‐rays coherently. It appears that such units in these materials are larger than previously expected and for vitrinites and fuzinites they are in the form of turbostratic layers. The process of formation of stacks of layers was observed in components with the lowest degree of o

ISSN:1600-5767    

DOI:10.1107/S0021889887009907

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

In this paper a heatable stretching device is presented which enables the performance of real‐time X‐ray experiments during deformation. The construction of the oven makes it possible to perform experiments with detectable X‐ray diffraction angles (2θ) of up to 30°. It is shown that the apparatus can be used to study morphological changes occurring during the deformation/drawing of polymer materials, using the benefits of synchrotron facilities. A series of small‐angle (SAXS) and wide‐angle (WAXS) X‐ray scattering patterns obtainedduringthe drawing of melt‐crystallized ultra‐high‐molecular‐weight polyethylene (UHMW‐PE

ISSN:1600-5767    

DOI:10.1107/S0021889887010057

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

High‐resolution solid‐state27Al NMR spectra have been obtained from ALON,β′‐sialon and yttrium aluminate phases. The usefulness of27Al magic‐angle spinning NMR in phase characterization is shown by comparing different preparations of the yttrium aluminates. The sensitivity of the27Al resonance position to the local environment means different units (e.g.AlN4, AlO4, AlO6) can readily be distinguished allowing oxynitride structures, containing elements of similar scattering factor, to be refined. This is exemplified for theβ′‐sialon phase. For quadrupolar nuclei (nuclear spin,I≥ 1) such as27Al (I= ), interaction with the local electric field gradient can cause extensive broadening of the resonance such that the NMR spectrum may not precisely quantify the alumi

ISSN:1600-5767    

DOI:10.1107/S0021889887010069

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

The Porod law states thati(h), the intensity of X‐radiation scattered by an ideal multiphase noncrystalline system, for `large' momentum transfer values (≡h) approaches

ISSN:1600-5767    

DOI:10.1107/S0021889887010409

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

Neutron scattering profiles from superparamagnetic colloids have been reported by several authors seeking evidence of a difference between a single particle's spatial and magnetic sizes. Calculations are presented which show that the weighting factor present in summing scattering functions over the particle size distribution arising from size‐dependent particle magnetization can mask the single‐particle transforms sought for. Three experimental precautions are suggested: (1) study only samples of known narrow dispersion (ΔD/D≤ 30% for magnetite); (2) perform experiments at several values of magnetic field spanning several orders of magnitude; (3) measure both incident and final polarization states of the neutrons. Numerical calculations based on a model allow quantitatively experimentally acceptable limits of sample dispersion to be d

ISSN:1600-5767    

DOI:10.1107/S0021889887010410

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

A crystal diffraction theory has been developed and compared with experimental measurements in order to model the integrated reflectivity of multilayer structures. This study shows that the most important defect reducing the integrated reflectivities of the multilayer structures studied was correlated roughness (root mean square value about 7 Å). The theory describes correlated roughness as a probability distribution of the substrate surface displacement. A computer simulation and an analytical solution have been used to calculate the reflectivity of multilayer structures. The calculations using a rectangular distribution of correlated displacement result in a good first‐order approximation of the experimental data. A Gaussian probability distribution for the substrate surface displacement results in calculations inconsistent with the measured reflectivities, although such a distribution has been assumed in other studies. Although other defects were studied, only roughness could explain the experimental

ISSN:1600-5767    

DOI:10.1107/S0021889887010434

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY