ISSN:1600-5767    

DOI:10.1107/S0021889888008258

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

The compositional depth profile in semiconductor heterostructures can be determined from X‐ray diffraction patterns. Different grading profiles were studied through theoretical simulations with regard to their features in the rocking curve. It was found that the thickness and the grading of a particular layer cannot be determined independently of each other. A linear grading gives rise to an increased peak width of the layer diffraction peak whereas an exponential grading can be detected from the damping of high‐order interference fringes. The exponential model can be applied to determine the abruptness of the heterointerfaces. The proposed evaluation method of experimental rocking curves includes the case of overlapping peaks of the layer and the substrate diffraction. The simulation results are discussed for a GaAs/Ga1−xAlxAs/GaAs[001] double heterostructure. When the experimental resolution is taken into account, the sensitivity of the interface width determination was 100

ISSN:1600-5767    

DOI:10.1107/S0021889888003188

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

A practical method to correct X‐ray powder diffraction patterns for the effects of preferred orientation in the case of monoclinic crystals is presented. The method is based on the use of symmetrized harmonics to describe the preferred orientation. To obtain a texture‐free result the diffraction pattern must be measured both in symmetrical transmission and in symmetrical reflection modes using a spinning sample. The method is tested by measuring pole figures for a cellulose powder sample. The analysis of the results shows that by averaging the measured intensities properly the effect of texture can be diminished by one order of magnitude. This accuracy is sufficient for applying the method, for example, in quantitative determination of crystallinity of cellul

ISSN:1600-5767    

DOI:10.1107/S0021889888003371

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

A range of `normal' divergence (0.25, 1.0°) and receiving (0.1, 0.2, 0.4 mm) slit dimensions has been used to determine their effect on the peak profile parameters obtained by Rietveld analysis of X‐ray powder diffraction data from a conventional X‐ray source. The instrument is a standard Bragg–Brentano diffractometer equipped with a diffracted‐beam monochromator and Soller slits. The instrumental profile characteristics are provided by a sample of coarsely crystalline Si, and the effects of small particle size and isotropic strain by fluorite, CaF2. Results are discussed for refinements of the full diffraction patterns (24 to 140°), and of the individual peaks at 28, 47 and 88° in each pattern. The use of a wide receiving slit aperture (i.e.0.4vs0.1 mm) provides (i) increased peak intensities, (ii) much improved values of the Rietveld agreement indices (especially at low angles), (iii) slightly more Gaussian peak shapes, and (iv) slightly wider peaks. A wider divergence slit (i.e.1.0vs0.25°) has only a small effect on the refinement results. The Voigt and pseudo‐Voigt peak shape models are seriously deficient for low‐angle peaks when axial divergence is larger than about 2°, but are otherwise satisfactory. The CaF2peaks are wider and more Lorentzian in character (at high angles) than those obtained for Si, owing to the presence of very small (350 Å) and strained crystals; in this case, the peak shape/width characteristics can be adequately modelled at all diffraction angles, regardless of axial divergence. Quantitative information about crystal size and strain can be extracted from the CaF2pattern by direct incorporation of the instrumental profile into the parameters of the Voigt function dur

ISSN:1600-5767    

DOI:10.1107/S0021889888003474

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

Pseudo‐hexagonally twinned orthorhombic crystals generally exhibit three domain variants related by mirror planes or (approximately) by 120° rotations. The reciprocal lattices of the three domains coincide partially. Those reciprocal‐lattice pointshklwithh+k= 2Ncoincide or are split depending upon the deviation of thea:bratio from orthohexagonality (i.e.a:b= tan 60°). With (white‐beam) synchrotron radiation these domains are simultaneously imaged by Laue X‐ray topography for these reflections. Lattice pointshklwithh+k= 2N+ 1 are `single' and only one of the three domain variants is imaged in such reflections. Two examples of these twins are presented, LiNH4SO4and (NH4)2SO4. In the former case the lattice is closer to the ideal ratioa:b= tan 60° than the latter. These two cases are contrasted. Both materials are subject to some extent to radiation damage in the synchr

ISSN:1600-5767    

DOI:10.1107/S002188988800408X

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

It is demonstrated with computer simulations that a bimodal distribution (narrow and broad) of reflection profile broadening effects, such as crystallite size and microstrain, can lead to several spurious results when the profiles are modeled by single pseudo‐Voigt functions in Rietveld refinement (RR),e.g.mixing parameterη>1 (`super‐Lorentzian'), a few observed data points lying consistently above the calculated peak maxima for some reflections, misrepresentation of the background level and shape, and in special cases well defined small observed peaks not seen in the calculated pattern as though they belonged to an additional minor phase. With a bimodal model (two pseudo‐Voigt functions) the RR's accurately returned the profile, mixing, and background parameters used in the data synthesis both without and with counting statistical fluctuations simulated in the data. The RR method thus demonstrates remarkable ability to sort out complex specimen characteristics when accurate modeling functions ar

ISSN:1600-5767    

DOI:10.1107/S0021889888004091

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

A procedure is described for the transfer of protein crystals from an ammonium sulfate‐containing mother liquor to a solution with another precipitant, such as polyethylene glycol. The suitable concentration of the alternative precipitant is establishedviaa novel protocol, using a hanging‐drop equilibration method. This crystal transfer procedure is illustrated by experiments with crystals of trypanosomal triosephosphate isomerase and bacterialp‐hydroxybenzoate hydroxylase, but it might have more general applicab

ISSN:1600-5767    

DOI:10.1107/S0021889888004108

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

Incident‐beam monochromators have been available commercially for many years, but the significant advantages of their use in conventional divergent‐beam diffractometry are not widely appreciated. The improvement in the quality of diffraction data which results from the removal of theKα2component ofKαradiation at source has important consequences in most applications of powder diffraction. In particular, there is a simple well characterized instrumental function with a predictable angular variation. The parameters which define this function are independent of any sample and provide a basis for comparing instrumental configurations and for assessing the quality of reference materials. At low angles the profiles are predominantly Gaussian and at high angles they tend to be Lorentzian, owing to the dominant effect of wavelength dispersion. The main features of using a strictly monochromatic radiation in conventional powder diffractometry, namely improved resolution, simple instrumental shape function and better evaluation of the background level at higher angles, are described. The benefits of these properties in current applications of powder diffractometry,e.g. indexing, line‐profile analysis and structure analysis, are discussed

ISSN:1600-5767    

DOI:10.1107/S002188988800411X

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

Experimentally measured scattering data differ from theoretical curves because of departures from point geometry in a real instrument. In a small‐angle neutron scattering (SANS) instrument, there are essentially three contributions to the smearing of an ideal curve: (1) the finite divergence of the beam, (2) the finite resolution of the detector, and (3) the poly chromatic nature of the beam. Where the scattering is azimuthally symmetric about the incident beam, indirect Fourier transform (IFT) methods may be used not only to smear an ideal scattering curve, but also to desmear an observed pattern. For experiments where the assumption of azimuthal symmetry cannot be made, alternative procedures based on Monte Carlo (MC) techniques have been developed which simulate the smearing of a given theoretical scattering function. This procedure permits the evaluation of smearing effects in anisotropic systems. Both IFT and MC procedures are illustrated with a range of applications from data taken on the 30 m SANS facility at Oak Ridge National Laboratory. It is shown that for experiments with scattering dimensions<200 Å smearing effects are small (<5%) and that dimensions up to ~ 1200 Å may be accurately resolved after proper evaluation of resolution effects. The procedures described may also be extended to include small‐angle X‐ray scattering, and an example of one such applica

ISSN:1600-5767    

DOI:10.1107/S0021889888004273

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

The crystal structure of α‐CrPO4, previously solved from high‐resolution synchrotron powder X‐ray data, has been refined by the Rietveld method. The peak shape variation is well fitted by a convolution of Gaussian and Lorentzian terms, which incorporate both instrumental and sample‐broadening contributions. The structure has also been refined using medium‐resolution neutron diffraction data, and the results of the two experiments are compared with a reported single‐crystal study. The three sets of coordinates are found to be in good agreement, demonstrating the potential of structure determination and refinement using

ISSN:1600-5767    

DOI:10.1107/S0021889888004625

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY