摘要:

On the basis of the expressions given by Rossmanith [Acta Cryst.(1992), A48, 596–610; (1993), A49, 80–91], a simple approximation is derived for the half‐widths of Bragg intensity profiles measured with a triple‐crystal diffractometer at a synchrotron‐radiation source. This new formula facilitates insight into the effects of four parameters – divergence, wavelength spread, mosaic spread and mosaic block size – on the widths of

ISSN:1600-5767    

DOI:10.1107/S002188989300439X

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

The practical usefulness of the expressions for the half‐width proposed in paper I [Rossmanith (1993).J. Appl. Cryst.26, 753–755] are demonstrated for three examples: a spherical YIG (yttrium iron garnet) crystal, a spherical silicon crystal and a plane‐parallel perfect silicon plate. The new concept for the calculation of the width results in theoretical widths that are in excellent agreement with experimental widths observed in parallel and antiparallel arrangements of the sample with respect to the second monochromator cr

ISSN:1600-5767    

DOI:10.1107/S0021889893004388

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

This paper describes the characterization of an image‐plate readout system used to calculate small‐angle‐scattering intensities as cross sections per unit volume. The raw data are the latent phosphorescent excitations stored on an image plate. Self‐decay, self exposure, resolution, smearing, dynamic range and sensitivity are measured. Preliminary experiments show that an inexpensive neutron‐imaging system can be obtained when the plates are placed behind a suitable neutron‐to‐X

ISSN:1600-5767    

DOI:10.1107/S002188989300487X

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

An intensity enhancement obtained from asymmetric diffraction with a fixed incident angleαhas been studied. Parallel‐beam synchrotron radiation withλ= 1.54 Å (Stanford Synchrotron Radiation Laboratory) andλ= 1.53 Å (Photon Factory) was used to collect powder diffraction patterns of Si, CeO2(α= 5 and 10°) and monoclinic ZrO2(α= 10°). The synchrotron‐radiation data were analyzed using single‐reflection profile fitting and whole‐powder‐pattern fitting techniques. The integrated intensities in the asymmetric diffraction were compared with those of symmetric diffraction obtained by the conventionalθ–2θscanning technique. An intensity, after correction for a limited height of counter aperture, was enhanced by factors of 1.8 (α= 5°) and 1.7 (α= 10°) at the maximum in asymmetric diffraction and its magnitudes agreed well wit

ISSN:1600-5767    

DOI:10.1107/S0021889893004881

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

An approximate experimental correction of electron diffraction intensities from an epitaxically crystallized phospholipid bilayer for dynamical scattering is described. This correction, which is useful for certain low‐angle centrosymmetric data sets, compares intensities recorded at high and low electron‐accelerating voltages to ascertain which reflections are most affected byn‐beam interactions. When applied to experimental intensity data from 1,2‐dihexadecyl‐sn‐glycerophosphoethanolamine (DHPE), the correction facilitates a direct phase determination based on the probabilistic estimate of three‐phase invariants because a more accurate estimate of the hierarchy of |El| values is obtained. When a multisolution technique is used, incorporating algebraic unknowns for certain phase values, the best phase assignment can be assessed by comparison of the single convolution of phased structure factors to the observed structure‐factor magnitudes for the low‐voltage data. This approach exploits an approximate analogy made earlier by Moodie between the Sayre equation and the phase grating series and is valid as long as the single convolution adequately models experimental low‐voltage data (a condition favored by light‐atom structures in a low‐angle region of reciprocal space). In real space, the correct structure can also be readily identified as the one having the sm

ISSN:1600-5767    

DOI:10.1107/S0021889893005394

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

A direct (i.e.noniterative) method for desmearing the beam‐height effect in small‐angle X‐ray scattering is discussed. The method is applicable to rectangular collimation systems with arbitrary beam‐height intensity profiles. The process involves the construction of an upper‐triangular matrix of terms containing the resolution information. A straightforward back‐substitution process can then be used to determine the ideal pinhole‐collimated curve for any experimental curve obtained with the given resolution. The principal advantage of the method lies in its simplicity, which facilitates an examination of the propagation of random errors through the desmearing process. A comparison between the direct method and the iterative approach of Glatter [J. Appl. Cryst.(1974),7, 147–153] is made to illustrate the efficiency o

ISSN:1600-5767    

DOI:10.1107/S0021889893005527

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

An algorithm has been developed for the automatic interpretation of a given set of observed reciprocal‐lattice points. It extracts a reduced cell and assigns indices to each reflection by a graph‐theoretical implementation of the local indexing method. All possible symmetries of the observed lattice compatible with the metric of the reduced cell are recognized and reported, together with the unit‐cell constants and the linear index transformation relating the conventional to the reduced cell. This algorithm has been incorporated into the program XDS [Kabsch (1988).J. Appl. Cryst.21, 916–924], which is now able to process single‐crystal area‐detector data without prior knowledge of the symmetry and the unit‐c

ISSN:1600-5767    

DOI:10.1107/S0021889893005588

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

X‐ray diffraction rocking curves of a strained‐layer superlattice structure have been measured at symmetric and asymmetric Bragg reflections and compared with simulated diffraction curves. The calculations are based on dynamical scattering theory. The experimental and theoretical curves exhibit a discrepancy with regard to the angular position of the higher‐order satellite reflections, which can be removed by introducing a new expression for the deviation parameter in the dynamical diffraction theory. The improved deviation parameter extends the range of validity of the two‐beam approximation, especially for asymmetric reflections with glancing exit geometry. Therefore, if the relaxation of strained‐layer heterostructures is to be determined by comparison with simulated rocking curves, only the improved parameter shoul

ISSN:1600-5767    

DOI:10.1107/S0021889893005643

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

Perturbations from crystal perfection will result in diffuse X‐ray scattering, which can be imaged by topography to yield defect information without the swamping contribution of perfect‐crystal scattering. This paper illustrates the importance of the probe shape for obtaining topographic images in this diffuse region not only to aid interpretation but to isolate the contributors to this scattering. The diffuse scattering in the vicinity of the Bragg peaks of GaAs, Si and Ge substrate crystals has been mapped to very high resolution and the diffuse scattering has been imaged by topography. It was found that the majority of the scattering emanates from surface damage and dislocations and not point defects or thermal diffuse scattering (TDS). These latter two components are found to be second‐order effects in general and are only just discernable as a very weak background intensity in highly perfect crystals. This topography method is very sensitive to surface damage. This is because the associated diffuse scattering close to a Bragg peak can be used to form an image. Therefore, this relatively intense scattering provides a topograph within a few hours for assessing substrate‐surface quality. The sensitivity of the method is illustrated with images of surface defects and dislocations in very perfect semiconductors. A procedure for measuring the diffuse scattering emanating from microdefects is also pr

ISSN:1600-5767    

DOI:10.1107/S0021889893006259

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

Three‐dimensional small‐angle scattering is discussed as a tool for investigation of anisometric particles in single‐crystalline alloys. The measurement and evaluation methods take advantage of the well defined orientation relation between the precipitates and the matrix in alloys. The determination of shape, size and orientation of precipitates from the scattering in several planes of reciprocal space is discussed for the example of ellipsoids and plates. Shape and orientation of ellipsoids with small aspect ratio can be determined by deviations of the scattering intensity from spherical symmetry in certain planes of reciprocal space. Particle size is obtained from one‐dimensional slices along certain directions. The small‐angle scattering of plate‐like particles with large aspect ratio is concentrated along radial streaks in reciprocal space. Slices along and across these streaks allow the determination of the plate thickness as well as size and aspect ratio of the plate surface. In addition, the evaluation of such `off‐center' slices provides a resolution enhancement of almost one order of magnitude compared to the conventional Guinier method applied to spherically

ISSN:1600-5767    

DOI:10.1107/S0021889893006296

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY