摘要:

TheROCKSsystem of computer programs is available to carry out most of the calculations involved in data acquisition and structure determination by the isomorphous replacement and molecular replacement methods.ROCKSis designed to be general, easy to use, reliable and efficient. It now includes facilities for precession and oscillation film scanning, data reduction, Wilson statistics, structure‐factor calculations, isomorphous‐replacement phasing, atomic parameter least‐squares refinement, Fourier, Patterson and difference‐Fourier calculations, contouring and peak searching, local scaling, Patterson superpositions, product andR‐factor translational searches, phase combination and phase refinement by electron density modification. It is available at present in versions for IBM series 360 and 370 computers and for DEC VAX

ISSN:1600-5767    

DOI:10.1107/S0021889884011171

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

A procedure based on successive convolution operations is developed and associated with the Warren–Averbach method. Applications to cold‐worked lithium fluoride samples are presented. The microstrain results are compared with those obtained in the classical way including the use of the direct deconvolution method. It is shown experimentally that the difficulties involved in the deconvolution operation have no significant effect on the evaluation of microstrains by the classical X‐ray line‐profile a

ISSN:1600-5767    

DOI:10.1107/S0021889884011183

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

Equations are derived for the intensity ratioRof secondary to primary scattering in the fixed‐sample transmission case with an incident‐beam monochromator crystal that results in a polarization ratioKmfor unpolarized incident radiation. A table allowsRto be calculated forKm= 0. This table, along with the one forKm= 1 published previously for the case without a monochromator, allowsRto be estimated before an experiment is done and thus an optimum experiment can be designed. A simple computer program allows direct calculation ofRonce experimental data are obtained if it is desired to avoid interpolation. Determination ofRinvolves some approximation, but the values obtained are sufficiently accurate for most purposes. However,Rcan be improved using exact equations, if desired. If synchrotron radiation polarized normal to the plane of scattering of the monochromator is used,Rdoes not depend onKm, and the value ofRforKm= 0 applies despite the true value ofKm. As is expected from symmetry considerations,Ris independent ofKmin the small‐angle scattering

ISSN:1600-5767    

DOI:10.1107/S0021889884011195

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

One of the main sources of error when studying weakly absorbing materials using an X‐ray diffractometer with Bragg–Brentano focusing geometry is specimen transparency. Its undesired effects (peak displacement and loss of intensity in the medium angular range) can be avoided by using a slab‐like specimen in symmetrical transmission, in which case the type of diffractometer is more or less immaterial except that a non‐focusing diffractometer offers a series of advantages,e.g.a very simple lining‐up procedure and the possibility of using rod‐like specimens. Such a diffractometer with a flat monochromator in the primary beam is described and both the equatorial aberration and the axial aberration ar

ISSN:1600-5767    

DOI:10.1107/S0021889884011201

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

Hexagonal crystals of cowpea mosaic virus in space groupP6122 (or its enantiomorph) witha= 451 andc= 1038 Å have been analyzed using the synchrotron X‐ray source at LURE. Data were collected using an X‐ray beam of size 0.18 × 0.22 mm permitting resolution of reflections along thec* direction. The beam was defined using a multiple slit system at both the monochromator and at the collimator. A helium path was used to reduce air scatter as a specimen‐to‐film distance of 175 mm was employed. Data to 4.3 Å resolution were recorded on each film. The films were processed to 6 Å resolution using the oscillation photograph processing package developed by Rossmann [J. Appl. Cryst.(1979),12, 225–238] with minor modifications to the normal procedures. One film was processed to 4.3 Å resolution establishing the feasibility of intermediate resolution analysis. Statistical analysis of the processed data showed excellent correlation between symmetry‐equivalent reflections. Post refinement of oscillation angles and lattice constants was very stable and led to significantly improved scaling of pa

ISSN:1600-5767    

DOI:10.1107/S0021889884011213

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

The measurement of the spectral intensity distribution of the incident beam in energy‐dispersive X‐ray diffractometry has been examined by several methods. The spectral distribution estimated from the integrated intensity of a well defined reflection at several energies is the most reliable. Three direct measurements of the spectral distribution by a solid‐state detector are also examined. Of these only the measurement at very low tube current can be used in structure factor determination, but the accuracy is about 5% even for strong reflec

ISSN:1600-5767    

DOI:10.1107/S0021889884011225

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

A new linear position‐sensitive scintillation detector has been developed and installed on a neutron powder diffractometer. The physical properties of this detector are described; test powder diffractograms are presented. The special advantages of this detector for neutron powder diffractometry are discussed: high detection efficiency also in the region of short wavelengths (75% forλ= 1.3 Å), good spatial resolution (FWHM = 2.5 mm), and high data‐point density (Δ2θ= 0.02°). Special emphasis is placed on peak‐profile analysis for the powder diffraction patterns. Examples for position and intensity separation of overlapping reflections

ISSN:1600-5767    

DOI:10.1107/S0021889884011237

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

A 3 mm long double‐polytype region was found in a ZnS needle. The region consists of a fine mixture of 47% of each of the polytypes 20L(5753) and 60R(5537)3and about 6% of the polytype 60R(511553)3. The latter polytype is assumed to be the parent polytype transforming into the two others. It is the first time that traces of the parent polytype have been found in a double‐polytype region. The formation mechanism of double‐polytype regions is explained as a two‐stage transformation process: expansion of grown‐in stacking faults taking place at temperatures just below the hexagonal‐to‐cubic transition temperature, followed by a sudden formation and expansion of stacking faults at a lower temperature. It is suggested that a similar mechanism operates in the formation of single polytype regions and

ISSN:1600-5767    

DOI:10.1107/S0021889884011249

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

Structural disorder in severely ground MgCl2is interpreted on the basis of a Bernoullian model with four parameters, specifying both the size and the statistical sequence of the layers; their choice completely dictates the intrinsic shape of all the X‐ray diffraction lines. The X‐ray powder spectra are reasonably well reproduced over a wide range of Bragg distances (~1.7

ISSN:1600-5767    

DOI:10.1107/S0021889884011250

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

Intensities and shapes of the (02,11), (20,13) and (04, 22) diffraction bands are calculated for model structures and are fitted to the experimental patterns, in order to describe the structural characteristics of a two‐water‐layer Na‐beidellite (Rupsroth, Bavaria, Germany). The determined features are: the average dimensions and size distributions of coherent domains, the coordinates of the crystallographically bound water molecules, the nature and abundance of stacking defects. The localization of the water sites and the determination of their occupancy rates are obtained using a general and powerful procedure which is described. For the stacking, a quantification of the influence, on the X‐ray powder diagrams, of all crystallochemically possible modes given in the literature has been done. On the basis of such a quantification, it was possible to establish the layer succession law in the studied hydrated pa

ISSN:1600-5767    

DOI:10.1107/S0021889884011262

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY