摘要:

The method of crystallographic refinement using molecular dynamics [Brünger, Kuriyan&Karplus (1987).Science,235, 458–460] has been implemented to work with theGROMOSsimulation package. It has been tested by applying it to the structure of phospholipase A2. The structure of this molecule had previously been refined at high resolution with conventional methods. The new method successfully refined the initial multi‐isomorphous replacement structure, removing most of the errors, without any manual intervention. All the refinement was performed at 300 K. The use of lower‐resolution data allows greater conformational transitions to occur whereas the inclusion of high‐resolution data results in a more accurate

ISSN:1600-5767    

DOI:10.1107/S0021889888009550

出版商:International Union of Crystallography    

年代:1989

数据来源: WILEY

摘要:

A method has been developed based on Rossmann's [J. Appl Cryst.(1979).12, 225–238] treatment of oscillation camera data for processing of X‐ray diffraction data collected on a screenless Weissenberg camera for macromolecular crystals [Sakabe (1983).J. Appl. Cryst.16, 542–547]. Crystal orientation parameters and film orientation parameters are refined by minimizing the discrepancies between the calculated and observed positions of reflections on a film. A data processing example is presented. Intensities were collected using a synchrotron radiation source and the imaging plate as a detector. The results show that the quality of the data is good, as judged by the agreement of the equivalent reflections. An absorption correction based on the use of the empirical Fourier expansion method reduced theRvalue and improved the correlations between Bijvoet differences observed on the different imaging plate expo

ISSN:1600-5767    

DOI:10.1107/S0021889888009562

出版商:International Union of Crystallography    

年代:1989

数据来源: WILEY

摘要:

Drawings of molecular and solid‐state structures generated by the computer programSCHAKAL88 are presented. Automatic atom labelling and some of the graphical techniques used for shading of line and raster drawings are briefly discusse

ISSN:1600-5767    

DOI:10.1107/S0021889888009057

出版商:International Union of Crystallography    

年代:1989

数据来源: WILEY

摘要:

A crystal of dimensions 2.5 × 2.5 × 1.8 mm and of composition Mo3.25Ir was examined by neutron diffraction at 293 and 10 K. It has the A15 structure for which the space group isPm3nand the ideal composition would be Mo3Ir. Least‐squares refinements gaveR(F) values of 0.013 and 0.017 for the 293 and the 10 K data, respectively. No structural phase transition was observed at 10 K. The lattice constants found werea= 4.967 (2) Å at 293 K and 4.958 (2) Å at 10 K. The transition temperature to superconductivity of the crystal was measured by an inductive method. An onset of superconductivity was found at 9.25 (5) K and a main transition at 9.10 (5) K. Another, approximately cylindrical, sample of volume 117 mm3showed an onset at 8.6 (1) K and a main transition at 8.0 (1) K. About two

ISSN:1600-5767    

DOI:10.1107/S0021889888009574

出版商:International Union of Crystallography    

年代:1989

数据来源: WILEY

摘要:

In paper I [Groma, Ungár&Wilkens (1988).J. Appl. Cryst.21, 47–53] a theory was developed to interpret the asymmetric X‐ray line broadening of plastically deformed crystals. It was shown that the dislocation structure can be described by five distinct parameters, namely the dislocation density, the mean quadratic spatial fluctuation of the dislocation density, the effective outer cut‐off radius, the dipole polarization and the spatial fluctuation of the dipole polarization of the dislocation structure. In this paper a procedure is developed to evaluate these parameters from the Fourier transform of the line profiles. The theory and this procedure are tested by applying it to the asymmetric line profiles of tensile‐deformed Cu single crystals orientated for ideal multiple slip. The asymmetry of these profiles is assigned to the dipole polarization of the dislocation cell structure and is directly correlated to residual long‐range internal stresses. It is shown that the data can be interpreted in terms of the quasi‐composite model of the dislocation cell structure developed earlier for the s

ISSN:1600-5767    

DOI:10.1107/S0021889888009720

出版商:International Union of Crystallography    

年代:1989

数据来源: WILEY

摘要:

Multilayer structures, analogous to broadband optical filters, may be used to reflect X‐rays or neutrons at angles larger than the total external reflection angle intrinsic to the bulk mirror material. These so‐called supermirrors have generally been designed using continuum theories, which predict that the thickness of thejth layer in the multilayer should vary smoothly asj−1/4. A new approach is proposed, based on considering the contribution of each bilayer to the extinction in a given stack of bilayers, and the discrete equations governing the choice of layer thicknesses are derived and solved. In the limit of zero layer thickness, the continuum result is recovered. The design produces essentially perfect reflectivity over the entire supermirror range. An optimal technique for trading reflectivity to gain angular range also emerges naturally from the physics of the d

ISSN:1600-5767    

DOI:10.1107/S0021889888010003

出版商:International Union of Crystallography    

年代:1989

数据来源: WILEY

摘要:

A new four‐circle X‐ray diffractometer (RMD) for single crystals is described that has only one (horizontal) axis of specimen rotation and a movable X‐ray tube and detector. The diffractometer is characterized by the equatorial geometry. The equatorial plane rotates about theχaxis lying in this plane whereas theϕaxis is fixed. This permits the use of various external (electromagnetic, magneticetc.) fields applied to specimens along theϕaxis. The angles between this direction and all the other crystallographic directions in the specimen remain constant in the course of intensity measurements. The designed RMD diffractometer allows the study of structural changes and electron distribution variations in a crystal subjected to external actions. The geometry and the design of the goniometer are considered in detail. Experiments carried out on the RMD diffractometer have demonstrated its efficiency for precision X‐ray structur

ISSN:1600-5767    

DOI:10.1107/S002188988801060X

出版商:International Union of Crystallography    

年代:1989

数据来源: WILEY

摘要:

Mass attenuation coefficients of seven elements between atomic numbers 22 and 30 were measured at 20 X‐ray energies between 4.5 and 11 keV. These energies correspond to fluorescence radiation excited by monochromatic MoKαradiation. The measurements were made on thin metal foils, which were spread flat and perpendicular to the beam scattered from the fluorescent sample. In this arrangement, the measured attenuation includes photoelectric absorption plus average elastic and inelastic scattering. The mass per unit area of a foil was determined by weighing and the impurity concentration by fluorescence analysis. The spectral lines corresponding to the components of the fluorescent radiation and those of the secondary radiation were resolved by the so‐called singular value decomposition (SVD). The probable errors of the attenuation coefficients were typically 1%. The general agreement with previous experimental and theoretical values is good, but at low energies and at energies just above the absorption edges the present values are about 5% larger than theoretical values based on relativistic HFS (Hartree–Fock–Slater) ca

ISSN:1600-5767    

DOI:10.1107/S0021889888010817

出版商:International Union of Crystallography    

年代:1989

数据来源: WILEY

摘要:

An automatic procedure has been developed for indexing reflections on oscillation photographs produced by crystals with very large unit cells (cell dimensions larger than about 100 Å). The procedure works in a similar fashion to the auto‐indexing used in four‐circle diffractometry. Features includeab initiocell parameter and orientation matrix determination, reduced cell calculation and transformation of the reduced cell to one with higher sy

ISSN:1600-5767    

DOI:10.1107/S0021889888010878

出版商:International Union of Crystallography    

年代:1989

数据来源: WILEY

摘要:

High‐pressure X‐ray diffraction studies have been performed on ThP using synchrotron radiation and a diamond‐anvil cell. The bulk modulusB0and its pressure derivativeB′0have been determined (B0= 137 GPa;B′0= 5.1). A phase transition from the NaCl structure to the CsCl structure was observed at a

ISSN:1600-5767    

DOI:10.1107/S002188988801091X

出版商:International Union of Crystallography    

年代:1989

数据来源: WILEY