摘要:

It is shown how the angles of incidence and exit may be controlled in a surface diffraction experiment using a four‐circle diffractometer. The procedure may be readily incorporated into existing diffractometer control program

ISSN:1600-5767    

DOI:10.1107/S0021889887008148

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

A fixture is described for collecting X‐ray diffraction data from biomolecule crystals under gas pressures up to 300 atm (30 MPa). The fixture is completely portable and can be used on conventional diffractometers. Normal procedures for crystal mounting, alignment and data collection require only slight modification. A test data set of the protein sperm whale metmyoglobin under a gas pressure of 145 atm of N2gas has been collected to a nominal resolution of 2.0 Å using CuKαradiation and refined to anR‐factor of 16.2%. Data collection beyond 2.0 Å resolution is hampered by the presence of strong powder diffraction from the

ISSN:1600-5767    

DOI:10.1107/S0021889887008173

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

Syncrystallization can be analysed through a new approach: the determination of the `tensor of compositional deformation'. It is a convenient way to take structural anisotropy into account and to point out the directions of strongest and weakest modifications of the intermolecular interactions due to the substitution process. The example of 2‐chloronaphthalene‐2‐methylnaphthalene alloys is given in order to illustrate the interest of this new

ISSN:1600-5767    

DOI:10.1107/S0021889887008185

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

Aspects of the optics of the energy‐dispersive scheme for X‐ray absorption spectroscopy are discussed. The idea of a set of monochromatic focus points related to a set of local Rowland circles is introduced to account for the source‐size effect on the energy resolution. It is shown that there exists an optimized location of the position‐sensitive detector where the energy resolution is no longer source‐size dependent. In addition, the stability of the dispersive optical system has been estimated and a 10 meV energy‐scale reliability is curre

ISSN:1600-5767    

DOI:10.1107/S0021889887008239

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

A technique has been developed for the simultaneous analysis of several powder diffraction data on the basis of the Rietveld method. Counting rates from one specimen at a given temperature taken at neutron, synchrotron or X‐ray powder diffractometers are joined to one single data set with weights given by the counting statistics. The structure is refined from this data set with a parameter field containing one structural model and individual zero points, scale factors and FWHM parameters for each of the methods and data sets. A new definition of the residuals is given. The residuals and goodness‐of‐fit values are calculated for all as well as for the individual data

ISSN:1600-5767    

DOI:10.1107/S0021889887008331

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

The first test of a position‐sensitive photomultiplier in a thermal neutron diffraction topography experiment is reported: a spatial resolution was obtained of ≃200 μm in FWHM over a sensitive area of size ≃8 × 8 mm (≃1600 pixels) and a detection efficiency of the order of 15% when using a 0.25 mm thick ZnS–LiF (NE 426) scintillator. The advantages of this detector, its foreseeable improvements, and the scientific problems it could help to

ISSN:1600-5767    

DOI:10.1107/S0021889887008343

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

A new version of a SANS instrument based on two bent perfect Si crystals in the nondispersive (1, −1) setting is presented. Unlike an earlier proposal which employed bent crystals in symmetric diffraction geometry [Kulda&Mikula (1983).J. Appl. Cryst.16, 498–504], use of the crystal analyser in the fully asymmetric geometry enables the angular dependence of the SANS intensity to be transformed into the positional dependence along its longest edge. This makes it possible to employ a one‐dimensional position‐sensitive detector and thus significantly increase the speed of collection of experimental data. Momentum‐transfer resolution may be easily controlled by an adjustment of the bending radii of the crystals. Use was made of diffraction by the (111) crystal planes at a neutron wavelength of 0.2 nm to measure powder samples of Si3N4, poly(vinyl chloride)/poly(methyl methacrylate) and CrO2in order to demonstrate the practical efficiency of the prop

ISSN:1600-5767    

DOI:10.1107/S0021889887008653

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

The determination of the full three‐dimensional stress tensor in a specimen by X‐ray diffraction requires that the strain be measured with the sample tilted in both a positive and a negative sense for different rotations of the sample in its own plane. The error introduced in the measured strain as a function of the position of the specimen relative to the X‐ray diffractometer axis is investigated and it is shown that the negative tilt values are more sensitive to sample position than the positive tilt values. The determination of the shear stress components uses the difference between the measured strain at equivalent positive and negative tilts and hence it is shown that displacement of the sample could lead to incorrect deductions concerning the presence and magnitude of the shear stress compo

ISSN:1600-5767    

DOI:10.1107/S0021889887008860

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

Single crystals of Ni3(Al,M) were grown by the Bridgman method, whereMis Ti, V, Cr, Mn, Fe, Nb, Mo and Ta. The composition was controlled to be about Ni75Al20M5so that the alloying element,M, substitutes mainly for Al. With these crystals conventional X‐ray structural analysis was performed. The measured static displacements of atoms from the average lattice points depended largely on the alloying elements and varied in the range 0.00–0.13 Å for Ni atoms and 0.09–0.18 Å for Al atoms. It was found that these atomic displacements correlated well with the atomic radius of the alloying element,M. For example, when the atomic radius ofMis larger than that of Al, the static displacements are large for the atoms in the Al sublattice but small for the atoms in the Ni sublattice. By contrast, when the atomic radius ofMis smaller than that of Al, the displacements are more enhanced in the Ni sublattice than in the Al sublattice. Thus, there is an interesting correlation between the atomic displacements in both the Al and Ni sublattices in the presence of alloying elements. This seems to be one of the characteristics of alloyed compounds with several

ISSN:1600-5767    

DOI:10.1107/S0021889887008975

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY

摘要:

X‐ray diffraction line profiles of plastically deformed Cu single crystals orientated for ideal multiple slip were recently found to be markedly asymmetric. A theory is developed to interpret this kind of asymmetric line broadening in terms of the average dislocation density, the dipole polarization of the dislocation structure and the mean square fluctuation of the dislocation densit

ISSN:1600-5767    

DOI:10.1107/S0021889887009178

出版商:International Union of Crystallography    

年代:1988

数据来源: WILEY