摘要:

The derivation of the conventional Lorentz factor for a powder diffraction measurement is made assuming a rectangular acceptance function for the detector aperture and by assuming that the curvature of the diffraction cone can be ignored within this aperture. Neither of these assumptions is valid for neutron diffraction measurements, particularly at low scattering angles, and corrections are therefore derived and evaluated for the case of a Gaussian acceptance function. Similar corrections are also derived and evaluated for detectors which have been displaced out of the scattering plane perpendicular to their axis of rotation.

ISSN:1600-5767    

DOI:10.1107/S0021889876010650

出版商:International Union of Crystallography    

年代:1976

数据来源: WILEY

摘要:

Amplitude and intensity distributions within the outgoing beams of the triple‐Laue‐case (LLL) interferometer have been calculated for the case of zero absorption, which in practice is predominantly encountered with thermal neutrons. The type‐1 wavefield with antinodes on the atomic sites, which with X‐rays is quite frequently anomalously attenuated in the lattice, is here fully taken into account together with the type‐2 wavefield of anomalous low absorption. After the combined diffraction by beam splitter, mirror, and analyser crystal of the interferometer for an incident plane wave has been solved, the solution for an incident spherical wave is developed by Fourier expansion, by a similar method to that first given by Kato [Acta Cryst. (1961),14, 526–532] for just one diffracting crystal plate. In order to optimize the interferometer geometry spatial intensity profiles as functions of the geometric dimensions of the interferometer and of the phase shift between the interfering beams are calculated. The influence of deviations from the ideal geometry is investigated. Deviations of the order of the extinction length can result in a drastic reduction of interference contrast. Very good energy‐converging and contrast properties are found iftM= 2tS= 2tAwheretM,tS,tAare the thicknesses of mirror, beam splitter, and analyser respectively. The calculated intensity profiles are in agreement with preliminary experimental profiles obtained recently with a silicon interferometer at the HF

ISSN:1600-5767    

DOI:10.1107/S0021889876010662

出版商:International Union of Crystallography    

年代:1976

数据来源: WILEY

摘要:

A procedure is described for the digital processing of fibre diffraction patterns to yield a quasi‐continuous map representing a central section through the cylindrically averaged intensity transform of the specimen. No assumptions are required about the nature of the specimen other than that it has fibre‐type symmetry. The specimen intensity transform can be used to obtain integrated intensities for discrete reflections from microcrystalline fibres that are potentially as accurate as those obtained from crystals. Improved procedures for extracting structure amplitudes from specimens with continuous layer lines are also descri

ISSN:1600-5767    

DOI:10.1107/S0021889876010674

出版商:International Union of Crystallography    

年代:1976

数据来源: WILEY

摘要:

The unreliability of diffuse‐reflexion recordings obtained from oblique sections of axially extended diffuse‐reflecting regions in reciprocal space is demonstrated by reference to a commonly used diffraction geometry with which theanomalous `spike' X‐ray reflexions of natural diamond are studied. A comparison between a sharp `spike' pattern (produced by a region of the crystal containing relatively large platelet precipitates on { 100}) and a more diffuse `spike' pattern (produced by a region containing smaller platelets) shows how the combination of spike diffuseness with fairly high obliquity in sectioning of the spike by the Ewald sphere produces marked discrepancy between apparent and predicted positions of the maxima of the more diffuse `sp

ISSN:1600-5767    

DOI:10.1107/S0021889876010686

出版商:International Union of Crystallography    

年代:1976

数据来源: WILEY

摘要:

The density, composition, X‐ray powder data, unit‐cell parameters, and stability fields are reported for five phases of the AgI–NH4I–H2O chemical system. The determinations were made by X‐ray powder diffraction but with some aid from polarizing optical microscopy. Provisional cell assignments are: phaseAn(4AgI.NH4I) is cubic, phaseBn(3AgI.NH4I.6H2O) monoclinic, phaseCn(AgI.2NH4I) orthorhombic, phaseEn(AgI.2NH4I.H2O) orthorhombic, and phaseFn(2AgI.NH4I.2H2O) monoclinic. Upon combustion of an acetone solution of silver iodide and ammonium iodide, phaseAnforms and, when injected into the atmosphere, transforms to phasesFnorBnas determined by the available moisture and te

ISSN:1600-5767    

DOI:10.1107/S0021889876010698

出版商:International Union of Crystallography    

年代:1976

数据来源: WILEY

摘要:

It is known that errors are introduced in an X‐ray powder diffractometer experiment due to instrument parameters such as the divergence of the incident X‐ray beam, the width of the detector slit and the spacing of the Soller slits. A method to correct for these errors is developed. This allows the use of larger slit widths and incident divergence, leading to increased detected intensities. The method will be of use in powder line profile analysis and particularly in the study of liquids and amorphous solids. The errors introduced in the measured intensity are corrected by expanding the scattered intensity as a power series in the scattering angle. The theory for the simple case of a one‐dimensional spectrum, scanned by a one‐dimensional slit, is developed. In this case, to the second order, the required correction to the measured intensity, is proportional to its second Taylor coefficient. The efficiency of the correction technique is studied through computer studies on model intensity profiles. Some results on an experimental line profile are also presented. Finally, the correction formula for the case of the general powder diffractometer geometry is

ISSN:1600-5767    

DOI:10.1107/S0021889876010704

出版商:International Union of Crystallography    

年代:1976

数据来源: WILEY

摘要:

An algorithm has been developed and implemented to determine automatically the indices and other reciprocal‐lattice information from the positions of a dozen or so arbitrary reflections on a four‐circle diffractometer. Output includes the indices, reduced cell dimensions, and the reduced cell scalars for Bravais lattice determination. Illustrations of the use of the indexing procedure both with the four‐circle diffractometer and with precession camera techniques are given. A comparison to some other autoindexing procedures is also

ISSN:1600-5767    

DOI:10.1107/S0021889876010716

出版商:International Union of Crystallography    

年代:1976

数据来源: WILEY

摘要:

An Enraf‐Nonius CAD‐4 four‐circle diffractometer with kappa geometry and an Elliott‐GX6 rotating anode generator have been linked to form a single‐crystal X‐ray data collection system which exploits the high intensity and instrument resolution available from rotating anode sources. A detailed description of this instrument and its construction is given. Experimental results illustrate the potential of the system for the rapid collection of diffraction data of high accuracy and resolution, particularly in the intended application to crystals of proteins, nucleic acids, and other macromolecules. The source can be operated at 3500 watts, and the crystal to source distance is 360 mm implying very narrow peak widths and high instrument resolution. The measurement of symmetry related reflections (Rsym= 2.2% based on intensity) indicates satisfactory overall system

ISSN:1600-5767    

DOI:10.1107/S0021889876010728

出版商:International Union of Crystallography    

年代:1976

数据来源: WILEY

摘要:

The 001 X‐ray line profile of Ni3Fe1−xAlxordered alloys is analysed by taking into account the variation of the structure factor along the profile. Between the 001 and 002 reciprocal nodes, this variation is steep enough to give a significant modification of the mean thickness of the ordered domains in the [001] direct

ISSN:1600-5767    

DOI:10.1107/S002188987601073X

出版商:International Union of Crystallography    

年代:1976

数据来源: WILEY

摘要:

Neutron Weissenberg photographs have been obtained in a time and of a quality comparable with a X‐ray photographs. The 104times lower neutron flux at the sample position can be compensated by a factor of 103in higher film speed and a larger crystal volume. The quality of the neutron photographs is shown by recording satellite reflections from labradorite crystal

ISSN:1600-5767    

DOI:10.1107/S0021889876010741

出版商:International Union of Crystallography    

年代:1976

数据来源: WILEY