摘要:

Integrated X‐ray intensities of Bragg peaks have been measured with an NbC0.72single‐crystal which contains a large number of non‐stoichiometric carbon vacancies. By standard structural analysis the mean‐square displacements were determined to be (u2Nb) = 0.0041 (7) Å2for Nb atoms and (u2C) = 0.0111 (20) Å2for C atoms. This (u2Nb) is comparable to the value of 0.0045 Å2, extrapolated from the compositional dependence of (u2Nb) proposed by Metzger, Peisl&Kaufmann [J. Phys. F.(1983).13, 1103–1113]. It was confirmed that the existence of carbon vacancies induces larger static displacements of neighbouring niobium atoms, and the contribution of thermal displacements to the observed (u2Nb) appears to be rather small in a non‐stoichiome

ISSN:1600-5767    

DOI:10.1107/S002188988608901X

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

An apparatus has been developed which permits rapid collection of X‐ray powder diffraction data for structure refinement by pattern analysis. The apparatus is built on a cylindrical position‐sensitive detector with an angular aperture of 120° and resolution of 0.05° in 2θ. The detector uses a metal blade as anode and works in the self‐quenching streamer mode of gas ionization. A wide‐angle powder pattern can be recorded simultaneously from a stationary specimen, where geometrical broadening of reflection lines is suppressed by limiting the angular width of the incident beam. A test made with standard corundum powder shows that a diffraction pattern with a substantially better resolution is obtainable in one‐eightieth the time compared with a normalθ–2θdiffractometer. The usefulness of the apparatus in high‐temperature work has been demonstrated in profile refinement of structural parameters for theδphase of bism

ISSN:1600-5767    

DOI:10.1107/S0021889886089008

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

A small‐angle neutron scattering spectrometer, suitable for the study of structural and magnetic inhomogeneities in materials in the 10 to 1000 Å range, has been constructed at the National Bureau of Standards Research Reactor. The instrument is 8 m long and utilizes a mechanical velocity selector and pinhole collimation to provide a continuous incident beam whose wavelength is variable from 5 to 10 Å. The neutron detector is a 65 × 65 cm position‐sensitive proportional counter which pivots about the sample position to extend the angular range of the spectrometer. Features unique to this instrument include a multibeam converging collimation system for high‐resolution measurements and an interactive color graphics terminal with specialized software for the rapid imaging and analysis of data from the two‐dimensional detector. The design and characteristics of the spectrometer and data acquisition system are described in detail and examples of data are presented which illustra

ISSN:1600-5767    

DOI:10.1107/S0021889886088994

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

The procedure of whole‐powder‐pattern fitting without reference to a structural model, first proposed by Pawley for use with neutron powder data [J. Appl. Cryst.(1981).14, 357–361], was extended as a tool for analyzing X‐ray whole‐powder data. X‐ray data forα‐SiO2,α‐Al2O3and CaCO3used as test samples were first analyzed by individual profile‐fitting techniques to establish the angle‐dependent profile‐shape model. Then the results were incorporated into the profile model of whole‐powder‐pattern fitting, where peak maximum positions of individual reflections are a function of unit‐cell parameters while integrated intensities are independent variables. Whole‐powder‐pattern fits for the test samples showed that the correction for peak shift has a large influence on the overall fit of the pattern and the accuracy of refined parameters. It is also important to take into account the asymmetry and the angular dependence of the intensity decay rate in modelling the diffraction profile. The method developed was applied to an analysis of the powder pattern of a

ISSN:1600-5767    

DOI:10.1107/S0021889886088982

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

A multiwavelength X‐ray diffraction study was performed at 6 Å resolution on the native cytochromec′ crystal fromRhodospirillum rubrum, which contains heme irons. The X‐ray intensity data from the crystal were collected on a four‐circle diffractometer with three wavelengths,λ1= 1.077,λ2= 1.730 andλ3= 1.757 Å, using the synchrotron radiation produced by the storage ring in the Photon Factory, National Laboratory for High Energy Physics.λ2andλ3are above and below the energy of theKabsorption edge of iron (λe= 1.743 Å) respectively, whileλ1is far from the edge. The positions of the two iron atoms in the crystal could be determined from the difference Patterson maps based on either the real or imaginary components of the anomalous scattering effect caused by iron. The best phase angles for the crystal were calculated from the X‐ray intensity data measured withλ1andλ2by the method which is basically the same as the isomorphous replacement method. Although the resulting best phase angles differed 75° on the average from those obtained by the multiple isomorphous replacement method, the molecular boundary andαhelices could be recognize

ISSN:1600-5767    

DOI:10.1107/S0021889886088970

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

The divergence and intensity distribution of X‐rays transported through a conical‐type X‐ray guide tube (XGT) were measured. Diverging X‐rays from a point source were condensed by use of the conical‐type XGT. The parallelism of X‐rays through the XGT was better than that encountered for a cylindrical‐type XGT proposed previously [Nakazawa (1983).J. Appl. Cryst.16, 239–241]. The intensity of the X‐ray beam around the central axis of the conical‐type XGT was exceedingly high in comparison with that measured without the tube and almost uniform over a cross‐sectional area 60 μm in diameter. The high intensity provides the possibility of performing diffraction experiments with crystals as small as 20 μm in diameter with a conventional X

ISSN:1600-5767    

DOI:10.1107/S0021889886088969

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

A unit for holding and centring a closed‐cycle refrigerator on an Eulerian cradle is described. TheX, Ytranslations orthogonal to theϕrotation are obtained by tilting motions, and blocked by a constant springload. Locking is therefore not necessary, and this facilitates the centring of the crystal. In addition, the unit is very easy and cheap to fabricate. The cryostat is in routine operation for neutron diffraction work, and a similar design should be applicable in X‐ray diffractom

ISSN:1600-5767    

DOI:10.1107/S0021889886088957

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

An evaluation of the concept of a line profile is provoked by truncation of the range of intensity measurement in practice. The measured truncated line profile can be considered either as part of the total intensity distribution which peaks at or near the reciprocal‐lattice points (approach 1), or as part of a component line profile which is confined to a single reciprocal‐lattice point (approach 2). Some false conceptions in line‐profile analysis can then be avoided and recipes can be developed for the extrapolation of the tails of the truncated line profile. Fourier analysis of line profiles, according to the first approach, implies a Fourier series development of the total intensity distribution defined within [l−½,l+ ½] (lindicates the node considered in reciprocal space); the second approach implies a Fourier transformation of the component line profile defined within [−∞, + ∞]. Exact descriptions of size broadening are provided by both approaches, whereas combined size and strain broadening can only be evaluated adequately within the first approach. Straightforward methods are given for obtaining truncation‐corrected values for the average

ISSN:1600-5767    

DOI:10.1107/S0021889886088945

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

At the moment, routine texture analysis is limited by the time required for data recording. A method allowing simultaneous acquisition of several pole figures with the use of several detectors or a position‐sensitive detector is described. Because of the simultaneity, the movements of the texture goniometer used for one pole figure are also used at the same time to obtain the other pole figures. The complete geometric arrangement and the scanning of pole figures are explained for reflection and transmission measurements. The blind area, the absorption intensity correction and the defocusing phenomenon are discussed with respect to the sample. For some examples, the results obtained by this method are compared with those obtained by the usual method employed for the measurement of a single pole figur

ISSN:1600-5767    

DOI:10.1107/S0021889886088933

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

The influence on the diffraction pattern of the angle α between the crystal surface and the major set of planes used in the diffraction has been studied. A triple‐crystal diffractometer in the (+ 1, − 1, + 1) nondispersive mode has been used to measure the diffraction for several values of α. The experimental results are in agreement with dynamical t

ISSN:1600-5767    

DOI:10.1107/S0021889886088921

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY