摘要:

The elimination of the ambiguity in the assignment of a space group for the spinel structure depends upon the detection of very weak `forbidden reflexions' in the presence of substantial multiple diffraction. Attention is drawn to the importance of the latter in the case of neutron diffraction when the wavelength is suitable for revealing structural detail. Practical experience shows that single‐crystal electron diffraction has distinct advantages for the observation of the forbidden reflexions from spinels, although high‐order intensity data are essential for a proper structure‐parameter refin

ISSN:1600-5767    

DOI:10.1107/S0021889877013788

出版商:International Union of Crystallography    

年代:1977

数据来源: WILEY

摘要:

Neutron diffraction has been used to obtain complete and normalized direct pole figures of recrystallized titanium sheets; only the transmission method was used for two kinds of samples, the first spherical and the second cylindrical with diameter equal to the height. The orientation distribution function of the crystallites was computed from four measured pole figures. Results of pole figures and of distribution functions are compared with those obtained by X‐ray diffraction, by a reflection–transmission method on a thin sample, and only in reflection on a composite sam

ISSN:1600-5767    

DOI:10.1107/S002188987701379X

出版商:International Union of Crystallography    

年代:1977

数据来源: WILEY

摘要:

A development of a conic fitting procedure is described which enables the X‐ray source point in Kossel diffraction to be determined solely by analysis of shadows cast on the recording film and so dispenses, in subsequent analysis of the pattern, with the need to have prior knowledge of the crystal structure of the specimen or to have accurately manufactured cameras. The technique involves a conic fitting analysis of the shadows cast by spheres positioned between the recording film and X‐ray source. Two or more shadows provide data from which the pattern centre and specimen to film distance are evaluated, so locating the X‐ray source. The technique has been used in conjunction with new experiments in the development of suitable procedures for adaptation of Kossel diffraction for use in scanning electron microscopes. The paper includes a numerical analysis of errors involved in source location and in subsequent evaluation of lattice spacing and the technique is tested with studies on iron, copper and copper‐bismuth alloy. It is shown that the standard deviation of errors in locating the X‐ray source amounts to 1 part in 3000 and in lattice‐spacing measurement, to 1 part in 7000. The lattice parameters found for the three materials tested were for iron 2.8653±0.0005 Å, for copper 3.61715±0.0005 Å, for copper–bismuth 3.6155±0.0005 Å. The crystal volume yielding the patterns for which these data were obtained is e

ISSN:1600-5767    

DOI:10.1107/S0021889877013806

出版商:International Union of Crystallography    

年代:1977

数据来源: WILEY

摘要:

A 35‐position Philips sample changer has been added to a PDP‐8/I computer‐controlled X‐ray powder diffraction system which has operated in a single‐sample mode for the past four years [Medrud, Harter, Stephenson&Kane, (1972),Program and Abstracts Amer. Cryst. Assoc. AnnualMeeting, Albuquerque, NM, p. 58]. The system operation, instrumentation and data processing are discussed. The computer controls the diffractometer and the sample changer as well as the data acquisition and on‐line processing. Subroutines provide for sample identification, correction of 20 values from an external standard, and shutdown of the system in the event of hardware malfunction. The diffraction data can be recorded on either a teletype or magnetic tape as well as on an analog strip‐chart recorder. For ease of interpretation, each sample run is automatically started on a major grid division of the strip chart with an interrupt generated by a recorder‐mounted photoelectric device. All essential elements of conventional manual diffractometer operation were included in the system plan. Digital intensity profiles taken at fast scan rates (2 to 4 deg min−1) are more precise than the corresponding analog profiles. As a result, the precision of peak location can be increased up to

ISSN:1600-5767    

DOI:10.1107/S0021889877013818

出版商:International Union of Crystallography    

年代:1977

数据来源: WILEY

摘要:

The detailed measurement of the Bragg line profiles of several X‐ray reflections from six single‐crystal samples of LiF irradiated under different and well defined conditions in the core of a swimming‐pool type nuclear reactor are reported. The crystals were irradiated with three different spectra: (a) total neutron spectrum plus the γ background radiation, (b) same as (a) without thermal neutrons, the crystal being wrapped in a cadmium foil, and (c) the background γ radiation alone. The experimental results presented display the changes in the profiles due to the different radiation spectra used and to the change in irradiation temperature within the interval from 113 to about 173°C. That peak intensities decrease while integrated intensities increase with irradiation is a well known result amply confirmed here. Integrated intensities in these experiments increase from 4 to 14 times with respect to the unirradiated crystal. The strongest reflections (111, 200 and 220) experienced the highest increases. The influence of the temperature of irradiation has been verified through measurements of reflections 111, 200 and 220 from crystals irradiated at temperatures of 113, 148 and 173°C. As the irradiation temperature rises the peak intensities clearly tend to those of the unirradiated crystal while integrated intensities attain a maximum at about 150°C. The results also indicate that at least under the conditions prevailing in the present experiments the highest amount of damage is due to thermal neutrons (E

ISSN:1600-5767    

DOI:10.1107/S002188987701382X

出版商:International Union of Crystallography    

年代:1977

数据来源: WILEY

摘要:

Preparation procedures are examined by which the length of amosite fibres may be reduced so that preferred orientation is removed thereby providing samples suitable for quantitative X‐ray diffraction analysis. Grinding in methyl alcohol produced samples from which X‐ray diffraction gave major peak intensities reproducible to within ±2%. X‐ray analysis has been carried out on samples of either calcium silicate powder or a lagging material which consists largely of a calcium silicate matrix reinforced with rockwool fibres which contain known additions (≤ 10 wt %) of amosite. The limits of detectability of amosite in both of these matrix materials, based on three diffraction peaks 110; 060:310; 240, have been established. It is concluded that for a mixture of amosite in calcium silicate powder, the limit of detectability is 0.4± 0.1 wt % and for amosite in lagging material, the limit of detectability is 0.2± 0.1 wt %. These are discussed in terms of differences in the physical distribution of the matrix

ISSN:1600-5767    

DOI:10.1107/S0021889877013831

出版商:International Union of Crystallography    

年代:1977

数据来源: WILEY

摘要:

A study of stacking‐fault parameters, microstrains and domain size has been made using X‐ray line profile analysis on manually prepared austenitic stainless steel (18% Cr, 9% Ni and 0.07% C) filings. Studies were carried out at room temperature and on filings annealed at 180, 350 and 500°C. Results obtained through studies based on integral breadths, variance analysis and Fourier methods are comp

ISSN:1600-5767    

DOI:10.1107/S0021889877013843

出版商:International Union of Crystallography    

年代:1977

数据来源: WILEY

摘要:

Data have been collected from a standard silicon sample with CuKβ radiation selected by means of a pyrolytic graphite monochromator mounted in the scattered beam. The peak shapes can be described reasonably well by Lorentz‐type functions, but not by a Gaussian function. These results have been applied to the profile refinement of data for rhombohedral La0.75Sr0.25CrO3, a perovskite‐type compound which is slightly distorted from the ideal cubic structure. The analysis, which employs a slightly modified version of a neutron profile program [Rietveld (1969),J. Appl. Cryst.2, 65–71] indicates that the best compromise for the peak shape is a modified Lorentz function of the formA[1 +C(Δ2θ)2]−1.5fitted over a range of two halfwidths on each side of the Bragg peak position. The results are compared with a similar refinement of neutron data collected from the s

ISSN:1600-5767    

DOI:10.1107/S0021889877013855

出版商:International Union of Crystallography    

年代:1977

数据来源: WILEY

摘要:

Polystyrene latex spheres of 1.011 and 0.234 μm diameter were used to observe low‐angle X‐ray scattering. For particles of about 1 μm in diameter the central scattering has not been observed accurately with characteristic X‐rays such as CuKα and MoKα radiations, because it is strongly affected by the primary beam and the parasitic scattering of the slits. The present results show that the central scattering of particles of this size can be measured within 22′ without such influences by the use of CKα radiation (wa

ISSN:1600-5767    

DOI:10.1107/S0021889877013867

出版商:International Union of Crystallography    

年代:1977

数据来源: WILEY

摘要:

A new numerical method is presented for simultaneous smoothing, desmearing and Fourier transformation of X‐ray and neutron small‐angle scattering data. The method can only be applied to scattering curves from dilute particle systems,i.e.for scattering media whose distance distributions are zero beyond a certain value. The distance distribution of the scattering medium is approximated by a linear combination of about 20 to 30 cubicB‐splines. These spline functions have a restricted extension in real space. Their coefficients are adjusted by a weighted least‐squares operation so that the series, after being Fourier transformed and smeared according to the geometry and wavelength distribution, represents an optimum smoothed approximation of the experimental data. Tendencies towards oscillations in the least‐squares operation are suppressed by a new stabilization routine. The method offers a new possibility for the estimation of the radius of gyration, which is generally superior to the Guinier ap

ISSN:1600-5767    

DOI:10.1107/S0021889877013879

出版商:International Union of Crystallography    

年代:1977

数据来源: WILEY