摘要:

Procedures of X‐ray diffraction line profile analysis for the evaluation of the dislocation content in plastically deformed hexagonal materials were tested by means of conventional powder diffractometry on polycrystalline zirconium deformed under tension at 77 K. In order to obtain a representative picture of the dislocation‐induced X‐ray line broadening a series of reflections was measured. The integral breadths and the Fourier coefficients were evaluated by both direct profile‐shape analysis and profile fitting with analytical functions. The results show a significant anisotropy of the line broadening. The 0001reflections are clearly less broadened than most of the others. According to the theoretical calculations presented previously such a phenomenon can be expected if the plastic deformation favours generation of dislocations with Burgers vector

ISSN:1600-5767    

DOI:10.1107/S0021889889001585

出版商:International Union of Crystallography    

年代:1989

数据来源: WILEY

摘要:

Following the discovery of systems with diffraction patterns exhibiting icosahedral symmetry by Shechtman, Blecht, Gratias&Cahn [Phys. Rev. Lett.(1984),53, 1951–1953], which is forbidden in classical crystallography, interest has centred on Penrose tiling and its generalizations as a model for the explanation of these patterns, although Pauling [Nature(London) (1985),317, 512–514;Phys. Rev. Lett.(1987),58, 365–368] has proposed an alternative explanation in terms of crystal twinning. In an attempt to reconcile these two points of view a computer algorithm has been developed, following the methods outlined by Wolny, Pytlik&Lebech [J. Phys. C(1988),21, 2267–2277], to grow a number of different structures which all tile the two‐dimensional (2D) plane without defects. With the same two types of basic unit, it is shown that it is possible, on the one hand, to produce structures similar to the Penrose tiling pattern and, on the other, different types of twinned and disordered structures, with a continuous sequence of intermediates. Optical diffraction patterns of these structures have been obtained for comparison with real quasi

ISSN:1600-5767    

DOI:10.1107/S0021889889002098

出版商:International Union of Crystallography    

年代:1989

数据来源: WILEY

摘要:

The model of the instrumental function of a four‐circle X‐ray diffractometer suggested earlier [Chulichkovet al.(1987).Kristallografiya,32, 1107–1114] is complemented by the introduction of the crystal mosaicity. This improved model is used to develop a method for the reconstruction of the mosaicity function profilef(ω) from the experimental intensity profileI(ω) measured on a diffractometer. The method consists of the reduction of the experimentalI(ω) distribution to the form it would have if it were measured on a diffractometer with an instrumental function close to theδfunction. The suggested method for determiningf(ω) is tested on Si crystals with dislocation densitiesNd= 3 × 1010and

ISSN:1600-5767    

DOI:10.1107/S0021889889002591

出版商:International Union of Crystallography    

年代:1989

数据来源: WILEY

摘要:

Two time‐of‐flight powder diffractometers have operated at the Intense Pulsed Neutron Source (IPNS) since August 1981. These instruments use dedicated microcomputers to focus time‐of‐flight events so that data from different detectors can be summed into a single histogram. Thus, large multidetector arrays can be employed at any scattering angle from 12 to 157°. This design permits data to be collected over a uniquely wide range ofdspacings while maintaining high resolution and count rates. The performance of the two instruments is evaluated by analyzing data from a standard Al2O3sample by the Rietveld method. These instruments provide the capability for moderate‐ to high‐resolution measurements with the duration of a typical run being

ISSN:1600-5767    

DOI:10.1107/S002188988900289X

出版商:International Union of Crystallography    

年代:1989

数据来源: WILEY

摘要:

To measure local lattice deformation on crystal surfaces, a monolithic monochromator system for MoKα1radiation was designed which uses successive 444 and 022 reflections from an Si crystal. The angular spread of the monochromated beam is 1.7′′ and the wavelength spread (Δλ/λ) is 1.9 × 10−5. The system has been applied to precise measurement of the Bragg‐case diffraction curve from Ge and GaAs crystals with a triple‐crystal arrangement. The percentage reflection and half‐width of the diffraction curve at various points on semi‐insulating GaAs wafers were measured before and after annealing at 1123 K, with the results that annealing processes are found to be very effective for improving the qu

ISSN:1600-5767    

DOI:10.1107/S0021889889003080

出版商:International Union of Crystallography    

年代:1989

数据来源: WILEY

摘要:

General strategies are described to obtain crystals of low‐molecular‐weight compounds suitable for X‐ray structure determination. A survey is given of a variety of crystallization techniques together with their advantages and drawbacks, illustrated by experiences with notoriously crystallization‐resisting compounds. The methods discussed range from preliminary investigations using evaporation, batch crystallization and liquid–liquid diffusion methods,viathe most frequently used methods such as sitting‐drop vapor‐phase diffusion and change of temperature, to methods such as gel crystallization, sublimation and solidification. The most successful method appears to be the sitting‐drop vapor‐phase diffusion. Complete crystallization routes are described, taking into account the results of preliminary investigations. The hard‐to‐crystallize vecuronium bromide is pres

ISSN:1600-5767    

DOI:10.1107/S0021889889003894

出版商:International Union of Crystallography    

年代:1989

数据来源: WILEY

摘要:

A mechanical model is presented for the calculation of the stress depth distribution and the curvature radius in multilayer crystalline structures. The model is compatible with the perpendicular strain and thickness determined for each lamella by simulation of a double‐crystal X‐ray rocking curve, and is suitable for coherence at all the interfaces as well as for coherence loss at one interface. An example of an application to a hypothetical practical case shows that the comparison between experimental and calculated curvatures and asymmetric rocking curves allows one to determine if loss of coherence occurred at an interface as a consequence of sample processing, and to determine the amount of incoherence and the depth position of the incoherent interf

ISSN:1600-5767    

DOI:10.1107/S0021889889003900

出版商:International Union of Crystallography    

年代:1989

数据来源: WILEY

摘要:

Bovine serum albumin (BSA) was dissolved in a mixture of deuterated glycerol and heavy water. The clusters formed by the 3300 proton spins in each BSA molecule were dynamically polarized up toP= 40%. Spin‐contrast variation in small‐angle neutron scattering was studied at several target polarizations. Zero contrast, and hence minimum polarized neutron small‐angle scattering, is expected atPH= 60% from extrapolation of present data. The three basic scattering functions of spin‐contrast variation look very similar because the shape of the BSA molecule and its proton distribution are congruent. Neutron small‐angle scattering of BSA is similar to X‐ray small‐angle scattering at room temperature, indicating no deterioration of the molecular structure of BSA on s

ISSN:1600-5767    

DOI:10.1107/S0021889889004073

出版商:International Union of Crystallography    

年代:1989

数据来源: WILEY

摘要:

Methods which determine the number and disorder of lattice planes in a crystal from the Fourier cosine coefficients of the intensity profile of an X‐ray reflection use only the low harmonics and require that the coefficients be normalized so that the zero harmonic is unity. Experimentally, the profiles can only be recorded over a smaller range of scattering angle than required by the theory, and it is necessary to subtract background, which is likely to be estimated with considerable error, before determining the coefficients. It is shown that with polymer fibres this causes serious errors in the normalization, and in the values of those low harmonics used in the size and disorder determination, and prevents reliable values being obtained. Methods which avoid normalization and use only high harmonics are needed. It is shown that disorder may be obtained in such a way, but not size, for which low‐order normalized coefficients are essential. A method of extrapolation is described and tested which enables the accurate high harmonics to be used to improve the estimates of the low ones. Whilst this will yield more reliable values of crystal size than are obtainable from existing methods, the accuracy depends entirely on the validity of the extrapolation, which cannot be tested in many cases of inter

ISSN:1600-5767    

DOI:10.1107/S0021889889004085

出版商:International Union of Crystallography    

年代:1989

数据来源: WILEY

摘要:

The temperature dependence of the lattice constants of AlxGa1 −xAs/GaAs superlattices MBE‐grown on (001) oriented GaAs substrates was determined by X‐ray diffractometry. The thermal expansion coefficients of these materials become negative at low temperatures, like that of GaAs and other tetrahedrally bonded covalent solids. The temperature dependence of the stress in these structures was also studied; although its value increases as temperature decreases, strain remains elastic down t

ISSN:1600-5767    

DOI:10.1107/S0021889889004607

出版商:International Union of Crystallography    

年代:1989

数据来源: WILEY