摘要:

The average number of reflections for radiation transmitted by a straight cylindrical channel can be determined analytically for an input beam for which the divergence has polar symmetry. For the case of an incident beam with a divergence of Cartesian symmetry (for example, the output from a rectangular neutron guide), the number of reflections can be calculated only if the maximum divergence is less than the critical angle of the cylindrical channel. For larger divergences, the requirement that the glacing angle is less than the critical angle does not allow a simple analytic solution. This paper reports the calculation of the number of reflections using an approximation and makes comparisons with a numerical simulation that gives an estimate of the exact result.

ISSN:1600-5767    

DOI:10.1107/S0021889894008812

出版商:International Union of Crystallography    

年代:1995

数据来源: WILEY

摘要:

The absolute optical chirality of sodium ammonium (+)‐tartrate tetrahydrate crystals has been determined. The wavelength dispersion of the linear birefringence and the optical gyration tensor components were found at room temperature with the use of a high‐accuracy universal polarimeter. The wavelength dispersion of the gyration is presented over the range 350–650 nm. The gyration coefficients at a wavelength of 500 nm were found to beg11= 12.5 × 10−6,g22 = −36.0 × 10−6and g33= 6.7 × 10−6. The results of the present work are in good agreement with values inferred by Koralewski&Szafrański [Ferroelectri

ISSN:1600-5767    

DOI:10.1107/S0021889894009222

出版商:International Union of Crystallography    

年代:1995

数据来源: WILEY

摘要:

A new method of area‐detector peak integration based on the statistical analysis of pixel intensities is described. The presence of the peak in the integration box causes the skewness of the pixel intensity distribution to increase compared to the skewness of the noise distribution. After the pixel intensities have been smoothed, the peak mask, assumed to be contiguous, grows from a `seed' until the skewness of the pixel intensity distribution outside the seed area reaches a minimum. The new method has been successfully applied to the processing of imaging‐plate data from a buckminsterfullerene derivative and from sodium nitroprusside collected by the oscillation method at a synchrotron source. In both cases, the seed‐skewness procedure proved to be superior to the standard‐box and dynamic mask methods, especially for weaker reflections. Similar results were obtained with rotating‐a

ISSN:1600-5767    

DOI:10.1107/S0021889894009696

出版商:International Union of Crystallography    

年代:1995

数据来源: WILEY

摘要:

Orthorhombic (O1,O2andO3) and monoclinic (M1andM2) phases are the crystalline approximants of the Al–Cu–Fe–Cr decagonal quasicrystal. The structures of these crystalline phases can be described using three subunits, which are derived from theO3structure. As an example, the structural model of theO1phase is presented in detail. This model agrees with the X‐ray powder diffraction pattern and the high‐resolution electron image of t

ISSN:1600-5767    

DOI:10.1107/S0021889894009714

出版商:International Union of Crystallography    

年代:1995

数据来源: WILEY

摘要:

A numerical calculation method for determining the resolution functions for radially symmetric collimation and scattering is described. In the present approach, the total number of integrations is reduced to four by use of the radial symmetry of the geometry. Furthermore, the beam‐stop shadowing effect is included exactly. A typical calculation can be completed in minutes on current personal computers. An interactive computer program allows the user to enter the experimental parameters such as aperture size and wavelength spread, allowing smearing calculations to be handled routinely as a `black box' operation. The exact smearing treatment is compared both to an improved technique involving Gaussian resolution functions where corrections for the beam stop are included and to results from Monte Carlo simulations. In most experiments, the use of a Gaussian distribution to approximate the resolution is preferred on account of its ease of calculation. But in a few cases, such as Porod scattering, the present more extensive numerical calculation or inclusion of the developed beam‐stop‐shadowing correction factors into a Gaussian scheme are needed for an adequate prediction of the smearing ef

ISSN:1600-5767    

DOI:10.1107/S0021889894010095

出版商:International Union of Crystallography    

年代:1995

数据来源: WILEY

摘要:

On the basis of known equations for calculating X‐ray diffraction intensities from a given number of unit cells of a crystal phase in polycrystalline material, as due to: (i) Bragg reflections; (ii) average diffuse scattering caused by thermal plus first‐kind disorder; and (iii) incoherent scattering, a relationship has been found that ties, in the Rietveld analysis, the Bragg scale factor to a scale factor for `disorder' as well as incoherent scattering. Instead of fitting the background with a polynomial function, it becomes possible to describe the background by physically based equations. Air scattering is included in the background simulation. By this means, the refinement can be carried out with fewer parameters (six fewer than when a fifth‐order polynomial is used). TheDBWS‐9006PCcomputer program written by Sakthivel&Young [(1990), Georgia Institute of Technology, Atlanta, GA, USA] has been modified to follow this approach and it has been used to refine the crystal structures of the cubic form of Y2O3and ofα‐Al2O3. Peak asymmetry has been described by a function based on an exponential approximation. The results from refinements using polynomial physically based background function are, in terms of final structural parameters and reliability indices, very close to each other and in agreement with results reported in the literature. The reconstruction and optimization of the background scattering by means of physically based equations helps the implementation in the Rietveld code of other possible specific diffuse scattering contributions, such as that due to an amorp

ISSN:1600-5767    

DOI:10.1107/S002188989401037X

出版商:International Union of Crystallography    

年代:1995

数据来源: WILEY

摘要:

Rietveld analysis has enabled a determination, by an internal approach, of the degree of crystallinity of a sample of polyethylene terephthalate (PET) and three samples of zirconium oxide obtained by precipitation from ZrOCl2followed by crystallization at a different temperature. After subtraction of the air scattering, a global diffuse scattering produced by the amorphous phase, by the thermal and lattice disorder and, lastly, by the incoherent or Compton scattering, was built up in the frame of theDBWS‐9006PCcomputer program written by Sakthivel&Young [(1990), Georgia Institute of Technology, Atlanta, GA, USA], modified as described by Riello, Fagherazzi, Clemente&Canton [J. Appl. Cryst.(1995),28, 115–120]. All these contributions (and the corresponding scale factors) were simultaneously optimized, together with the crystalline peak profiles. On the basis of the modified Rietveld analysis, an equation has been suggested that calculates the degree of crystallinity. The value found in the examined PET sample is very close to that already calculated [Polizzi, Fagherazzi, Benedetti, Battagliarin&Asano (1990).J. Appl. Cryst.23, 359–365] with a nonstructural fitting proc

ISSN:1600-5767    

DOI:10.1107/S0021889894010381

出版商:International Union of Crystallography    

年代:1995

数据来源: WILEY

摘要:

A preferred‐orientation analysis for powder diffraction data is proposed, based upon an extension of the Wilson‐plot intensity analysis, in order to correct the integrated intensities before the structure determination is started. The technique is based on an empirical exponential model for preferred orientation proposed earlier in the literature, of which a formal derivation is now given. Prerequisites are disc‐shaped behaviour of the crystallites and spinning of the specimen. Furthermore, the indexed powder pattern and the unit‐cell contents are required, but neither a structural model nor pole‐figure observations are necessary. The influence of preferred orientation on the statistics of the normalized structure factor are assessed. It is shown that preferred orientation diminishes the quality of the triplet‐phase‐sum statistics. Improvements of the normalization procedure for powder diffraction intensities are proposed. Finally, it is shown by means of simulated powder data under which conditions the (most likely) preferred‐orientation direction and corresponding strength can be established. Test results indicate that a reconstruction of the preferred‐orientation model imposed is feasible provided the maximum individual intensity enhancement (or decrease) as a result of imposed preferred orientation is at least 70%, when compared with data in which preferred orie

ISSN:1600-5767    

DOI:10.1107/S0021889894010629

出版商:International Union of Crystallography    

年代:1995

数据来源: WILEY

摘要:

This paper reports synchrotron X‐ray powder diffraction and high‐resolution electron microscopy studies of the structural arrangement in γ‐Fe2O3. Powder diffraction data as well as high‐resolution electron microscopy show that the basic structure of γ‐ferric oxide possesses cubic symmetry (space groupP4332), whereas the ordered distribution of the cation vacancies on the octahedral positions results in the formation of the tetragonal superlattice (space groupP41212). The character of the superstructure orientation with respect to the developed planes of the γ‐Fe2O3microcrystals obtained by different preparative techniques is

ISSN:1600-5767    

DOI:10.1107/S0021889894010113

出版商:International Union of Crystallography    

年代:1995

数据来源: WILEY

摘要:

A method of X‐ray line‐broadening analysis is presented whereby the coherence length and microstrain contributions can be calculated using a Monte Carlo interference‐function‐fitting algorithm. The method is based on the `column‐like' crystal model and can be applied to both single and multiple‐order reflections. Examples on simulated diffraction peaks, deformed face‐centred‐cubic palladium powder and heavily textured deformed body‐centred‐cubic low‐carbon‐steel sheets are presented and comparisons are made with cu

ISSN:1600-5767    

DOI:10.1107/S0021889894009027

出版商:International Union of Crystallography    

年代:1995

数据来源: WILEY