摘要:

The equations describing X‐ray diffuse scattering in crystals with the pseudobinary sphaleriteF3mstructure (e.g.Hg1 −xCdxTe and other pseudobinary semiconductors) are presented accurately up to the second order in local atomic displacements. This treatment considers the local properties from the noncentrosymmetric structure as well as complex scattering factors. The result contains additional terms related to the noncentrosymmetric crystal properties and the imaginary parts of the atomic scattering factors. Expressions are also presented that are applicable to an experiment involving difference anomalous dispersion techniques. Equations relating the various terms at symmetry‐related points in reciprocal space are included as well as the least‐squares method required to solve the matrix equations from diffraction data taken in a volume of reciproca

ISSN:1600-5767    

DOI:10.1107/S0021889893000299

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

Indirect Fourier transformation is a widely used technique for the desmearing of instrumental broadening effects, for data smoothing and for Fourier transformation of small‐angle scattering data. This technique, however, can only be applied to scattering curves with a band‐limited Fourier transform,i.e.separated and noninteracting scattering centers. It can therefore not be used for scattering data from partially ordered systems. In this paper, a modified technique for partially ordered systems working in reciprocal space is presented. A peak‐recognition technique allows its application to scattering functions with narrow peaks, such as the scattering functions of layered systems like lamellar stacks or strongly interacting particles. Arbitrary geometry effects and wavelength effects can be corrected. Examples of simulations show the merits and limits of this new method. One example shows its applicability to real

ISSN:1600-5767    

DOI:10.1107/S0021889893000561

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

In the present paper, it is shown that the fractal notion can be used in neutron optics. More specifically, self‐similar neutron multilayer mirrors can be made according to the pattern of the Cantor ternary set. These new multilayers have a fractional Hausdorff dimension. The self‐similarity gives them new reflectivity properties. Compared with periodic neutron multilayers and supermirrors, these fractal multilayers, with a large number of generations, allow high reflectivity of the order of unity at very short neutron waveleng

ISSN:1600-5767    

DOI:10.1107/S0021889893000755

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

In powder diffraction measurements, the errors in integrated intensities caused by preferred orientation can be corrected by using some suitable analytical model as the representation of the orientation distribution of the crystallites. In this paper, a model based on symmetrized harmonics expansion is described in detail. The harmonic functions in different crystal symmetries are given and the influences of various diffraction geometries are examined. Applications of the method in quantitative phase analysis and particularly in the Rietveld method are described. The method is clarified using corundum as an example.

ISSN:1600-5767    

DOI:10.1107/S0021889893001219

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

The structures of superlattices on misoriented (001) substrates are described by means of two modified models. From both models, it can be concluded that there is an intensity distribution rod in the reciprocal space perpendicular to the surface and the symmetry of the diffraction pattern is reduced from tetragonal to monoclinic. However, in the first model the diffraction pattern near the substrate reflections and in the second model that near the so‐called `average lattice' reflections remains tetragonal. Neglection of both of these facts can cause systematic errors in the determination of the lattice parameters of superlattices. These errors are estimated. Additionally, 60° misfit dislocations are discussed as a reason for the tilt of the layer system with respect to the substrate. The validity of the theoretical predictions was proved by X‐ray diffraction studies on symmetric and asymmetric reflections on three GaAs/Ga1 −xInxAs superlattices grown on (001) GaAs. The first sample has a large misorientation and the diffraction behaviour corresponds to the above‐mentioned models. The second sample shows a diffraction pattern that can be explained by a tilt of the layer system in relation to the substrate. The third sample has a large misorientation and a large tilt of the layer system and so its diffraction pattern shows an overlap of both

ISSN:1600-5767    

DOI:10.1107/S0021889893000305

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

Single‐ and double‐crystal spectrometer measurements of the CuKαdoublet have been carried out after the selection of optimum measurement procedures and parameters. These include the influences of the X‐ray‐tube arrangement, horizontal and vertical divergences and the slowθorλdependence of the reflection curve. The influence of the apparatus function on the measured intensity distribution was minimized. The measured intensity profile could be approximated by a convolution of the instrumental function and the spectral line, with the inclusion of two additional terms. The intrinsic spectral line was fitted by Lorentz functions and, thus, described by 12 parameters. Good agreement between the results of the different measuring procedures and those of other authors is obtained. The apparent differences are

ISSN:1600-5767    

DOI:10.1107/S0021889893000160

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

The refractive indices and the optical activity along and perpendicular to the optic axis have been determined in the room‐temperature phase of KLiSO4. The values of all these quantities have been successfully connected with the crystal structure by using a classical point‐dipole polarizability theory. This permits determination of the absolute optical chirality of the material,i.e.provides a link between the absolute configuration and the sign of the optical rotation. The polarizabilities of the different atoms have been determined. The conclusion is reached that the main contributors to the optical activity are the O atoms, in particular those occupying general positi

ISSN:1600-5767    

DOI:10.1107/S002188989201286X

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

The absolute optical chirality of potassium lithium sulfate (KLiSO4) at room temperature has been determined by anomalous X‐ray dispersion combined with observation of the rotation of plane‐polarized light. The crystal used for this study was dextrorotatory along [00.1] and was carefully checked for the possibility of twinning; it was found to be single domain to within the limit of error. Finally, it was established that this crystal has the same structural chirality as that of the crystal whose structure was published by Schulz, Zucker&Frech [Acta Cryst.(1985). B41, 21

ISSN:1600-5767    

DOI:10.1107/S0021889893001128

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

An automated system for protein crystallization that is based on the vapor diffusion method has been developed using novel sitting‐drop supports for standard Linbro plates. Laboratory robotics have been designed for liquid handling of stock solutions, silicone‐grease dispensing and coverslip transfer. The system can handle up to four crystallization plates (96 wells) and ten stock solutions, without any user attendance. The software allows specification of several experimental parameters and records experiments, which may be accessed by the use of a bar‐code label. Experiments may be conducted according to a user's specific design, a screening test or an incomplete factorial design. The system is capable of pipetting viscous solutions such as polyethylene glycol with good reproducibility. Automatic well‐sealing techniques are discussed and preliminary protein‐crystallization results are

ISSN:1600-5767    

DOI:10.1107/S0021889893001633

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

The INEL diffractometer, equipped with a CPS120 curved detector and set up in a Debye‐Scherrer geometry, is a unique tool for carrying out powder diffraction studies on air‐sensitive and/or small‐volume samples. Although it has routinely been used in powder diffractometry because of its minute acquisition times, its accuracy ind‐spacing and intensity measurements has not been clearly demonstrated before now. Concerning thedspacings, proper linearization of the CPS120 with a cubic Na2Ca3Al2F14standard allowed a meanδ2θdifference of 0.006°. Intensity accuracy was measured with different highly and poorly absorbing samples. The accuracy is fairly good for the latter but poor for the former, except when special procedures such as the dilution of the sample with boron powder are used. A Rietveld calculation carried out on TI4V207showed a very good agreement between the INEL Debye‐Scherrer‐geometry results and those obtained with a Philips diffractometer and Bragg‐B

ISSN:1600-5767    

DOI:10.1107/S0021889893001670

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY