摘要:

LaGaO3single crystals, grown by the method of zone melting without a crucible, with radiation heating, were investigated by X‐ray topography and diffractometry. It has been found that at 293 K these crystals have an orthorhombic unit cell witha= 5.520 (2),b= 5.490 (2),c= 7.770 (1) Å and twinning in the {110}/ 〈10〉 and {112}/〈10〉 systems simultaneously. Regions of cross‐twinning have been discovered in these systems. A first‐order phase transition from orthorhombic to rhombohedral has been observed at 412.5 K, the cell parameters beinga= 3.889 Å,α= 89.50° (423 K). The rhombohedral phase has a developed twin structure in the {100}/〈001〉 and {110}/〈001〉 systems. During transformation the structure undergoes an inherited change to the

ISSN:1600-5767    

DOI:10.1107/S0021889892008744

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

Methods of minimizing the uncertainties in temperature and position that are inherent in high‐temperature X‐ray powder diffraction measurements made using strip heaters are discussed, with particular reference to the Anton Paar HTK 10 heating stage. Modification of the strip geometry is recommended to largely eliminate temperature gradients across the sample area and the use of the strip heater as an internal standard is recommended to eliminate positional uncertainties and test temperature calibration. Data on the thermal expansion of Al2O3are used to evaluate the success of these procedu

ISSN:1600-5767    

DOI:10.1107/S002188989200877X

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

The processing and analysis of fibre diffraction data is often cumbersome. Presented here is a compact interactive computer program,PROFIDA(processing of fibre diffraction data), for fast processing of fibre diffraction data from biological molecules without many of the difficulties. Interactive rotation and centring of the diffraction pattern allow four‐quadrant averaging and background subtraction. Furthermore, film coordinates may be transformed into reciprocal‐space coordinates for any orientation of the specimen. Specimen tilt towards the incident‐beam direction may be determined and the appropriate correction made. In addition, a routine that scales and merges data sets from a film pack is available. Data analysis is supported by an integration routine that allows layer‐line profiles to be examined and integrated and layer‐line plots to be produced under control of the cursor. The program is written inCto run on the Silicon Graphics Iris workstation with a button‐and‐dial box using the Iris gra

ISSN:1600-5767    

DOI:10.1107/S0021889892008781

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

The PIPEX system, an inexpensive pipetting station for the preparation of protein crystallization buffers, is described. The system, built around a computer‐controlled motorized pipette, contains a spreadsheet application for the planning of experiments in, for example, standard Linbro plates, a program for the control of the pipette and a customized electronic interface. The PIPEX system performs most of the software tasks offered by current automatic crystallization system

ISSN:1600-5767    

DOI:10.1107/S0021889892008793

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

With the assumption that both size‐ and strain‐broadened profiles of the pure‐specimen function are described with a Voigt function, it is shown that the analysis of Fourier coefficients leads to the Warren–Averbach method of separation of size and strain contributions. The analysis of size coefficients shows that the `hook' effect occurs when the Cauchy content of the size‐broadened profile is underestimated. The ratio of volume‐weighted and surface‐weighted domain sizes can change from ~1.31, for the minimum allowed Cauchy content, to 2, when the size‐broadened profile is given solely by a Cauchy function. If the distortion Subscripts coefficient is approximated by a harmonic term, mean‐square strains decrease linearly with increasing the averaging distance. The local strain is finite only in the case of purely Gaussian strain broadening, because strains are then independent of a

ISSN:1600-5767    

DOI:10.1107/S0021889892008987

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

The one‐dimensional contrast modulation along thebaxis of [001] high‐resolution reverse images of the compounds Bi2Sr2(Ca1 −xNdx)Cu2O8 + δ(x= 0.05 + 0.1n;n= 1, 2, 6 and 7) is examined closely using the density distribution recorded from a wide area of the negative film by microphotometric densitometry. Three typical short units of the density distribution, of lengths 4.5b0, 5b0and 4b0, characterized by twin peaks or a single maximum peak with subpeaks in the middle of the unit, are discerned. The density distributions of the three units and the contrast modulations of their images are reproduced well by computer simulations, using three structure models modulated with longitudinal displacement waves along thebaxis of the metal atoms. The one‐dimensional contrast modulation is attributed to sinusoidal changes in the effective scattering amplitudes for the [001] electron beam of the metal‐atom chains along thecaxis. The results of the simulation are applied to investigation of the (2, 1)5and (3, 1)4modulation modes in the modulated

ISSN:1600-5767    

DOI:10.1107/S0021889892009142

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

By the application of Rodrigues parameters, crystal orientations are represented as points on the unit semi‐hypersphereS4+⊃4or equivalently on the projective hyperplaneH3⊃4. For the statistical analysis of orientation data, probability models onS4+≡H3are required. Among different hyperspherical analogs of the normal distribution in Euclidean space, corresponding to various of its characterizations, the Bingham model distribution is characterized as the hyperspherical analog for statistical purposes. Maximum‐likelihood estimates of its parameter matrices and single‐sample significance tests of uniformity and rotational symmetry are presented. In terms of probability theory and stochastic processes, the hyperspherical Brownian‐diffusion distribution is the analog of the normal distribution in Euclidean space. An orientation distribution derived by Savelova [Ind. Lab.(USSR), (1984),50, 468–474] and erroneously related to a central‐limit‐type theorem for rotations by Matthies, Muller&Vinel [Textures Microstruct.(1988),10, 77–96], with the Brownian‐diffusion distribution onS4+≡H3, recalls the fact that no simple analog of the central limit theorem in Euclidean space exists for arbitrary spaces and manifolds,e.g.forSP,SP+,SO(p),p≥ 2. Therefore, interpretations of preferred orientations in terms of mechanisms of texture development related to an imaginary simple analog of the central limit theorem would generally be misleading. Individual crystal‐orientation measurements may initially require different nonparametric methods of analysis. As a counterpart of pole‐to‐orientation density inversion of diffraction data, hyperspherical kernel orientation density estimation is suggested for individual orientation measurements. With the application of non‐negative kernels, the estimated orientation density is always non‐negative. The critical smoothing parameter of this method is determined on information‐theoretical grounds. For patterns of preferred orientation too complex to be sufficiently approximated by the parametric Bingham model distribution, a clustering of individual orientation measurements into disjunct classes is suggested that relates similarity of orientations to multimodality of the estimated orientation density; each class of orientations may then be indi

ISSN:1600-5767    

DOI:10.1107/S0021889892009270

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

(011) twin boundaries ofY‐cut synthetically grown quartz crystals are studied by means of X‐ray section topographs. The images show an anomalous contrast and no extinction condition can be found. Simulated images demonstrate that the twin boundaries are not simple planar defects. They may be described as lamellae of a non‐diffracting material. Their thickness is of the order of 15 μm and, within these boundaries, the Fourier components of the dielectric susceptibility are drastically reduced by one order of m

ISSN:1600-5767    

DOI:10.1107/S0021889892009415

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

Theoretical calculation shows that suitable approximations of the line‐shape asymmetry in powder diffraction profiles should be represented by functions having limited codomain

ISSN:1600-5767    

DOI:10.1107/S0021889892009725

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

The feasibility of using soft X‐rays from the Aladdin storage ring for small‐angle X‐ray scattering (SAXS) experiments was demonstrated through the study of the diffraction patterns of polyethylene, paraffin and polystyrene latex spheres using monochromated radiation in the range 4.0

ISSN:1600-5767    

DOI:10.1107/S0021889892008732

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY