摘要:

A computer‐oriented procedure for the generation of the symmetry of the multiple implication function has been developed. The symmetry of the multiple implication function, referred to its own conventional unit cell, is tabulated for all space group

ISSN:1600-5767    

DOI:10.1107/S0021889890000322

出版商:International Union of Crystallography    

年代:1990

数据来源: WILEY

摘要:

Synchrotron radiation has been used to measure the rocking curves from a (GaIn)(AsP) single layer grown on an InP (100)‐oriented substrate. For angles of incidenceΦB= (ΘB−ϕ) ≤ 0.7° measured at the Bragg angleΘB(ϕis the angle between the diffracting lattice plane and the surface), the rocking curve (RC) is very strongly influenced by total external reflection (TER). This causes a decrease in the full width at half‐maximum (FWHM) and an asymmetrical shape for the RC for smallΦB. Both of these effects are due to the Bragg‐angle shift from the actual incident angleΦwhich is not considered in the conventional dynamical theory. In this paper the essential influence of TER on any thin‐layer rocking curve is investigated using a numerical solution of the extended dynamical theory. The pattern can be interpreted up toΦB≥ΘC(ΘCis the angle of TER) by semiempirical incorporation of the results of the extended theory with the coupling formalism of Barrels, Hornstra&Lobeek [Acta Cry

ISSN:1600-5767    

DOI:10.1107/S0021889890001704

出版商:International Union of Crystallography    

年代:1990

数据来源: WILEY

摘要:

The reproducibility of data collected on a Seimens/ Nicolet area detector has been tested with respect to crystal orientation, X‐ray collimation and scaling procedure. In addition, a comparison between area detector data and diffractometer data was made in order to determine the accuracy of the area detector data. The reproducibility of the area detector data, although acceptable, is somewhat affected by crystal orientation (shape effect) and appears to be insensitive to X‐ray collimation, contrary to what would be expected. The scaling procedures used in the tests wereXENGEN[Howard, Gilliland, Finzel, Poulos, Ohlendorf&Salemme (1987).J. Appl. Cryst.20, 383–387] (one, two and three parameters),FBSCALE[Weissman (1982). InComputational Crystallography, edited by D. Sayre, pp. 56–63. Oxford: Clarendon Press] andSIMPLAD[Takusagawa (1987).J. Appl. Cryst.20, 243–245]. It was found that, although these procedures do an adequate job of scaling area detector data, none can totally correct for crystal‐shape effects. From our experience,XENGENmulti‐parameter orSIMPLADscaling on data sets having low redundancy must be used with care. When compared to diffractometer data, area detector data scaled by any of the above procedures is in good agreement, with mergingR‐factors (onF) in the range 3.45 to 4.55%. A plot of 〈Fad〉versus〈Fd〉 shows a straight line with slope of 1(3)%. No saturation

ISSN:1600-5767    

DOI:10.1107/S0021889890001881

出版商:International Union of Crystallography    

年代:1990

数据来源: WILEY

摘要:

A series of experiments have been undertaken on the Oak Ridge National Laboratory (ORNL) 10 m small‐angle X‐ray scattering (SAXS) camera to provide quantitative data on the level of background (parasitic) scattering generated by different types of bevelled collimating slits. The addition of a third (guard) slit, positioned close to the sample, resulted in a reduction of over an order of magnitude in the parasitic background generated by the best two‐slit combination of collimating slits. This made it possible to reduce the size of the beamstop, permitting useful data to be collected down to a value of the scattering vectorQ= 4πλ−1sinθ≃ 3 × 10−3Å−1, whereλis the wavelength, and 2θis the angle of scatter. This permits the resolution of distan

ISSN:1600-5767    

DOI:10.1107/S0021889890001984

出版商:International Union of Crystallography    

年代:1990

数据来源: WILEY

摘要:

A new procedure for simultaneous refinement of structural and microstructural disorder parameters for polycrystalline materials is proposed. It is based on the Rietveld method combined with Fourier analysis for broadened peaks. Crystallite size and shape and r.m.s. microstrain are regarded as fitting parameters, replacing the well known formula of Caglioti, Paoletti&Ricci [Nucl. Instrum. Methods(1958),3, 223–228] for the angular dependence of the peak width. In particular, from these microstructural disorder parameters, by inverting the Warren–Averbach procedure [Warren&Averbach (1950).J. Appl. Phys.21, 595–599; (1952),23, 1059] for a single peak, it is possible to obtain the parameters of the pseudo‐Voigt (pV) functions employed to fit the experimental data. The anisotropy of the crystallite size and microstrain is also taken into account. The method has been tested on three materials with different degrees of crystallization: tetragonal ZrO2(P42/nmc,a= 3.5961,c= 5.1770 Å,Vc= 66.95 Å3,Z= 2,Rwp= 0.077,M≃100 Å, 〈ɛ2〉1/2≃ 3 × 10−3); tetragonal Zr0.82Ce0.18O2(P42/nmc,a= 3.6419,c= 5.2440 Å,Vc= 69.556 Å3,Z= 2,Rwp= 0.0654,M≃1000 Å, 〈ɛ2〉1/2≃8 × 10−4);α‐Al2O3(Rc,a= 4.7605,c= 12.9956 Å,Vc=

ISSN:1600-5767    

DOI:10.1107/S0021889890002382

出版商:International Union of Crystallography    

年代:1990

数据来源: WILEY

摘要:

A method is presented which indexes spots recorded on single oscillation images without anya prioriknowledge of cell parameters. The strategy is similar to that used in four‐circle diffractometry and the method works in a fully automatic manner. It is applicable to multiple oscillation images or multiple stills. A complementary method is also described to obtain orientation angles for the case where cell parameters have already been determine

ISSN:1600-5767    

DOI:10.1107/S0021889890002461

出版商:International Union of Crystallography    

年代:1990

数据来源: WILEY

摘要:

An autoclave cell has been designed for performing time‐resolved neutron diffraction analyses of the dynamic processes occurring during hydrothermal syntheses under hostile (corrosive and explosive) conditions: such conditions include those of hot NaOH/NaOD solutions and pressurized steam. The cell is also capable of measuring differential pressures and accurate sample temperatures as required for the study of reactions which are temperature sensitive. The cell is described and examples of its successful use are given illustrating the synthesis of basic zeolites and a layered calcium silicate hydrate. This technique has considerable potential for studying a variety of synthesis processes of industrial importance, such as in the production of catalysts and the hydration of cement

ISSN:1600-5767    

DOI:10.1107/S0021889890002473

出版商:International Union of Crystallography    

年代:1990

数据来源: WILEY

摘要:

Microcrystalline vaterite, CaCO3, has been synthesized by decomposition of ikaite, CaCO3.6H2O, crystals at room temperature. Scanning electron micrographs show that vaterite occurs as arborescent aggregates ~30 to 40 μm in size. This growth form has not been described before. It is of interest that the overall morphology of the vaterite is reminiscent of some dendritic calcite tufas, although on a smaller scale. This similarity opens up the possibility that the calcitic tufas such as that associated with the Quaternary Lake Lahonton, Nevada, may have been deposited as vaterite that changed to calcite, while preserving the original growth fo

ISSN:1600-5767    

DOI:10.1107/S0021889890002485

出版商:International Union of Crystallography    

年代:1990

数据来源: WILEY

摘要:

The decomposition process of the supersaturated solid solution Cu–15wt%Ni–8wt%Sn prepared by powder metallurgy has been studied by anomalous small‐angle scattering of X‐rays. The ageing has been followed at 623 K. The analysis of the small‐angle scattering intensities shows that the spinodal decomposition model accounts neither for the measured contrast of the electronic densities nor for the anomalous effects observed near the absorption edges of the elements contained in the sample. The measured intensity variation is large and of opposite sign when the Cu and Ni edges are approached from the lower energy side: a decrease of 30% for an energy variation from 292 to 8 eV below the NiK‐absorption edge and a sharp increase of 140% from 300 to 7 eV below the CuKedge. A comparison with previous studies on the same system indicates that the decomposition process investigated in our samples corresponds more to the nucleation and growth regime than to the spinodal one. We found that the assumption of (Cu0.52Ni0.48)3Sn precipitates havingD022structure is in agreement with our exp

ISSN:1600-5767    

DOI:10.1107/S0021889890002655

出版商:International Union of Crystallography    

年代:1990

数据来源: WILEY

摘要:

The ferroelectric ferroelastic phase transitions of tetramethylammonium tetrachlorozincate (HTMA) were observed by standard X‐ray topography and by synchrotron topography. Observation of the phase boundary is possible in the latter case. The different kinds of domain walls observed in the ferroelastic phase cast doubts on the commonly assumed symmetry group of this phas

ISSN:1600-5767    

DOI:10.1107/S0021889890002709

出版商:International Union of Crystallography    

年代:1990

数据来源: WILEY