摘要:

An energy‐dispersive small‐angle X‐ray scattering technique is presented. The incident beam was theBremsstrahlungfrom the X‐ray tube and the detector a silicon–lithium diode. Experimental results for a quenched AlAg alloy are compared with those obtained by the classic small‐angle X‐ray scattering method. The results are in agreement wi

ISSN:1600-5767    

DOI:10.1107/S0021889893002511

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

This paper describes a mirror furnace that has been developed for neutron diffraction work at temperatures up to 2300 K. It is based on a reflecting rotational ellipsoid, in which the heating element, a halogen lamp, is placed at one focus and the sample at the other. It works in a normal,i.e.oxidizing, atmosphere, but can also be used in a vacuum. It has been developed for experiments of long duration. The stability and reproducibility of the temperature are better than 1% of the setting temperature. Further main characteristics are applicability to single‐crystal and powder work, very low background, low power consumption and very easy and cheap handling. Several experiments have been carried out with the furn

ISSN:1600-5767    

DOI:10.1107/S0021889893002596

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

The structure of zeolite ZSM‐23 (structure‐type code MTT) loaded with NH4F [ao= 11.129 (1),bo= 5.025 (1) andCo= 21.519 (1) Å andβ= 89.85 (4)°] was refined from high‐resolution synchrotron X‐ray powder data. The Rietveld refinement converged in space groupP1211 (No. 4) toRF= 0.090 andRwp= 0.085, confirming the proposed model of the silicate framework. The use of complementary methods (synchrotron X‐ray powder diffraction,29Si magic angle‐spinning nuclear‐magnetic‐resonance spectroscopy, and distance and angle least‐squares refinements) was essential for the determination of the true space‐group symmetry, a prerequisite for the successf

ISSN:1600-5767    

DOI:10.1107/S0021889893002006

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

High‐resolution X‐ray diffraction was used to evaluate a number of three‐layer structures of InAlAs/ InGaAs/InAlAs, which were grown on (001) InP by molecular‐beam epitaxy. The aim was to determine the influence on the rocking curve of the InGaAs layer between the two InAlAs barriers. Very sharp and well defined thickness oscillations up to the 20th order show the high quality of the samples. It was possible to prove the existence of an InAs‐rich InAsP layer with a thickness of 0.2–0.8 nm between the substrate and the first InAlAs layer. The transitions between the InGaAs layer and the two InAlAs barriers are characterized by a concentration gradient. This structure, which deviates from the ideal one, was very well suited to the matching of rocking‐curve simulations based on the dynamical theory of X‐ray diffraction with the experimental 004 and 115

ISSN:1600-5767    

DOI:10.1107/S0021889893002390

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

The inversion of a real‐space density profile from specular reflection data is a highly nonlinear transformation and, in the absence of phase information, a direct inversion is not possible. Fourier synthesis techniques used in crystallography (where a similar phaseless problem is encountered) have been applied to the analysis of reflectivity data. The approach involves approximating the density profile by a Fourier series and then using the correspondence between the Bragg‐peak intensities and the corresponding Fourier coefficients to fit the reflectivity data. Although the phase is not determined at each point, the overall shape of the peak provides a measure of the relative magnitudes of the sine and cosine coefficients and hence a partial measure of the relative phase. This technique provides an excellent tool for the inversion of reflectivity data from regular periodic structures and is a promising step towards the final goal of establishing the uniqueness of the inverted density profile. The Fourier method of analysis has been further applied to the analysis of reflectivity from diblock copolymer fi

ISSN:1600-5767    

DOI:10.1107/S0021889893002791

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

The time‐ and position‐resolving scintillation detector JULIOS was used for diffraction experiments in a pulsed neutron beam at the ISIS spallation source at the Rutherford Appleton Laboratory. Angle‐dispersive time‐of‐flight measurements with samples positioned in the primary white beam were performed on standard materials and on zinc‐containing spinel oxides; cation distributions of these compounds were determined by least‐squares intensity refinements. The experimental setup was used for a comparison of results obtained using white‐beam diffractometry at a pulsed spallation source and results obtained using conventional diffractometry with monochromatic neutrons at a continuous reactor beam. A gain by a factor of about 50 in intensity in favour of the white‐beam technology was obtained when results were extrapolated for an equal time‐averaged mean neutron flux. A JULIOS‐type diffractometer was designed to take full advantage of the time struct

ISSN:1600-5767    

DOI:10.1107/S0021889893003267

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

Integrated intensities of Bragg reflections have been measured at room temperature for Ni3Al alloys containing various amounts of boron, hafnium, zirconium and titanium and the data have been used to evaluate the mean Debye–Waller factors (〈B〉), Debye temperatures (ΘM) and long‐range‐order parameters (S). The Debye temperatures are observed to show a slight decrease with the decrease of aluminium content within the Ni3Al phase. The results for the Debye–Waller factors indicate that additions of the ternary elements hafnium, zirconium and titanium cause the mean Debye–Waller factor to increase linearly, while boron addition results in a nonlinear increase. The long‐range‐order parameter of stoichiometric Ni3Al is 0.97, which decreases with decreasing aluminium concentration from the stoichiometric composition. The long‐range‐order parameter of Ni3Al increases until 0.75 at.% B has been added, beyond that it remains constant. The long‐range‐order parameter of Ni3Al increases initially with the addition of hafnium, zirconium and titanium up to 0.50 at.% Hf and 0.52 at.%Zr, with further addition of hafnium and zirconium, it decreases. The results are discussed in relation to the crystal structure and

ISSN:1600-5767    

DOI:10.1107/S0021889893003395

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

Dislocation contrast in silicon carbide has been examined using white‐radiation synchrotron topography. It was found that, in the case of dislocation bundles, the existing theory had to be extended to take into account the overlapping of dislocation strain fields. With this modification, the agreement between theory and observation is greatly improve

ISSN:1600-5767    

DOI:10.1107/S0021889893004017

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

Recurrence relations are derived for calculating closed‐form expressions for the Fourier–Bessel transform of electronic charge densities represented by Gaussian‐ and Slater‐type atomic orbitals. The derivation is based on hypergeometric and Legendre function representations of the transform and the use of their recurrence properties. The scattering‐factor calculations are discussed in detail. A table of closed‐form expressions for Gaussian‐type orbitals, not hitherto available, has been calculated using the new method and is availab

ISSN:1600-5767    

DOI:10.1107/S0021889893004091

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

Owing to the two‐dimensional periodicity of a superstructure on the crystal surface, the intensity in reciprocal space is continuously distributed along rods normal to the sample surface. The analysis of rod scans in surface X‐ray diffraction provides information about the structure parameters normal to the sample surface. For high resolution to be achieved, the measurements must extend to momentum transfersq⊥that are as large as possible. At large exit angles, the conventional Lorentz factor must be modified to take account of the finite aperture of the detector and the continuous intensity along the lattice rod. For two types ofZ‐axis diffractometer used in surface X‐ray crystallography, an analytical expression for the resolution correction of rod‐scan intensity data has been developed. It takes into account an anisotropic detector resolutionT(ΔΘ, ΔΦ), the finite width of the diffracted beam and the primary‐beam divergence parallel to the sample surface,Δτ. The calculation of the convolution functions is simplified by a projection onto theq⊥= 0 plane. The effects of different detector settings and the influences of the primary‐beam divergence and the sample quality on the measured intensity are demonstrat

ISSN:1600-5767    

DOI:10.1107/S0021889893004364

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY