摘要:

A new radiative microfurnace has been developed for high‐temperature single‐crystal work on the four‐circle diffractometer. The microfurnace is fairly easy to construct and can be operated at temperatures up to 1270 K with a precision of ± 6 K. A calibrated shunt in the circuitry of the microfurnace is used to determine the microfurnace's resistance, current draw and power usage for any given temperature during a diffraction experiment. A procedure for mounting single crystals in a refractory cement that can be used as an internal calibrant of temperature is also described. The thermal expansion properties of periclase (MgO) have been reinvest

ISSN:1600-5767    

DOI:10.1107/S002188988609009X

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

The lattice parameters of a single‐crystal of BaTiO3have been measured under high hydrostatic pressure up to 5 GPa using a diamond‐anvil high‐pressure cell of a new design. The lattice becomes cubic at 2.08 (8) GPa and stays cubic at higher pressures. The pressure dependence of the tetragonal–cubic transition temperature is estimated to be −4

ISSN:1600-5767    

DOI:10.1107/S0021889886090088

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

Crystal‐structure parameters and their associated estimated standard deviations (e.s.d.'s) have been determined for α‐Al2O3by Rietveld analysis of CuKα X‐ray powder diffraction data collected on a standard diffractometer with profile step widths ranging from 0.01 to 0.32° 2θ. Estimates of the true precision of the parameters have been obtained by replication of the experiment five times. The degree of serial correlation between adjacent points in the profiles has been quantitatively determined by the use of the Durbin–Watsondstatistic. Values of the conventional profile agreement indices,Rp,Rwpand goodness of fit, are not influenced by the scan‐point density, but the BraggR‐factor decreases to a limiting value as the step width is decreased. The structural parameters obtained at all step widths are statistically indistinguishable at the 1σlevel, although there is some loss of accuracy relative to single‐crystal values at widths larger than 0.16°. The least‐squares e.s.d.'s decrease linearly with the reciprocal square root of the number of steps in the pattern, as expected, but the e.s.d.'s of the unit‐cell dimensions are the only ones that are significantly smaller than estimates of their true precision obtained from repetitions of the experiment. In the case of α‐Al2O3, and possibly in general, the optimum step width for Rietveld analysis corresponds approximately to the minimum value of the full‐width at half‐m

ISSN:1600-5767    

DOI:10.1107/S0021889886090076

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

The variation of the atomic scattering factor near the X‐ray absorption edge of an element can be an important tool for determining the distribution of that element in any sample. For defining experimentally the possibilities of anomalous small‐angle X‐ray scattering (ASAXS) in metallurgy, three compositions of a phase‐separated aluminium‐zinc alloy have been studied at different energies below the Zn‐absorption edge, using a synchrotron radiation source. In order to test the reliability of these measurements and of the data treatment, the variation off′, the real part of the scattering factor, has been measured with a precision comparable to that of more direct

ISSN:1600-5767    

DOI:10.1107/S0021889886090064

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

Scattering exponents are discussed for systems of power‐law polydisperse surface and mass fractals. It is found that the scattering exponent for power‐law polydisperse mass fractals lies on the interval [0,d] and always depends on the mass fractal dimension. The scattering exponent for power‐law polydisperse surface fractals lies on [0,d+ 1] and is independent of the surface fractal dimension for a large range of the polydispersity exp

ISSN:1600-5767    

DOI:10.1107/S0021889886090052

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

This paper describes a powerful preliminary to various least‐squares programs for finding the exact crystal orientation for the oscillation method. It is well suited to cases of large initial misorientation. It uses the real‐time rotation and clipping capabilities of the high‐performance graphics system PS300 from Evans&Sutherland. A program has been written that performs a geometrical inversion of reciprocal space through its origin and manipulates this transformed image in place of the direct one. The Ewald sphere is thus transformed into a plane that can be superimposed on the film. The crossing of any reciprocal‐lattice point through the Ewald sphere is then replaced by the crossing of its transform through the film plane. Clipping allows elimination of all the points too far away from this plane,i.e.those that are not in a diffraction condition. Determination of the correct orientation is achieved by superimposing the inverted reciprocal‐lattice image on the corresponding transformed image of the obse

ISSN:1600-5767    

DOI:10.1107/S0021889886090040

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

The convolution of the various contributions to X‐ray intensity distribution curves measured in lattice‐parameter determination leads to peak shifts and, hence, to systematic errors in the case that at least one of the contributing curves is asymmetric. For perfect crystals, depending on the collimator divergence, this effect may reach the order of magnitude of the refraction correction. It is caused by the asymmetry of the spectral distribution curve, which includes intrinsic asymmetry, dispersion and angle dependence of intensity (ADI). For imperfect crystals characterized by orientation and interplanar‐spacing distributions, the distribution curves should be measured in order to correct accurately the peak shift. Examples of simulation calculations are given for Si, CuKα1, Bond method for various reflections of perfect crystals and for crystals having orientation distributions, 444 refl

ISSN:1600-5767    

DOI:10.1107/S0021889886090039

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

The aim of this paper is to assess the potential accuracy of structure analyses based upon solution X‐ray scattering experiments, before the structure problem is solved. In other words, an algorithm is sought expressing accuracy in terms of the experimental observations and associated errors, independently of the value of the structure parameters. The mathematical treatment involves five steps: (a) Data reduction, namely the definition in experimental space of the degrees of freedom of the problem and the determination of their probability law. (b) Choice of representation of the structure consistent with the coarse resolution of solution scattering experiments; a set of sharp pseudo‐atoms is adopted, so that the degrees of freedom in real space are the position and electron density of each pseudo‐atom. (c) Introduction to a `virtual experiment' defined by a noise distorting the structure parameters, and determination of the corresponding probability law in the experimental space. (d) Comparison of the real and the virtual experiments, or more precisely of the probability laws of the degrees of freedom in experimental space, yielding a relation between the variances of the position and electron density uncertainties, the number of the pseudo‐atoms and the value of the structure parameters. (e) Attenuation of the structure dependenceviathe introduction of a hypothesis of quasi‐spherical symmetry, whose effect is to reduce the expression of the variances to a function of the experimental observations, independent of the knowledge of the structure of the particle. This analysis is applied to solution X‐ray scattering experiments at variable contrast, interpreted within the framework of the invariant volume hypothesis. The final result takes the form of a curve relating the variances of the position and electron density uncertainties, divided by the number of pseudo‐atoms. The example of a low‐density serum lipoprotein is used to illustrat

ISSN:1600-5767    

DOI:10.1107/S0021889886090027

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

This paper is an extension of the preceding one and deals with the problem of assessing the potential accuracy of low‐resolution crystallographic analyses restricted to the same angular range as solution scattering studies. The mathematical treatment is very similar in the two papers; similarly, the final result takes the form of a curve relating the normalized variances of the positional and chromatic structure parameters. In the absence of `small‐angle' data – these are utterly disregarded in routine protein crystallographic analyses – an ideal experiment performed on a low‐density serum lipoprotein, under conditions similar to those that prevailed in the experimental study of methionyl t‐RNA synthetase, is used as an illustration. The potential accuracy estimated for this putative crystallographic analysis is found to be quite close to that assessed for the solution scattering experiment. In the case of protein crystallographic analyses, the projection information associated with the `small‐angle' reflections is estimated and it is shown that it amounts to a substantial fraction of the whole of the experimental information. More precisely, the average information per reflection is almost 40 times larger for the `small‐angle' than for the other reflections. The conclusion is reached that 'small‐angle' reflections should be involved in protein crystallographic analyses and it is suggested that in the early stage of the phasing process `small‐angle' crystallographic data and solution scattering experiments be combined together to locate and orient the molecule(s) in the cell and to draw the

ISSN:1600-5767    

DOI:10.1107/S0021889886090015

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY

摘要:

Screen‐less oscillation photography is the method of choice for recording three‐dimensional X‐ray diffraction data for crystals of biological macromolecules. The geometry of an oscillation camera is extremely simple. However, the manner in which the reciprocal lattice is recorded in any experiment is fairly complex. This depends on the Laue symmetry of the reciprocal lattice, the lattice type, the orientation of the crystal on the camera and to a lesser extent on the unit‐cell dimensions. Exploring the relative efficiency of collecting X‐ray diffraction data for different crystal orientations prior to data collection might reduce the number of films required to record most of the unique data and the consequent amount of time required for processing these films. Here algorithms are presented suitable for this purpose and results are reported for the 11 Laue groups, different lattice types and crystal orientations often employed in data c

ISSN:1600-5767    

DOI:10.1107/S0021889886090003

出版商:International Union of Crystallography    

年代:1986

数据来源: WILEY