摘要:

Two crystals of monoclinic hen egg‐white lysozyme were irradiated in a monochromatic synchrotron X‐ray beam (λ= 1.488 Å), the first as a rapidly frozen crystal mounted at the end of a glass fiber at low temperature (120 K) and the second mounted in a capillary tube at ambient temperature (298 K). Comparison of oscillation photographs, extending in resolution to 1.85 Å, and taken from both crystals at zero time and again after a period of exposure in the synchrotron beam (60 min exposure at 120 K; 8 min at 298 K), reveals that radiation‐induced decay is not observed at 120 K but is observed, particularly at high resolution, at 298 K. In a separate set of experiments, data sets to 1.9 Å resolution at 100 and 298 K were collected from two monoclinic and two tetragonal hen egg‐white lysozyme crystals using a rotating‐anode source (λ= 1.5418 Å). Before inclusion of solvent molecules, the monoclinic and tetragonal structures at low temperature, where data were collected from rapidly frozen crystals, refined toR= 27.5 and 25.2%, respectively. The structures at ambient temperature, however, where crystals were mounted in capillary tubes, refined to significantly lower values ofR= 20.9 and 20.6%. After inclusion of solvent, theRvalues at convergence were 20.3 and 17.6% for the monoclinic and tetragonal low‐temperature structures and 17.9 and 16.2% for the room‐temperature structures. Approximately twice the number of water molecules were included in the low‐temperature structures at convergence (406 and 237) than could be assigned in the room‐temperature structures (191 and 100). These results suggest that data sets from rapidly frozen crystals might generally be expected to yield higher initialRfactors, compared to similar room‐temperature structures, but that this difference should diminish appreciably as ordered solvent is included in the model. Apart from the general reduction in atomic temperature factors, the enhancement in resolution observed in diffraction patterns obtained from rapidly frozen crystals is probably due, to some significant degree, to the increase in the ordered

ISSN:1600-5767    

DOI:10.1107/S0021889892010070

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

A sample environment apparatus, such as a diamond‐anvil cell (DAC), cryostat or furnace, presents a unique problem for the crystallographer because some of the diffraction data are shadowed by its components. The first algorithm for sorting diffraction data from a sample consisting of several crystals enclosed in a sample environment apparatus is described. This algorithm requires no previous knowledge of the crystal structure of the sample, so it can be used to solve the structures of substances when the growth of one unstrained crystal is unsuccessful. An example is given to illustrate the development and the implementation of this algorithm: the identification of the unit cell of a high‐pressure phase of oxygen known as epsilon‐oxygen (ɛ‐O2). The ɛ‐O2sample contains at least seven individual crystals and is enclosed in a DAC. The ɛ‐O2unit cell is monoclinic, it contains eight molecules per unit cell and the lattice constants at 19.7 GPa and room temperature area= 3.642,b= 5.491,c= 7.705 Å andβ= 116.2°. In the example, monochromatic X‐ray diffraction data from this high‐pressure sample are sorted, but the algorithm is a general‐purpose technique; it can also be used to sort single‐crystal time‐of‐flight neutron diffraction data. Additionally, this method may be used to sort reflections from several crystal samples containing a mixture of materials. The algorithm is given in symbolic logic so that it can be translated and inserted into available c

ISSN:1600-5767    

DOI:10.1107/S0021889892008422

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

This work describes the conditions required to observe the tunneling phenomenon in the grazing‐angle reflection configuration of both polarized and unpolarized monochromatic neutron waves. According to the computed results, this neutron‐tunneling phenomenon manifests itself by the existence of sharp resonances in the total reflection plateau. Their angular positions depend on the polarization state of the incident neutron waves. We treat the case where all these tunneling resonances (high and low orders) in the total‐reflection plateau are observed. This can be achieved by using a Fabry–Perot cavity deposited onto an appropriate substrate. The substrate must be nonmagnetic, having a negative nuclear scattering‐length density. The numbers of these resonances change mainly with the thickness of the spacer layer of the Fabry–Perot cavity, while their bandwidths are linked to the thicknesses of the reflec

ISSN:1600-5767    

DOI:10.1107/S002188989201032X

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

Diffusion in Ni–Ti multilayers with periods of 120 Å is studied in the temperature range 293–743 K by grazing‐angle unpolarized neutron reflectometry. The effective diffusion coefficient,Deff, of nickel into titanium and its corresponding activation energy,Qa, are determined by measurement of the decay of the reflectivity of the first Bragg peak, which arises from the nuclear scattering‐length density modulation, as a function of annealing temperature at constant time. The direction of diffusion is determined by simulation of the Kiessig fringes located between the total‐reflection plateau and the first Bragg peak. Two diffusion regimes with a transition temperatureTc≃ 543 K are observed in this Ni–Ti multilayer; the corresponding activation energy values are 0.21 and 0

ISSN:1600-5767    

DOI:10.1107/S0021889892010355

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

An automatic procedure for recovering a complete crystal structure after a direct phasing process is described. The procedure consists mainly of a Fourier recycling method that can be implemented in any direct‐methods package. The residualRvalue attained at the end of the process provides an estimate of the degree of success of the structure determination. The procedure can also be applied using a small molecular fragment as prior information. The procedure has been implemented intoSIR92, the successor toSIR8

ISSN:1600-5767    

DOI:10.1107/S0021889892010331

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

A mathematical model for the correction of intensity aberrations in Bragg–Brentano X‐ray diffractometers is described. The model considers tube self‐absorption, the inverse square law, sample transparency, a slit passage function for the diffraction cones and axial displacement of the diffracting elements. A program calledBBCCURV, based on the model, calculates an intensity correction curve for a given sample type (transparency) and diffractometer dimensions. The corrections are significant, especially for samples with low‐angle intensi

ISSN:1600-5767    

DOI:10.1107/S0021889892010781

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

The results of multiple small‐angle neutron scattering (SANS) experiments carried out using a low‐resolution instrument on two bidisperse Al2O3samples, with particle sizes much larger than the limit accessible under the single‐scattering approximation, have been analyzed in the light of a recently developed formalism for multiple small‐angle scattering. It is shown that the extraction of realistic structural parameters that are in good agreement with those calculated from prior knowledge of the samples is only possible when both multiple scattering and bidispersity of the samples have been accoun

ISSN:1600-5767    

DOI:10.1107/S0021889892011063

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

Analysis of the X‐ray powder diffraction profiles of an isomorphous series of inorganic solids has been carried out for two series of complex oxides: La3LnBaCu5O13 +δ(Ln = Y, Nd or Gd) and LnBa2Cu3O7 +δ(Ln = Y, Nd, Pr, Gd or Dy). In this method, profile and structure refinements of a reference compound are followed by a difference Fourier synthesis to locate the positional parameters of a substituent atom. A linear correlation exists between the Fourier peak height in the difference Fourier map and the atomic number of the substituent atom. By an analysis of relative peak height in the correlation plot, the extent of cation disorder, if any, can be determi

ISSN:1600-5767    

DOI:10.1107/S0021889892011762

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

The study of poorly crystallized compounds by X‐ray powder diffraction is sometimes very difficult because structural information is not easily obtained. One way to determine these structural data is to test more or less complex crystallographic models and compare simulated wide‐angle X‐ray scattering (WAXS) and small‐angle X‐ray scattering (SAXS) spectra with experimental ones. This paper presents new software for the simulation of X‐ray patterns. Three modules are available;PRECRAY,SIMVAXandSIMVAX1.PRECRAYallows the definition of the structural model for the solid and different conditions for the calculation of the X‐ray diagram, such as the type of anode and the angular domain.SIMVAXandSIMVAX1 calculate all the interatomic distances according to the Debye formula; the contributions of the different kinds of atoms may be displayed. Applications to different sorts of material are presented; the influences of several kinds of crystallographic defects on the X‐ray diagrams have been studied. The relevance of this simulation approach to the interpretation of small‐angle X‐ray scattering patterns, by the generation of complex models of particles and calculation of the scattered intensity

ISSN:1600-5767    

DOI:10.1107/S0021889892011944

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY

摘要:

Single‐crystal Laue diffraction patterns have not hitherto been used for the precise determination of unit‐cell dimensions, although axial ratios and interaxial angles have frequently been derived. Here, we use an attenuator such as 0.1 mm palladium foil in the white synchrotron‐radiation beam incident on the crystal; the absorption edge (e.g.palladium at 0.509 Å) then sharply defines the minimum wavelength present. With gnomonic projections in the manner described by Carr, Cruickshank&Harding [J. Appl. Cryst.(1992),25, 294–308], the unit‐cell dimensions on an absolute scale can be derived. Three tests show that these dimensions are corre

ISSN:1600-5767    

DOI:10.1107/S0021889892012391

出版商:International Union of Crystallography    

年代:1993

数据来源: WILEY