摘要:

The small‐angle neutron scattering technique is applied to a hot stretched 5% perdeuterated polystyrene sample in order to investigate the residual macromolecular orientation at various length scales. The observed orientation is left over after fast quenching (in ice water) of the hot stretched sample. Previous investigations focused on how much polymer chains follow the external stretching. It is found that, even when macromolecules follow the external stretching affinely on the average, they do not deform uniformly. Moreover, over the time scale of the temperature quench, small chain portions relax more rapidly than large one

ISSN:1600-5767    

DOI:10.1107/S0021889889008484

出版商:International Union of Crystallography    

年代:1990

数据来源: WILEY

摘要:

A computer‐graphics system has been used interactively to study the powder diffraction pattern (CuKβ) of a `perfect' quartz sample. The poly crystalline specimen was obtained by powdering a thin slide of synthetic quartz. The reticular planes were indexed and the cell parameters refined using theUSGSLSprocedure. A comparison with the literature suggests the presence of systematic effects in the measuring equipment; evaluation of these by means of Wilson's model permitted the listing and indexing of 44 reflections and determination of the Bragg angle as well as the cell and the instrumental parameters. The refinement of the cell parameters performed directly by the optimizing procedure came to a precision of 1 × 1

ISSN:1600-5767    

DOI:10.1107/S0021889889009210

出版商:International Union of Crystallography    

年代:1990

数据来源: WILEY

摘要:

The contribution of multiple scattering in wide‐angle X‐ray scattering experiments on isotropic samples using MoKαand CuKαradiation is estimated. The portion of doubly and triply scattered intensity was calculated with a tedious Monte Carlo simulation method and the portion of doubly scattered intensity was calculated with a fast approximate numerical integration procedure. In the case of light elements the correction was found to be important. The approximate method proved to be reliable for thin samples in cases where the cross section for inelastic scattering is small. The effect of the molecular structure of the material on the multiply scattered intensity was demonstrated in the case of liquid sulfur and found to be meaningful at small scattering angles. The correction method was applied to the radial atomic density function (RDF) analysis of water and sulfur. The correction improved the behaviour of the reduced interference function but its effect on the RDF was small in both

ISSN:1600-5767    

DOI:10.1107/S0021889889009283

出版商:International Union of Crystallography    

年代:1990

数据来源: WILEY

摘要:

Anomalous X‐ray scattering terms for Rb+near theK‐edge measured in diffraction experiments with synchrotron radiation range up to 4.71 (6) forf′′ and down to −9.80 (11) forf′, in agreement with values derived from the abso

ISSN:1600-5767    

DOI:10.1107/S002188988901023X

出版商:International Union of Crystallography    

年代:1990

数据来源: WILEY

摘要:

Anisotropic diffuse scattering of the one‐dimensional conductor K‐hollandite has been measured at a synchrotron source and compared with measurements with a rotating anode. The intensity gain of more than one order of magnitude and the ability to adjust the wavelength provide the counting statistics in a few hours that are necessary for aquantitativeanalysis of diffuse phenomena. The main advantage of a synchrotron source is the high resolution throughout reciprocal space (low and highQ). Thus, commensurate/incommensurate positions of diffuse layers may be distinguished, weak diffuse maxima become detectable and small peak shifts of diffuse modulations clearly visible. Correlation functions may be determined directly from the diffuse profiles without tedious resolution corrections. These features are essential for a dedicated instrument at a storage r

ISSN:1600-5767    

DOI:10.1107/S0021889889010514

出版商:International Union of Crystallography    

年代:1990

数据来源: WILEY

摘要:

X‐ray and neutron scattering provide valuable and specific information about the internal structure of macromolecules in solution. The excess electron distance distribution function of a macromoleculep(r), derived from the scattering by a Fourier transformation, is more directly structurally interpretable than the scattering curve itself. This is especially the case when a set of atomic coordinates is available as a `starting structure' and when thep(r) function is highly resolved. A procedure is proposed for calculation of the high‐resolutionp(r) function at distances greater than 0.4 nm. To reduce termination errors in the distance distribution function, the difference between the scattered intensity of the molecule, calculated using atomic coordinates, and the scattered intensity of the `independent' atoms of the macromolecule is modified by a Hamming window and then Fourier transformed. A more direct structural interpretation of changes in the highly resolvedp(r) function is possible by the correlation of known structural elements in proteins and nucleic acids with main features of the electron distance distribution functionviathe so‐called distan

ISSN:1600-5767    

DOI:10.1107/S0021889889010526

出版商:International Union of Crystallography    

年代:1990

数据来源: WILEY

摘要:

The X‐ray reflectivity of bent perfect crystals is calculated from a model where the crystal is approximated by a stack of perfect‐crystal lamellae which have a gradually changing orientation. A computer program is developed for calculation of the reflectivity of the composite crystal from the dynamical theory of diffraction. An approximate solution is also given where an analytical formula for the reflectivity of a non‐absorbing lamella is used and the effects of absorption are calculated separately. Typically, in the Bragg case, the reflectivity curve has a steep edge and an exponentially falling slope, while in the Laue case the curve is almost rectangular if the absorption is not too large. The width of the curve is inversely proportional to the bending radius in both cases. Reflectivity curves were measured for the 111 and 400 reflections of Si with MoKα1radiation. The agreement with analytical and computer calculations is good, particularly at small bending radii where the kinematical limit is appr

ISSN:1600-5767    

DOI:10.1107/S0021889889010800

出版商:International Union of Crystallography    

年代:1990

数据来源: WILEY

摘要:

An account is given of transmission electron microscope investigations of the phase transitions in single crystals and ceramics of the complex perovskite structured ferroelectric `relaxor' compound Pb(Sc1/2Ta1/2)O3. The crystal symmetries pertaining to both the non‐polar paraelectric (PE) and polar ferroelectric (FE) states have been studied by the technique of convergent‐beam electron diffraction. A new phase transition has been discovered in the temperature range for which the FE and PE states coexist. The new phase transition is interpreted as the creation of a modulated antiferroelectric state, and this is viewed as marking a departure from relaxor behaviour towards more `normal' ferroelectric behavi

ISSN:1600-5767    

DOI:10.1107/S002188988901085X

出版商:International Union of Crystallography    

年代:1990

数据来源: WILEY

摘要:

Atomic arrangements of long‐period superstructures in Au–24.5at.%Cd alloy with the basic h.c.p. structure have been investigated by high‐resolution electron microscopy. The alloy shows one‐dimensional or two‐dimensional long‐period superstructures in the close‐packed plane with a 6Hstacking sequence, depending on heat treatments. It is shown that the two‐dimensional incommensurate superstructure with a period larger than 9a0is composed of domains of the hexagonal commensurate structure (M= 9) separated by the antiphase‐boundary zone in which 33/2a0× 2a0× 3c0orthorhombic structure is formed (a0,c0being the lattice parameters of the h.c.p. structure). Microstructures of the alloy annealed at 620 K with the one‐dimensional long‐period superstructure and of the alloy quenched from 770 K showing diffuse scattering have also been investigated by high‐r

ISSN:1600-5767    

DOI:10.1107/S0021889889011441

出版商:International Union of Crystallography    

年代:1990

数据来源: WILEY

摘要:

A method of measuring differences in lattice spacing with an accuracy of 1 part in 108in a period as short as two minutes is described. The method uses one source of copper radiation and a triple‐axis arrangement. Two of these axes are double‐leaf silicon springs in monolithic crystal assemblies which achieve the high stability required by such measurements. Samples are easily changed and sequences of measurements are performed entirely under computer control. The method is demonstrated by a comparison of the lattice spacings of four samples which reveals differences of up to 60 parts in 108between silicon crystals of different orig

ISSN:1600-5767    

DOI:10.1107/S0021889889011623

出版商:International Union of Crystallography    

年代:1990

数据来源: WILEY