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  • American Physical Society (APS)
  • International Union of Crystallography (IUCr)
  • 2000-2004
  • 1995-1999
  • 1985-1989  (1,114)
  • 1986  (1,114)
Collection
Publisher
Years
  • 2000-2004
  • 1995-1999
  • 1985-1989  (1,114)
Year
  • 1
    Electronic Resource
    Electronic Resource
    Scattering exponents for polydisperse surface and mass fractals (1986)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 19 (1986), S. 25-27 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: 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 exponent.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889886090052
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  • 2
    Electronic Resource
    Electronic Resource
    A real-time interactive graphics program to determine crystal orientation for the analysis of oscillation diffraction photographs (1986)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 19 (1986), S. 28-33 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: 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 observed film.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889886090040
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  • 3
    Electronic Resource
    Electronic Resource
    Systematic errors in precision lattice-parameter determination of single crystals caused by asymmetric line profiles (1986)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 19 (1986), S. 34-38 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: 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, Cu Kα1, Bond method for various reflections of perfect crystals and for crystals having orientation distributions, 444 reflection.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889886090039
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  • 4
    Electronic Resource
    Electronic Resource
    Accuracy and resolution in small-angle crystallographic analyses. A comparison with solution scattering studies (1986)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 19 (1986), S. 51-60 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: 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 molecular envelope.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889886090015
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  • 5
    Electronic Resource
    Electronic Resource
    Information content and retrieval in solution scattering studies. II. Evaluation of accuracy and resolution (1986)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 19 (1986), S. 39-50 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: 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 dependence via the 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 illustrate these results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889886090027
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  • 6
    Electronic Resource
    Electronic Resource
    Powder diffraction peak shapes. Parameterization of the pseudo-Voigt as a Voigt function (1986)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 19 (1986), S. 63-64 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The Voigt function is now recognized to be one of the best mathematical descriptions of the symmetrical component of the peak shape in constant-wavelength X-ray and neutron powder diffraction. The simpler pseudo-Voigt function has been used previously to provide an empirical fit to observed peak shapes. In this paper, a parameterization of this function is obtained to make it, to a very good approximation, equivalent to a Voigt function, and, additionally, it is described by the half-width parameters of the Gaussian and Lorentzian components of that Voigt function. This new construction allows for greater flexibility in peak-shape description and, in particular, allows for a more straightforward convolution of additional functional contributions into the `Voigt' component of a peak shape description.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889886089999
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  • 7
    Electronic Resource
    Electronic Resource
    Strategies for collecting screen-less oscillation data (1986)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 19 (1986), S. 61-62 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: 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 collection.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889886090003
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  • 8
    Electronic Resource
    Electronic Resource
    Einfluss der Valenzelektronenkonzentration auf die Bildung der Phasen vom Ni2In-Strukturtyp am Beispiel der ternären Verbindungen Mn2Ga0.5As0.5(m),Fe3GaAs und Co2Ga0.5As0.5 (1986)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 19 (1986), S. 80-85 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The structures of the metastable phase Mn2Ga0.5As0.5 (m) and the stable phases Fe3GaAs and Co2Ga0.5As0.5 are isotypic with Ni2In. X-ray powder diffraction data are reported. Lattice constants and macroscopic densities were measured in the entire range of homogeneity. Crystal chemical parameters of these ternary phases and of the quasi-homologous isovalent phases are compared and discussed from the point of view of the valence-electron concentration and of the number of atoms in the unit cell. It is concluded that the valence-electron concentration is a very important but not a sufficient energetic factor for the stability of phases with the Ni2In structure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889886089884
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  • 9
    Electronic Resource
    Electronic Resource
    Lattice thermal expansion behaviour of spinel HgCr2Se4 (1986)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 19 (1986), S. 86-89 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: An accurate value of the lattice parameter of the spinel ferromagnetic semiconductor HgCr2Se4 has been determined at different temperatures, ranging from 298 to 770 K, by X-ray powder diffraction. The data have been used to evaluate the coefficient of thermal expansion, α, at various temperatures. The temperature dependence of the coefficient of expansion can be represented by the equation α = 10.059 × 10−6 − 1.862 × 10−8 T + 3.017 × 10−11 T2, where T is the temperature in K. The diffraction pattern of HgCr2Se4, obtained at 785(5) K, was completely different from that at room temperature. The high-temperature phase persisted up to 900 K, and above this the compound decomposed. Furthermore, the new phase was retained when the sample was cooled from 900 K to room temperature. These results suggest an irreversible structural phase transformation in HgCr2Se4 at 785 K. An attempt is made to explain this interesting high-temperature thermal behaviour of HgCr2Se4 in relation to its structure and other physical properties.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889886089872
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  • 10
    Electronic Resource
    Electronic Resource
    CAMAL – a new component of the Cambridge Structural Database software system (1986)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 19 (1986), S. 90-91 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The efficient use of crystallographic databases necessitates the use of statistical techniques. CAMAL is a library of Fortran subroutines for performing simple and multivariate statistical calculations. The library is intended for use with the Cambridge Structural Database and is distributed by the Cambridge Crystallographic Data Centre.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889886089860
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