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  • Articles  (3,284)
  • International Union of Crystallography (IUCr)  (1,211)
  • Cambridge University Press  (1,201)
  • American Meteorological Society  (872)
  • American Physical Society (APS)
  • Springer Nature
  • 2000-2004
  • 1980-1984  (3,284)
  • 1930-1934
  • 1984  (3,284)
  • Geosciences  (2,696)
  • History  (588)
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  • Articles  (3,284)
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  • 2000-2004
  • 1980-1984  (3,284)
  • 1930-1934
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  • 1
    Electronic Resource
    Electronic Resource
    Eigenvalue filtering in the refinement of crystal and orientation parameters for oscillation photography (1984)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 17 (1984), S. 238-243 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Selective application of only parameter shifts with large eigenvalues permits singular or near-singular least-squares problems to be solved. This `eigenvalue filtering' process has been applied to the refinement of the unit-cell, crystal-orientation and reflecting-range parameters needed to process oscillation films. Eigenvalue filtering permits automatic identification and determination of just those combinations of parameters that are most relevant for processing films from a particular crystal in a particular setting. The procedure may be carried out with alignment information from an individual data film or from multiple alignment films taken at different spindle angles. Eigenvalue filtering has been incorporated in a refinement program that minimizes discrepancies between observed and calculated fractions recorded for partially recorded reflections. This permits the reflecting range (combined mosaic spread, beam cross-fire, and wavelength spread) to be refined along with the unit-cell and orientation parameters. Observed fractions recorded may be obtained by visual estimation prior to film scanning, or the program may be used in a `post-refinement' mode with data obtained from actual intensity measurements. The relatively unreliable information represented by the positions of spots on the film is not used except for indexing. The program handles crystals of any symmetry in any setting and the oscillation spindle may be inclined from normal beam geometry. The mis-setting angles are defined in a crystal-fixed coordinate system, making it easy to compare refinement results for data taken at different spindle angles.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889884011444
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  • 2
    Electronic Resource
    Electronic Resource
    Optimization of the experimental resolution for small-angle scattering (1984)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 17 (1984), S. 249-256 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The instrumental optimization conditions for most small-angle scattering experiments in which the data are azimuthally symmetric require that the scattered flight path be equal to the incident flight path. This is in contrast to a recent analysis which shows that under some conditions the incident and scattered flight paths are in a ratio of two to one. The equal flight-path condition is also valid for experiments measuring sharp (Bragg-like) peaks, or where the intensity is required at specific scattering vectors, as in low-angle diffraction of ordered or semiordered systems. The implications of the optimization conditions on the resolution and count rates at the detector are discussed for both types of experiment, and the dependence of the resolution on the spectrometer geometry is considered.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889884011468
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  • 3
    Electronic Resource
    Electronic Resource
    High-resolution measurements of angle-resolved X-ray scattering from optically flat mirrors (1984)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 17 (1984), S. 257-264 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: With a compact arrangement using single-crystal collimator and analyzer (silicon 220 reflection) of channel-cut type, angle-resolved scattering (ARS) curves of X-rays (Cu Kα1) for optically flat mirrors have been measured at various glancing angles, ω, of X-rays to the mirror surface with an angular resolution of 4′′. Weak scattered intensity (10−3–10−5 of the specularly reflected beam) is observed over an angular range of a few hundred arc s below and above the direction of specular reflection. When ω is close to the critical angle for total reflection, the scattered intensity at the low-angle tail of ARS curves is higher than that at the high-angle tail. This asymmetric tail profile of ARS curves is explained by simply superposing intensities of specularly reflected beams from surface elements inclined to each other.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S002188988401147X
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  • 4
    Electronic Resource
    Electronic Resource
    Comments on Small-angle scattering at a pulsed neutron source: comparison with a steady-state reactor (1984)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 17 (1984), S. 293-294 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A recent comparison of data for a specific experiment taken on a time-of-flight small-angle scattering spectrometer on a pulsed neutron source and data for the same kind of experiment taken at a steady-state reactor suggests that the instruments have similar performances, and in particular comparable count rates at the detector. This similarity disappears when resolution is taken into account. The criterion for a valid comparison should be the detector count rate for instruments measuring over similar ranges of scattering vector and with similar resolutions, as required by the particular experiment.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889884011523
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  • 5
    Electronic Resource
    Electronic Resource
    Accuracy and resolution in protein crystallography: a probabilistic approach (1984)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 17 (1984), S. 273-285 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The purpose of this paper is to establish an explicit correlation between the values and probable deviations of the observed intensities and the potential accuracy of the structure eventually determined using those intensities. This problem involves several steps: the choice in real space of a reference stochastic model defining the ideal state of ignorance; the determination in reciprocal space of the probability density corresponding to that model and of the probability law corresponding to the experiment; the determination of the information; the transfer of the information from reciprocal to real space; the interpretation of the information in terms of structural resolution. A rigorous treatment of this problem is hindered by the inadequate knowledge of the statistical correlations between different reflections; more realistically, it is convenient to address a simpler problem, corresponding to an ideal experiment in which the intensities relevant to each reflection are measured independently of all the other reflections. In this case the mathematical problems can easily be solved and a parameter introduced – called projection information – whose value can be determined. This information is the sum of two terms, one associated with the modulus, the other with the phase of each reflection. By resorting to a mathematical model it is possible to make use of the projection information to determine the value of a parameter akin to structure resolution. The final result of this work is an operational definition of resolution, based upon the stochastic properties of the experimental observations. This resolution can be used to assess and compare the intrinsic quality of different structure analyses and of different stages of one structure determination, before the structure is solved.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S002188988401150X
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  • 6
    Electronic Resource
    Electronic Resource
    The effect of profile-step counting time on the determination of crystal structure parameters by X-ray Rietveld analysis (1984)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 17 (1984), S. 297-306 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Crystal structure parameters have been obtained for α-Al2O3, β-PbO2 and (Mg, Fe)2SiO4 by Rietveld analysis of Cu Kα X-ray powder diffraction data collected on a conventional diffractometer using counting times ranging from 0.01 to 5 s per step. For all but the 0.01 s data collected on (Mg, Fe)2SiO4 the structural parameters obtained at different counting times are statistically identical at the 3 σ level, and the spread in the values is essentially the same as that obtained by sample repacking at a fixed counting time of 1 s per step. The parameter e.s.d.'s and conventional agreement indices Rwp and RB decrease to values limited by residual model errors as the counting time increases, but the goodness-of-fit parameter becomes unacceptably larger than its ideal value of unity. When more than a few thousand counts are accumulated for the maximum step intensity in the diffraction profile a weighting scheme based solely on counting variance is inappropriate and the parameter e.s.d.'s are no longer a reflection of their accuracy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889884011547
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  • 7
    Electronic Resource
    Electronic Resource
    The many-beam moiré effect in electron micrographs of epitaxic Sn/SnTe layers (1984)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 17 (1984), S. 307-314 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The many-beam dynamical theory of moiré fringes of Gevers [Phys. Status Solidi (1963). 3, 2289–2297] has been applied to the parallel moiré case and the results have been verified on epitaxically grown Sn/SnTe specimens showing the sensitivity of moiré patterns to the presence of systematic reflections. The Fourier analysis of the calculated intensity profiles showed that each additional diffracted beam gives rise to a new term in the Fourier series so that the profile can be sinusoidal only in the two-beam approximation. The same result has been obtained recently by Pardo, Pariset & Renard [Phys. Status Solidi A. (1981). 64, 283–295], who applied a slightly different theoretical approach to Sb/Bi specimens. It is shown that the corrections to scattering factors for temperature effects are not important since they do not affect the general character of the moiré profiles. Some experiments were performed showing the dependence of the moiré pattern on the number of beams that are transmitted through the objective aperture. The effect of the gradual thinning of one layer on the moiré pattern could be reproduced by computer simulation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889884011559
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  • 8
    Electronic Resource
    Electronic Resource
    The true unit cell of ammonium hydrogen sulfate, (NH4)3H(SO4)2 (1984)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 17 (1984), S. 331-333 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A combination of X-ray powder diffraction and single-crystal Laue photography was used to determine the unit-cell parameters for letovicite. μr = 247.25. The unit cell is triclinic, P1 or P{\bar 1}, and has dimensions a0 = 5.87(1), b0 = 10.17(3), c0 = 8.27(1) Å, α = 101.1(4), β = 111.1(1), γ = 89.9(2)°, V = 450.7 Å3, Z = 2, Dx = 1.82 Mg m−3. The M(20) cell reliability is 23.8. Many weak powder spectra were observed, which preclude the possibility of monoclinic cell symmetry previously reported. Projection of the letovicite Laue data along [001] reveals the striking pseudo-hexagonal symmetry observed optically. The JCPDS Diffraction File No. for ammonium hydrogen sulfate is 35-1500.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889884011596
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  • 9
    Electronic Resource
    Electronic Resource
    On the automatic extension of incomplete models by iterative Fourier calculation (1984)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 17 (1984), S. 364-366 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A program is presented that solves crystal structures completely using advanced Fourier methods starting from a small known fraction (about 8%) of the structure. The program uses a statistical test based on the R2 factor to check the correctness of the model obtained so far. The results obtained with four test structures are given.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889884011705
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  • 10
    Electronic Resource
    Electronic Resource
    A sample holder for cutting single crystals along any desired X-ray oriented plane (1984)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 17 (1984), S. 369-370 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889884011730
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