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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 31 (1975), S. 59-63 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The contrast of 180° domains of PbTiO3 depends on temperature and is caused by differences in the intensity of the diffuse background. The thickness extinction fringes of 180° domains taken at different temperatures between room temperature and near the Curie temperature are qualitatively explained with the use of many-beam dynamical theory, in which different anomalous absorption factors are assumed for each case. The origin of the contrast is interpreted and discussed in terms of the difference in thermal diffuse scattering between the 180° domains.
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 143-151 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Many-beam lattice images obtained at 200 kV from V2O3 crystals are discussed in comparison with those calculated in the Bloch-wave approach. The technique of optical diffractogram and equal-thickness fringes is utilized, if possible, to determine the defocus value and the crystal thickness which are the essential parameters for objective interpretation at atomic resolution. Whereas images observed in a thin region (∼ 50 Å) of crystal have been reproduced fairly well by simulation, there are others from thicker regions which are not always explained for lack of knowledge of the parameters. As for the effect of the partial coherence, the validity of the envelope-function approximation is examined with the aid of the first principle involving image-intensity summation, and under the experimental conditions used it proved to be satisfactory for qualitatively reproducing the observed image even for relatively thick (∼ 450 Å) regions.
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 36 (1980), S. 1033-1041 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Lattice images of high-temperature-phase vanadium sesquioxide (V2O3) were calculated within the Bloch-wave approach. At an accelerating voltage of 1000 kV the structure image can be produced while at 100 kV it cannot be obtained because of the small number of reflections available in the image formation, which demonstrates clearly an advantage of high voltages which are produced by improvement in the transfer condition of the objective lens. On the other hand, if an ideal phase-contrast lens is assumed in order to obtain the structure image at 100 kV, lattice-image calculations can be carried out with modified unit-cell size. The results show that the maximum allowable thickness to obtain the structure image varies more rapidly than in a linear manner with the required resolution. The maximum allowable thickness increases either with higher voltages (from about 20 Å at 100 kV to about 35 Å at 1000 kV) or with larger unit cells (e.g. a dilatation of 25% in unit-cell size increases the maximum thickness from about 20 to about 35 Å). However, the evolution of the image details as a function of crystal thickness is different for these two factors, due to the different dynamical interactions of electron waves.
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 825-837 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Tables of dynamic extinction lines which occur in convergent-beam electron diffraction patterns (GM lines) are given for all the space groups on the basis of the rules given by Gjønnes & Moodie [Acta Cryst. (1965), 19, 65-67]. It is found that 191 space groups can be identified by GM lines. A convenient experimental method which distinguishes a screw axis and a glide plane is demonstrated. Experimental results are shown in which the GM lines due to a screw axis and those due to a glide plane are separately observed. The GM lines appearing in a symmetrical four-beam pattern are demonstrated.
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 36 (1980), S. 350-352 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Three different space groups, C2/c, C2 and Cc, have been reported for the room-temperature form of the rare-earth orthoniobates, LnNbO4 (Ln = La, Nd). They belong to different point groups: 2/m, 2 and m, respectively. By means of convergent-beam electron diffraction, the point group has been determined to be 2/m in the present study. Therefore the true space group must be C2/c.
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 40 (1984), S. 721-721 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Tables 13 and 17 of the paper by Tanaka, Sekii & Nagasawa [Acta Cryst. (1983), A39, 825-837] are not perfect. The correct tables are given.
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  • 7
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 39 (1983), S. 357-368 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The method of point-group determination from convergent-beam electron diffraction patterns has been established by Buxton, Eades, Steeds & Rackham [Philos. Trans. R. Soc. London (1976), 281, 171-194]. However, Table 2 given by them is inconvenient for practical purposes, since many symmetries of the dark-field and ±G dark-field patterns are not given and are left for the reader's consideration. The table is improved and completed with the help of some new symmetry symbols and illustration of symmetries. The new table makes the point-group determination easy and quick. The symmetries of the symmetrical many- beam convergent-beam electron diffraction patterns have been studied by Tinnappel [PhD Thesis (1975), Tech. Univ. Berlin] using group theory. It is shown that the graphical method used by Buxton et al. can reveal the symmetries of these patterns. A method of point-group determination which uses three types of symmetrical many-beam patterns, the hexagonal six- beam, square four-beam and rectangular four-beam patterns, is described. This method requires only one photograph in determining most diffraction groups. This fact means that the method is more convenient and reliable than that of Buxton et al., since their method requires two or three photographs for most cases. Experimental results which verify the theoretical ones are given. The characteristic features of the symmetrical many-beam method are discussed.
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  • 8
    Electronic Resource
    Electronic Resource
    Oxford [u.a.] : International Union of Crystallography (IUCr)
    Acta crystallographica 48 (1992), S. 739-740 
    ISSN: 1600-5759
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 9
    Electronic Resource
    Electronic Resource
    Oxford [u.a.] : International Union of Crystallography (IUCr)
    Acta crystallographica 51 (1995), S. 2534-2537 
    ISSN: 1600-5759
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 10
    Electronic Resource
    Electronic Resource
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 30 (1997), S. 272-279 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Modulated structures formed in defect fluorite-type (Y2O3)1 − x(Nb2O5)x (0.21 ≤ x ≤ 0.27) solid solutions have been investigated by electron diffraction. [1{\bar 1}0] diffraction patterns showed a pair of diffuse spots in the [11{\bar 2}] direction at both sides of the ½½½ reciprocal-lattice point. The distance between the diffuse spots decreased from 0.064 to 0.045 Å−1 with increasing x from 0.21 to 0.27. High-resolution electron microscopy images revealed the existence of small domains with an antiphase structure. The two-dimensional antiphase structure projected on the (1{\bar 1}0) plane is described by compositional waves of Y3+/Nb5+ and O2−/vacancy and displacive waves of Y3+ + Nb5+ and O2− along the [11{\bar 2}] direction. The composition at cation sites is expressed by a sinusoidal wave with occupation changes between Y and 0.58Y + 0.42Nb. The occupational probability of O at anion sites is also expressed by a sinusoidal wave with occupation changes between 0.806 and 0.904. Amplitudes of the displacive waves for anions and cations were determined to be about 0.3 and 0.02 Å, respectively.
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