α-Bi4V2O10 crystal structure and oxidation mechanism. X-ray and electron diffraction analysis
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The principles of x-ray and electron diffraction by free molecules are discussed in view of the application of these methods to the determination of molecular structures. The influence of the chemical bond is not more disturbing for x-ray measurements than for electron measurements. The hydrogen atoms are practically nonexistent for x-ray diffraction while it is not so for electron diffraction. This is an advantage which has not yet been fully used. The following procedure is suggested: first an x-ray investigation in which the number of parameters to be determined is much reduced, and subsequently an electron investigation which will determine the remaining parameters, i.e., the position of the hydrogen atoms. Thus in some cases at least (CH3Cl e.g.), the use of both diffraction methods is necessary and is sufficient for determining completely the molecular structure, including the position of the hydrogen atoms. A discussion of x-ray measurements on SiHCl3 shows the precision and the certainty of the method. As for the disadvantages of the long times of exposure, which have been more than 100 hours, they have been reduced to, say, 8 hours for crystal-monochromatized radiation and they will probably be reduced by a new factor 10 in the near future. Although being less numerous than in the case of electron diffraction, the maxima given by x-ray diffraction present two fundamental advantages. In general they are actual, well-defined maxima. The effect of molecular vibrations on them, which is difficult to calculate, is the smallest just at the angles at which they are the most prominent.
Gas electron diffraction
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Crystal (programming language)
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A straightforward kinematic analysis of diffraction from metastable three-dimensional crystallites of Ge grown on Si(001) is presented. Low-energy electron diffraction data from these crystallites agree with diffraction images calculated for a structure determined from scanning-tunneling microscopy data. Additionally, reflection high-energy electron diffraction images predicted for these crystals agree with existing data.
Metastability
Low-energy electron diffraction
Reflection
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Electron-diffraction data obtained from cesium antimonide cathodes deposited on oxidized manganese are consistent with a composite layer structure of Cs3Sb and MnO in which no appreciable chemical reaction occurs. In constrast, the low sensitivity cathodes obtained on unoxidized manganese can be attributed to the formation of MnSb when antimony is deposited. No change in the diffraction pattern of Cs3Sb is observed when the cathode is superficially oxidized as would be expected if only surface oxidation takes place. Direct evidence was also obtained for essentially complete order in annealed samples of Cs3Sb by x-ray diffraction. Although diffraction lines, indicative of order, were observed in the electron diffraction patterns of some simple Cs3Sb cathodes, their presence could not be correlated with any changes in photoelectric sensitivity.
Antimonide
Photoelectric effect
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Orthorhombic crystal system
Powder Diffraction
Crystal (programming language)
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The role of substrate reconstruction in the Cu(100)-(2\ifmmode\times\else\texttimes\fi{}2)-S system has been investigated using x-ray diffraction. It is found that the first-layer Cu atoms are laterally displaced away from the S atoms by 0.03\ifmmode\pm\else\textpm\fi{}0.01 \AA{}. This result resolves a recent controversy between a low-energy electron-diffraction study and an angle-resolved-photoemission extended-fine-structure analysis in favor of the electron-diffraction result.
Gas electron diffraction
Low-energy electron diffraction
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Superstructure
Reflection
Space group
Trigonal crystal system
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Polycrystalline U3O7 powder was synthesized by oxidation of UO2 powder under controlled conditions using in situ thermal analysis, and by heat treatment in a tubular furnace. The O/U ratio of the U3O7 phase was measured as 2.34 ± 0.01. The crystal structure was assessed from X-ray diffraction (XRD) and selected-area electron diffraction (SAED) data. Similar to U4O9-ε (more precisely U64O143), U3O7 exhibits a long-range ordered structure, which is closely related to the fluorite-type arrangement of UO2. Cations remain arranged identical to that in the fluorite structure, and excess anions form distorted cuboctahedral oxygen clusters, which periodically replace the fluorite anion arrangement. The structure can be described in an expanded unit cell containing 15 fluorite-like subcells (U15O35), and spanned by basis vectors A = ap - 2bp, B = -2ap + bp, and C = 3cp (lattice parameters of the subcell are ap = bp = 538.00 ± 0.02 pm and cp = 554.90 ± 0.02 pm; cp/ap = 1.031). The arrangement of cuboctahedra in U3O7 results in a layered structure, which is different from the well-known U4O9-ε crystal structure.
Fluorite
Selected area diffraction
Powder Diffraction
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For the reconstructions present during epitaxial growth, the diffraction experiment of choice is reflection high-energy electron diffraction (RHEED). In the RHEED geometry, the momentum transfer normal to the surface is small and can be varied by changing the angle of incidence at constant incident energy. The resulting rocking curve, or diffracted intensity versus angle of incidence, is comparable to a low-energy electron diffraction IV profile. We present rocking curve results for the 〈001〉 and 〈11̄0〉 principle azimuths for 10-keV diffraction experiments from GaAs(110). This known surface structure is the best tested structure to develop the analysis. The measurements are compared to a fully convergent dynamical N-beam RHEED calculation which includes as many propagating and evanescent beams as needed. An important issue is to determine what extent the diffraction is dominated by bulk features and single-layer resonances. This is addressed by selectively eliminating beams in the analysis and comparing beam emergence predictions to experimental observations.
Reflection
Low-energy electron diffraction
Kikuchi line
Momentum transfer
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By using the techniques developed by Taylor et al. [(1975) J. Mol. Biol. 92, 165-167] (freezing of the hydrated specimen before its insertion into the electron microscope and keeping it frozen throughout the diffraction experiment), it was possible to obtain a high-angle electron-diffraction pattern from collagen fibrils. This pattern is in good agreement with that obtained by high-angle X-ray diffraction. Electron diffraction will be very useful to study collagen, because the diffraction pattern from a carefully selected area of one fibril is now feasible.
Selected area diffraction
Collagen fibril
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