Showing papers on "Electron backscatter diffraction published in 1983"

Many-Beam Calculation of Reflection High Energy Electron Diffraction (RHEED) Intensities by the Multi-Slice Method

Abstract: In order to calculate RHEED intensities by using the dynamic electron diffraction theory, crystals are sliced in thin stabs parallel to the surface. The crystal potential in each stab is taken as constant the normal to the surface. The RHEED intensities are calculated from the products of matrices obtained from Bethe's dynamic diffraction theory for each slab.

Polymer structure determination using electron diffraction techniques

Abstract: The crystallographic structure of organic crystals is most commonly studied using x-ray diffraction (XRD) techniques. Unfortunately, rather large crystals, at least 106 µm3. are required for XRD analysis, and it is often quite difficult and sometimes impossible to prepare such large crystals. On the other hand, electron diffraction techniques, although not nearly as precise as XRD, do afford the capability of studying much smaller crystals. The minimum size for electron diffraction is on the order of 10-3 µm3 (0.1 µm2 area by 0.01 µm thick). Since most polymer crystals are very sensitive to radiation damage caused by the beam in the electron microscope, special precautions must be taken to minimize beam damage to the specimen. Our approach to minimizing radiation damage, while still obtaining usable diffraction data, is described in terms of using the condenser-objective lens optics of the Philips 301 S(TEM) electron microscope. Three examples of the application of electron diffraction structure analysis are given. These include the structures of halogenated polysulfur nitride (SN)x, neutral α,α'-polypyrrole, and poly(p-hydroxybenzoic acid) (PHBA).

Use of high-symmetry zone axes in electron diffraction in determining crystal point and space groups

Abstract: A sequence of steps is given for making use of the information available on convergent-beam electron diffraction patterns from high-symmetry zone axes for crystal point- and space-group determination.

Neutron Diffraction Study on the Structure of Liquid Cs-Sb Alloys

Abstract: Abstract By neutron diffraction experiments the total structure factors and the total pair correlation functions of liquid Cs-Sb alloys containing 85, 75, 65, and 50 at% Cs, respectively, were determined. The structural results confirm the non metallic properties of Cs-Sb melts. The correspondence of the nearest neighbour atomic arrangement in liquid Cs75Sb25 and in the solid compound semiconductor Cs3Sb suggests a similar type of bonding, namely by valence bonds and ionic forces simultaneously. The stability of this compound in the molten state leads to a microsegregation tendency between compound forming regions and excess Cs in the concentration range from pure Cs up to 25 at% Sb, which is established by a small angle scattering effect. Proceeding from Cs75Sb25 to Cs50Sb50, a continuous change in the structure takes place. Covalently bonded Sb chains are formed just as found in the corresponding solid compounds ASb (A = alkali metal). An additional diffraction peak in front of the main peak of the structure factors within this composition range implies the formation of rather large molecular clusters in the alloys.

Dynamical electron diffraction from elastically bent organic crystals

Abstract: Calculation of dynamical structure factors from a periodically curved crystal lattice have been carried out for the paraffin n-hexatriacontane, C36H74, and the linear polymer anhydrous nigeran, poly[(1 → 3)-α-d- maltose], (C12H20O10)n. These are compared to dynamical structure factors from a fiat crystal, and also to kinematic structure amplitudes from a bent lattice in order to determine the validity of considering each experimentally observed data perturbation separately. Individual treatment of these effects is found to be a good approximation so long as the crystal thickness and bend deformations are each small.

Characterization of the current-blackening phenomena in scandia stabilized zirconia using transmission electron microscopy

Abstract: The nature of blackening of 8 mol % Sc2O3-ZrO2, caused by imposing high current densities on this material at 800° C and PO, < 10−7 atm, has been investigated by transmission electron microscopy. The material studied was in the form of hot pressed polycrystalline discs with equiaxed grains 2 to 4 μm in size with a cubic fluorite structure. There is also evidence of small coherent precipitates of tetragonal phase embedded within the matrix of the cubic fluorite phase. The current-blackened samples contained numerous polycrystalline aggregates, with crystallites 10 to 20 nm in size, and they were usually found at the grain boundaries. The lattice parameter of this phase is virtually identical to that of the original cubic-fluorite matrix phase. The electron diffraction pattern showed, however, weak reflections forbidden by the fcc symmetry of the parent phase. These reflections can only be indexed as primitive cubic, and it is proposed that grain boundary phase consists partly of non-stoichiometric zirconium suboxides (ZrO2−x). The analysis of energy dispersive X-ray spectrum and electron diffraction patterns of the polycrystalline phase shows no evidence of the presence of metallic zirconium, as has been postulated previously.

Complex Formation in an Aqueous ZnBr2 Solution Based on Electron Diffraction, X-ray Scattering and Raman Spectra

Abstract: Abstract Electron and X-ray scattering as well as Raman spectra of as aqueous solution of ZnBr2 with a Br-/Zn++ ratio of 3:1 have been studied. The existence of hexaaquo, di-, tri-and tetra-bromo complexes in solution has been established by all three methods. Octahedral hexaaquo and tetrahedral Zn(H2O)4-n Brn complexes with n=2, 3, 4 are consistent with electron and X-ray structure functions. The inner sphere type complexes have interatomic distances of 2.2 Å for Zn++ -H2O, 2.4 Å for Zn++ -Br-, 3.93 Å for Br--Br- and 2.91 Å for W-W interactions with average numbers of contacts 2.4, 2.22, 1.33 and 1.1 respectively.

The doping method in quantitative X‐ray diffraction phase analysis. Addendum

Abstract: A specific case of the doping method [Popovic & Gržeta-Plenkovic (1979). J. Appl. Cryst. 12, 205–208] is elaborated, in which the weight fraction of a crystalline component in the multicomponent system can be determined from the measurement of diffraction line intensities of that component only. Optimum conditions to attain maximum accuracy of the doping method are discussed.

Laser diffraction of oriented fibrinogen molecules

Abstract: Fibrinogen is an extended protein, organized in a few characteristic domains, which possesses various sites for precise and strong interactions with enzymes, metals and neighboring fibrin( ogen) molecules. The overwhelming information accumulated in recent years about the chemical and immunological characteristics of the molecule and the elucidation of the complete sequence of human fibrinogen provided a more detailed understanding of the fibrinogenfibrin conversion and the accommodation of the different domains in the aggregation process. However, the overall structure of fibrinogen is still a controversial subject and several contradictory models have been discussed. The three-nodular structure proposed by Hall and Slayter in 1959 and confirmed in recent publications 2 s reflects best our knowledge of fragments obtained by enzymatic or chemical cleavage and of the refined primary structure. X-ray crystallographic analysis is unquestionably the method of choice to obtain a detailed three-dimensional structure. Cohen and coworkers 4, succeeded in producing crystals of partially digested bovine and rabbit fibrinogen. An X-ray structure analysis of these crystals has not yet been published. Electron microscopy has been shown to be a reasonable approach to gain information about the gross structure and especially the alignment of the single molecules in fibrinogen aggregates and in fibrin fibers.6-8 In this paper we present electron microscopic studies on needle-like (para)crystals of native human fibrinogen and describe the evaluation and interpretation of these electron micrographs by laser diffraction and by autocorrelation.

Poly(ethylene sulfide): Reevaluation of the electron diffraction structure analysis

Abstract: The single-crystal electron diffraction data of poly(ethylene sulfide) reported by Hasegawa, Claffey, and Geil have been reinvestigated. Taking into account possible crystal bending effects and dynamical scattering leads to an improved structural model. The final agreement factor of R = 0.21 is considerably better than that determined in the original analysis, R = 0.33, attesting to the quality of the zonal electron diffraction data obtained by these investigators.

Electron self-energy in a metal measured from a beam-threshold effect in low energy electron diffraction

Abstract: Observation, pour la face (111) de Cu, d'une structure fine dans le diagramme de diffraction, que l'on relie a l'emergence des faisceaux et qui determine des points de la courbe potentiel interne-energie, virtuellement sans influence de la relaxation en surface

The symmetry of convergent-beam electron diffraction patterns from bicrystals: erratum

Abstract: A discussion of the relationship between the symmetry of bicrystals and the resulting symmetry of convergent-beam electron diffraction (CBED) patterns is presented. For this purpose bicrystal symmetry is defined from the symmetry of the dichromatic pattern or complex formed by the interpenetrating lattices or structures of the individual crystals in a bicrystal. The interrelation between the possible coloured point groups and the diffraction groups, characterizing the symmetry of a CBED pattern, is established. These results are illustrated by a determination of the symmetry of thin twinned Au crystals from suitable CBED patterns. These experiments give information about the state of relative translation of the two crystals at the grain boundary and are consistent with the state of zero translation expected for a (111) coherent twin boundary in Au. It is pointed out that the interpretation of the CBED pattern symmetry may be complicated by a non-ideal location of the boundary plane in a bicrystal.

X-ray diffraction study of short-range order in a disordered Au3Cu alloy

Abstract: A single crystal of 'orange' 1802 at 9.6 (3) GPa and 297 (1) K was produced in a Merrill-Bassett diamondanvil high-pressure cell and examined by X-ray diffraction. Pressure was determined with the rubyfluorescence method. The unit cell is orthorhombic, space group Fmmm, lattice constants a = 4.2151 (6), b = 2.9567 (4), c = 6.6897 (17)A, molar volume = 12.56 x 10 -6 m 3 mol -~ with four molecules per unit cell. The charge densities of the molecules overlap or distort to a considerable degree. Generalized scattering factors were used in an aspherical-atom least-squares * Work performed under the auspices of the US Department of Energy. 0567-7408/83/020153-05501.50 refinement. For five refined parameters and 24 observations the final R w is 0.056. A correction is made to our previous work on fl-O2 at 5.5 GPa and 299 K.

Analysis of the incommensurate structure in Sr2Nb2O7 by electron microscopy and convergent‐beam electron diffraction

Abstract: The incommensurate lattice modulation in Sr2Nb20 7 has been studied by the high-resolution dark-field technique, which revealed curved lattice fringes showing the irregularity of the modulation wave inside the crystal on a relatively large scale. Convergent-beam electron diffraction patterns were analyzed to derive the symmetry of the incommensurate lattice modulation and that of the fundamental lattice. The results were compared with the previously proposed atom-displacement model.