Showing papers on "Electron backscatter diffraction published in 1978"

Charge-density waves in NbSe3 at 145K: crystal structures, X-ray and electron diffraction studies

Abstract: The crystal structure of NbSe3 has been refined from single crystal X-ray diffraction data. It has a monoclinic symmetry with lattice parameters: a=10.009 AA, b=3.4805 AA, c=15.629 AA, beta =109.47 AA, space group P21/m and six formulae per unit cell. The crystal structure of NbSe3 as determined at 100K shows that the 145K transition in the electrical resistivity is not accompanied by a structural distortion. Except for the decrease due to thermal contraction all interatomic distances are found to be the same. This is compatible with the proposed model of the charge-density-waves formation which has been put forward in order to explain the physical properties of the 145K transition. Electron diffraction pictures taken above and below the transition give direct evidence of the charge-density-waves formation at the transition. At 120K in agreement with Tsutsumi et al. (1977) the pictures contain superstructure spots at the (h,k+or-0.243, l) positions. Contrary to what has been reported by the same authors, the authors have found that at 3K above the transition the electron diffraction pictures do not contain the diffuse scattering streaks corresponding to the planes at k'=k+or-0.243, which would indicate fluctuations of the CDW above the transition.

X-Ray Diffraction Measurement of Net Atomic and Molecular Charges

Abstract: A variety of physical methods lead to information directly or indirectly related to the net charge on a molecule or molecular fragment. But there is often a conceptual difficulty in interrelating such measurements to each other, or indeed in deducing a net charge from a property that is charge-dependent. For example, experimental bond lengths depend on the relative population of bonding and antibonding orbitals and are insensitive to the occupancy of orbitals that are nonbonding in the regions examined. Similarly, ESCA inner electron ionization potentials depend on the electrostatic interaction between electrons and are therefore not a direct measurement of total charge. Measurements of electron spin resonance or of the field dependence of the second moment of NMR lines give the spin density which can be related to total charge only under certain assumptions regarding spin pairing. The electronic charge will here be defined as the total number of electrons in the volume associated with a molecule or molecular fragment. Once this volume has teen defined, this charge may be derived from experimental time-averaged electron densities, which can be obtained from accurate diffraction data. We will review some aspects of this technique and compare pertinent results with those from other methods.

In situ intercalation of TaS2 in the electron microscope

Abstract: Electron diffraction has been used to study the formation of a variety of complex superlattices in the IT, 2H and 4Hb polytypes of TaS2 during intercalation with hydrazine gas (N2H4), in situ in the electron microscope. Results are presented over the temperature range 15 K-360 K and are interpreted in terms of scattering from periodic lattice distortions linked to charge density waves. Comparisons are also made with similar effects in both the as-grown crystals and TaS2 samples containing other adducts.

Neutron diffraction investigation and structural model for liquid gallium from room temperature up to 1 303 K

Abstract: 2014 Neutron scattering measurements on liquid gallium have been carried out at 293, 838 and 1 303 K on the H10 spectrometer of the EL3 reactor at Saclay. The obtained structure factors and pair correlation functions are compared to various published data. A quasi-crystalline model related to the 03B2 monoclinic form of gallium is proposed in order to explain the shape of the structure factor of liquid gallium and its behaviour versus temperature. LE JOURNAL DE PHYSIQUE TOME 39, MAI 1978,

Transmission Electron Diffraction Intensities from Real Organic Crystals: Thin Plate Microcrystals of Paraffinic Compounds

Abstract: Solvent-grown microcrystals of long polymethylene compounds packing in the O⊥ methylene subcell are found to have some of the defect structures seen for crystalline polyethylene. Crystals composed either of rectangular layers or of oblique layers both exhibit the growth around a screw dislocation characterized earlier by other workers. In addition, rectangular layer crystals of orthorhombic n-hexatriacontane give evidence of intralamellar edge dislocations. When rectangular layer crystals of cetyl palmitate are grown from hot ethanol, the same surface corrugations are found as seen earlier for polyethylene and orthorhombic paraffins. Oblique layer crystals have not demonstrated these latter two defect structures. All crystals show evidence of appreciable bending. As was found to be true for rectangular layers of n-paraffins, bending has some influence on the electron diffraction intensities from an oblique layer. Another important factor is a perturbation due to defects along long chain axes which disrupts the ordered packing of polar regions, and causes incoherent scatter from these regions. In addition, observed screw dislocations give a nonuniform end plane, causing isolation of layers from one another. Diffraction models accounting for bending or isolation of chain layers in an oblique layer both explain the observed near invariance of certain intensities which are due to the coherent scattering from the line grating formed by rows of carbon atoms in the tilted chain aggregate. Forbidden reflections predicted by a bending model are less important than those anticipated by the multiple scattering from a laminated layer model. As these forbidden reflections are mostly identified for multilayer crystals and since crystal structure analysis indicates most of the long chain in a oblique monolayer to be diffracting coherently, the concentration of Reneker-type defects in the centers of chains is probably not great enough to isolate small chain length laminae within a monolayer.

A description of electron diffraction from higher-order Laue zones

Abstract: The usual Bloch wave description of electron diffraction from higher-order Laue zones requires the solution of a quadratic eigenvalue equation. In this note we show that in the high-energy case this eigenvalue equation reduces to linear form.

Axial Channeling in Electron Diffraction

Abstract: Abstract Kossel patterns from Silicon and Niobium were obtained with a convergent electron beam. An intensity maximum in the direction of the zone axes [001] and [111] of Nb was interpreted as axial channeling. The intensity distribution in Kossel patterns was calculated by means of the Bloch wave picture of the dynamical theory of electron diffraction. Particularly zone axis patterns were calculated for different substance-energy combinations and they were compared with experimental observations. The intensity distribution in the calculated Kossel patterns was very sensitive to the model of absorption and it was found that a treatment of the absorption close to the model of Humphreys and Hirsch [Phil. Mag. 18, 115 (1968)] gave the best agreement with the experimental observations. Furthermore it is shown which Bloch waves are important for the intensity distribution in the Kossel patterns, how they are absorbed and how they change with energy.

Variation of the Yield of Electron Emission from Single Crystals with the Diffraction Condition of Exciting X-Rays

Abstract: The yield of photoelectrons and Auger electrons emitted from single crystals shows an anomalous change around the Bragg angle when X-rays satisfy the diffraction condition It is explained by the fact that the amount of electron emission is proportional to the intensity of X-ray wave fields formed on the atomic planes near the crystal surface The case of the symmetric 220 Bragg-case diffraction of silicon with Cu Kα radiation is studied in detail and several other cases are also described Finally the possible applications of this technique are discussed briefly

Single-crystal X-ray diffraction study on the hydrogen ordering in Ta2H

Abstract: A crystal of Ta2H with a single ordered domain has been prepared by the reaction of hydrogen gas with a tantalum single-crystal. Precession and Weissenberg photographs show the superlattice reflections which are indexed in terms of an orthorhombic unit cell with lattice parameters A = a[2(1 +cos γ)]1/ 2, B = c and C = a[2(1−cos γ)]1/2, where a(= b) = 3.359, c = 3.398 A and γ = 90.3° are the parameters of the host monoclinic tantalum lattice. Quantitative intensity measurements indicate that these reflections arise mostly from the systematic displacement of metal atoms caused by the ordered hydrogen arrangement. The amount of displacement has been determined as 0.086 A, which is a little larger than a previous neutron diffraction result (0.057 A) on Ta2D.

Structure of Amorphous Polymers

Abstract: A critical survey is made of experimental results obtained by the application of direct structural methods — X-ray, electron, and neutron diffraction, electron microscopy, and several other physical methods — to amorphous linear polymers. A list of 162 references is included.