Showing papers on "Electron backscatter diffraction published in 1968"

The diffraction of low-energy electrons by crystals

Abstract: A formulation of the problem of low-energy electron diffraction from crystal surfaces is given using multiple scattering theory. Only elastic processes are considered. The resultant formulae are straightforward to compute and can be applied to model surfaces of arbitrary complexity, provided they are regular.

Dynamical effects in high‐voltage electron diffraction

Abstract: Relativistic versions of dynamical theories are surveyed and it is shown that electron diffraction phenomena become more dynamical at very high voltages. The results of diffraction experiments obtained with a 500 kV electron microscope are outlined. Specimens were magnesium oxide wedges and molybdenite films. The dynamical many-beam interactions were enhanced at 500 kV. The variation of Bragg width with the accelerating voltage was in agreement with the relativistic theory. The intensity reversal of Kikuchi bands, which is not remarkable below 100 kV, was clearly observed at 500 kV. The 3n + 1,0,0 reflections of molybdenite were found to be vanishingly weak, although it is not yet clear whether or not this is due to a dynamical effect. The appearance of many diffraction spots at high voltages seems to be due mainly to the decrease of the background intensity.

Interpretation of Electron Diffraction Patterns

Abstract: Many minerals and synthetic inorganic phases of low symmetry occur only as fine powders or fibre aggregates. In such cases, determination of the unit cell from X-ray data alone can be difficult or impossible, because of failure to resolve multiple reflections or to detect weak ones on polycrystalline X-ray photographs, together with uncertainties about systematically absent reflections. However, electron diffraction of small single crystals can be used for determination of the unit cell, enabling the X-ray lines to be indexed. Alternatively, electron diffraction of polycrystalline platy textures can sometimes be used, both to determine the unit cell and to provide indexed intensity data for use in structure analysis. d-spacings and intensities measured on X-ray patterns are, however, generally more accurate and reliable than electron-diffraction data. Correlation of both sources of data can therefore yield information not available from either technique alone.

Electron diffraction of U4O9

Abstract: Thin foils of U4O9 prepared from bulk single crystals of UO2 were investigated at 20 °C and between 300 °C and 1000 °C in the electron microscope. At 20 °C the complete reciprocal lattice of U4O9 has been established. Between about 550° and 700°C an order-partial disorder transformation has been found in most of the specimens investigated. This leads to a modified U−O phase diagram but at present it is not known if this new diagram or the old one represents metastable phase relations. Crystallographic relations between the low temperature phase and the partly disordered high-temperature phase of U4O9 − y have been deduced.

On the accuracy of orientation determination by selected area electron diffraction

Abstract: A simple, analytical method of calculating unique, accurate orientations from selected area electron diffraction patterns which contain spots or Kikuchi lines from more than one zone is described. ...

Mixed oxides prepared with an induction plasma torch

Abstract: Pure and mixed aluminium and chromium oxides have been prepared by passage of the halides through a radio-frequency oxidising plasma. The nature of the materials produced has been assessed by electron microscopy, electron diffraction, X-ray diffraction, magnetic susceptibility, and electron spin resonance. The products ware finely divided solids, containing mostly spherical particles in the size ranga 0.01 to 0.15μm; however, the chromia particles were often hexagonal platelets up to 0.4μm in diameter. The X-ray diffraction patterns were well defined, showingδ-alumina andα-chromia as major phases, andθ-alumina as a minor phase. The maximum degree of solid solution of chromium ions in theδ-alumina was limited to about 6% of the total cation content; solid solution of aluminium ions in theα-chromia phase was below the detection limit of 10%. The nature of the particles, the phases produced, and the degree of solid solution are shown to be consistent with a sequence of condensation which can be predicted from thermodynamic data.

Diffuse scattering in electron diffraction patterns. I. General theory and computational methods

Abstract: The solution of the n-beam dynamical theory of the diffraction of electrons by crystals is generalized to cover the case of diffraction by crystals containing defects and disorders, including thermal motion The conditions and assumptions under which practical computer calculations of diffuse intensities can be made are explored on the basis of the slice approach of Goodman and Moodie, although matrix methods are equally applicable It is shown that, if the range of correlation of the deviations from the perfect crystal lattice is small, the total diffuse scattering can be expressed in terms of dynamical factors which multiply the intensities calculated using the kinematical approximation Simple expressions are derived for the absorption coefficients which must be applied to the sharp Bragg reflexions to take account of the energy lost from them into the diffuse scattering The possibility that the intensity of diffuse scattering may show dependence on the range of correlation of the defects is discussed

Occurrence of Olivine in a Type I Carbonaceous Meteorite

Abstract: A REPORT of the discovery of olivine in the Orgueil carbonaceous meteorite1 aroused considerable scepticism2,3. The identification was based on selected area electron diffraction patterns obtained from two mineral fragments observed during an electron microscope examination of a powdered meteorite specimen. Because, with the electron microscope in use at that time, it was only possible to examine particles in the orientation which they had arbitrarily taken up on the specimen support film, each diffraction pattern gave only limited information about the unit cell parameters and space group of any individual unknown mineral. The possibility of using such restricted data to identify mineral species rests on the fact that many minerals develop pronounced and specific faces, either by growth or cleavage, so that different fragments of the same mineral tend to lie in a common orientation, and thus to exhibit the same type of single crystal diffraction pattern in the electron microscope. The match between patterns from terrestrial olivine samples and those from olivine in Orgueil was good, but it was clearly desirable to put the identification on a firmer basis, if possible, by obtaining for a single mineral particle the full complement of unit cell parameters and the space group.

X-ray and neutron diffraction studies of the high-temperature ?-phase of the AgBiSe2/AgBiS2 system

Abstract: An X-ray and neutron diffraction study of the high-temperature, NaCl-type β-phases of the AgBiSe2/AgBiS2 system was performed to determine if the crystal structures are ordered or random arrangements. X-ray diffraction studies established that the Ag and Bi species are present at random on the cation sites, and neutron diffraction studies established that the S and Se species are present at random on the anion sites.

Neutron Diffraction from Liquid Carbon Tetrachloride

Abstract: The structure of liquid carbon tetrachloride has been studied by neutron diffraction. The theoretical basis of neutron diffraction from multiatomic liquids is discussed in terms of polyatomic and molecular assemblages. The data is analyzed using these models. The parameters obtained in this study agree with those obtained from x‐ray and electron diffraction.

The early history of electron diffraction

Abstract: (1968). The early history of electron diffraction. Contemporary Physics: Vol. 9, No. 1, pp. 1-15.

Electron diffraction study of the molecular structure of trisilylarsine

Abstract: The molecular structure of gaseous trisilylarsine has been determined by the sector-microphotometer method of electron diffraction. The heavy-atom skeleton is found to be pyramidal; the As–Si distance is 2·355 ± 0·001 A and the Si–As–Si angle 93·79 ± 0·17°. The Si–H distance is 1·488 ± 0·011 A.