Showing papers on "Electron backscatter diffraction published in 1975"

Structure of graphite by neutron diffraction

Abstract: THE structure of graphite1 consists of layers of linked hexagons of carbon atoms. The layers are stacked in a ⋯ ABAB ⋯ sequence so that half the atoms in a layer are directly above and below carbon atoms in the adjoining layers, and half are directly above and below centres of the hexagons. The hexagonal unit cell contains atoms at the (0, 0, 1/4), (1/3, 2/3, 1/4), (0, 0,−1/4) and (2/3, 1/3,−1/4) positions. Modifications to this structure have been proposed by several workers. Lukesh2 reported diffraction patterns indicating a lower symmetry, and Pauling3 has proposed an orthorhombic structure in which the interconnected six-membered rings are distorted so that two-thirds of the bonds are longer. Pauling based his model on the high compressibility of the basal planes, and considerations of stacking bonds in the layered structure. Ergun4 suggested that data on pyrolitic graphite indicated an apparent quinoid structure in the graphite layers, but errors in his analysis have been reported5. X-ray and neutron diffraction studies of graphite were undertaken in our laboratory to study the details of the electron charge density and atomic motion in the graphite structure. The results of the neutron diffraction work show that all in plane carbon–carbon distances have the same value 1.422±0.001 A.

Electron Diffraction in the Electron Microscope

Abstract: Electron diffraction patterns are routinely obtained in the electron microscope and are used to gain quantitative information on the following.

Neutron powder diffraction study of the low-temperature phase of solid acetylene-d2

Abstract: Acetylene-d2 (CzDz) has two solid phases. The crystal structure of the low-temperature phase has been determined at 4.2 K by neutron powder diffraction. The structure is orthorhombic, with a = 6.193 (3), b = 6.005 (3), c= 5"551 (3) A, space group Acam, Z= 4. The linear molecules lie in layers parallel to (001) in slightly off-diagonal directions. The final agreement index Rz = 0.071. Bond lengths are C-C = 1-18 (1), C-D = 1.06 (2),~. The apparent shortening of the C-C bond compared with the value calculated from highresolution Raman spectra in the gas phase is attributed to the effect of molecular libration.

System for reflection electron microscopy and electron diffraction at intermediate energies

Abstract: A system employing medium energy electrons (1–15 keV) for scanning microscopy and diffraction intensity measurements has been constructed. The specimen is situated in an UHV environment and can be cleaned by argon ion sputtering and heated up to 1000 °C. The resolution is currently about 300 A for scanning images formed when secondary electrons are detected. The use of diffracted beams to form images is shown to provide new types of information regarding the structure and defects of crystal surfaces. Intensities and intensity profiles of diffracted beams can be measured for comparison with theoretical data and structure analysis.

Dynamic X-ray diffraction technique for measuring rheo-optical properties of crystalline polymeric materials

Abstract: A dynamic X-ray diffraction technique, which can follow the responses of polymer crystals (crystallization, orientation, and lattice deformation) to mechanical excitation of sinusoidal strain induced to bulk specimen, was described. The descriptions for such responses are qualitatively made by using a narrow sector technique, which can measure the X-ray diffraction intensity distribution at a particular phase angle of the sinusoidal strain as a function of static and dynamic strains, temperature, and angular frequency. A typical result is demonstrated in terms of the investigation of orientation crystallization phenomena of natural rubber vulcanizates. More quantitative descriptions can be made by using a half-circle sector technique, which can measure the in-phase and out-of phase components of the dynamic X-ray diffraction intensity distribution. From these, one can obtain the dynamic strain-induced crystallization and orientation coefficients and the dynamic response of lattice deformation of specific crystal plane both as function of temperature and frequency. After a brief survey of the principle of the half-circle sector technique, frequency dependence of the dynamic strain-induced crystallization coefficients of the (002) and (200) crystal planes of natural rubber vulcanizates is demonstrated in terms of the two frequency dispersion regions around 10−2 and 101 Hz at a room temperature. The former and latter dispersions must be correlated with the crystallization processes of the so-calledα- andγ-filaments, respectively. In addition, frequency and temperature dispersions of the dynamic strain-induced orientation coefficient and the dynamic response of lattice deformation of the (110) crystal plane of polyethylene are demonstrated in relation to the so-calledα 1 andα 2 dispersions of dynamic mechanical modulus function of this material.

Energy‐dispersive spectroscopic methods applied to X‐ray diffraction in single crystals

Abstract: Two single-crystal energy-dispersive crystallographic methods (the fixed-crystal method and the rotating-crystal method) are described and investigated. Formulae for integrated intensities are derived for mosaic and perfect single crystals. Experimental results and a comparison between measured and calculated integrated intensities for a perfect germanium crystal are given. Special features and possible applications of the methods are discussed.

Electron microscopical study of pyro-carbon microneedles grown by high field pyrolysis

Abstract: Investigations with transmission electron microscopy and selected area electron diffraction demonstrated that high field polymerization products grown at room temperature with benzonitrile are of needle shape and have an amorphous structure. High field pyrolyzation products which originated at high temperatures (1200° C) display a morphologically similar, but mechanically more stable composition as well as a larger number of needles per unit area. Their structure corresponds to an arrangement of layers of graphitelike regions concentric to the needle axis. According to the evaluation of the electron diffraction diagrams obtained, it was possible to image directly in the electron microscope the curved lattice planes of the high field pyrolyzation products, in close agreement with the net plane distance of the 001 lattice planes of graphite (3.35 A).

Electron diffraction from areas less than 3 nm in diameter

Abstract: A method of obtaining rocking‐beam microarea electron diffraction patterns is described employing the electron optics of a single‐field condenser‐objective lens in a commercial scanning transmission electron microscope Theoretical and experimental evidence is presented showing that it is possible to obtain diffraction patterns from areas less than 3 nm in diameter with minimum lattice spacings of 014 nm This is demonstrated with a specimen consisting of small gold particles evaporated on a carbon substrate

X-ray Diffraction Study of Molten Te and Tl-Te Alloys

Abstract: Abstract X-ray diffraction patterns have been obtained from molten Te at 470, 520 and 570 °C. The heights of the peak maxima in the structure factor were much the same in contrast with those of typical molten metals such as sodium. Molten Tl-Te alloys have been studied by X-ray diffraction for the alloy compositions 25, 33.3, 50, 60 and 75 at% Te at 500 °C and at about 20 °C above the liquidus. The total structure factors for the 25 and 33.3 at% Te alloys were almost the same as that of pure Tl. This implies that the atomic arrangement of these molten alloys is very close to that of pure Tl. Although a drastic change is not found in the general form of the structure factor, the parameter of the range of local atomic order abruptly increases on passing from Tl2Te to more Te-rich alloys. The three partial structures were also evaluated from the observed X-ray intensities assuming that each partial structure is independent of the relative abundance of the constituent elements in the alloys.

Direct structure determination of asymmetric membrane systems from X-ray diffraction

Abstract: A theoretical analysis of X-ray diffraction from asymmetric planar systems is given. Phase information is obtained from the continuous intensity function from such a system. Although a unique phase function cannot be determined, it is possible to derive the relatively small number of phase solutions which are consistent with the observed diffraction.

Electron diffraction studies of biological membrane and lipids

Abstract: IN diffraction studies of biological membranes, X-ray diffraction has been used more frequently than electron diffraction in spite of certain advantages inherent in the latter technique. Electron diffraction enables the study of a small area (about 1 µm2) of a very thin specimen, and the collection of experimental data within a short time. Moreover, the morphology and degree of purity of the specimen can often be investigated. It seems possible to overcome the main problems arising from the application of the electron diffraction technique to biological materials, namely the difficulty of studying hydrated materials and the damage caused to the specimen by the electron beam, and a study of wet biological membranes has been reported1. In this investigation dry specimens have been studied. Radiation damage was largely overcome by using minimum beam current, by maximally overfocusing the first condenser of the electron microscope, and by moderately overfocusing the second condenser. To minimise the temperature rise of the specimen, caused by inellastically scattered electrons, a cooled specimen holder was used. To facilitate heat transport, the specimen grids were covered with aluminium instead of carbon. A Philips 301 G electron microscope equipped with an anti-contamination device was used and the diffraction patterns were recorded at 100 kV on a highly sensitive X-ray film, Kodirex. All diffractograms were recorded at a specimen holder temperature of about −60 °C, which was regarded as a fair estimate of the temperature of the specimen itself.