Embedded unit cell homogenization model for localized non-periodic elasto-plastic zones

ALCHEMI (Atom Location by CHanneling Enhanced Microanalysis) enables the site occupancy of atoms in single crystals to be determined. In this article the fundamentals of the method for both EDS and EELS will be discussed. Unlike HRTEM, ALCHEMI does not place stringent resolution requirements on the microscope and, because EDS clearly distinguishes between elements of similar atomic number, it can offer some advantages over HRTEM. It does however, place certain constraints on the crystal. These constraints are: a) the sites of interest must lie on alternate crystallographic planes, b) the projected charge density on the alternate planes must be significantly different, and c) there must be at least one atomic species that lies solely on one of the planes.An electron beam incident on a crystal undergoes elastic scattering; in reciprocal space this is seen as a diffraction pattern and in real space this is a modulation of the electron current across the unit cell. When diffraction is strong (i.e., when the crystal is oriented near to the Bragg angle of a low-order reflection) the electron current at one point in the unit cell will differ significantly from that at another point.

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The External Shape of Human γ GI Immunoglobulin from Crystal Sections

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s042482010006619x ◽

1971 ◽

Vol 29 ◽

pp. 428-429

Author(s):

L. W. Labaw

Keyword(s):

Molecular Weight ◽

Sodium Phosphate ◽

Osmium Tetroxide ◽

Unit Cell ◽

Monoclinic Space Group ◽

X Ray ◽

Large Molecular Weight ◽

Cell Dimensions ◽

Unit Cell Dimensions ◽

Crystallographic Axes

Crystals of a human γGl immunoglobulin have the external morphology of diamond shaped prisms. X-ray studies have shown them to be monoclinic, space group C2, with 2 molecules per unit cell. The unit cell dimensions are a = 194.1, b = 91.7, c = 51.6Å, 8 = 102°. The relatively large molecular weight of 151,000 and these unit cell dimensions made this a promising crystal to study in the EM.Crystals similar to those used in the x-ray studies were fixed at 5°C for three weeks in a solution of mother liquor containing 5 x 10-5M sodium phosphate, pH 7.0, and 0.03% glutaraldehyde. They were postfixed with 1% osmium tetroxide for 15 min. and embedded in Maraglas the usual way. Sections were cut perpendicular to the three crystallographic axes. Such a section cut with its plane perpendicular to the z direction is shown in Fig. 1.This projection of the crystal in the z direction shows periodicities in at least four different directions but these are only seen clearly by sighting obliquely along the micrograph.

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The orientation of sickle hemoglobin fibers within fascicles

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100104066 ◽

1988 ◽

Vol 46 ◽

pp. 400-401

Author(s):

Christopher A. Miller ◽

Bridget Carragher ◽

William A. McDade ◽

Robert Josephs

Keyword(s):

Sickle Cell ◽

Sickle Cell Anemia ◽

Relative Orientation ◽

Unit Cell ◽

Red Cell ◽

High Ph ◽

Sickle Hemoglobin ◽

Polar Crystal ◽

Helical Configuration

Highly ordered bundles of deoxyhemoglobin S (HbS) fibers, termed fascicles, are intermediates in the high pH crystallization pathway of HbS. These fibers consist of 7 Wishner-Love double strands in a helical configuration. Since each double strand has a polarity, the odd number of double strands in the fiber imparts a net polarity to the structure. HbS crystals have a unit cell containing two double strands, one of each polarity, resulting in a net polarity of zero. Therefore a rearrangement of the double strands must occur to form a non-polar crystal from the polar fibers. To determine the role of fascicles as an intermediate in the crystallization pathway it is important to understand the relative orientation of fibers within fascicles. Furthermore, an understanding of fascicle structure may have implications for the design of potential sickling inhibitors, since it is bundles of fibers which cause the red cell distortion responsible for the vaso-occlusive complications characteristic of sickle cell anemia.

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Kinematical and Dynamical CBED Pattern Models: Increasing the Accuracy of the Unit-Cell Parameter Measurements Based on CBED Patterns

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100136301 ◽

1990 ◽

Vol 48 (2) ◽

pp. 540-541

Author(s):

I.N. Yadhikov ◽

S.K. Maksimov

Keyword(s):

Reciprocal Lattice ◽

Unit Cell ◽

Reciprocal Lattice Vector ◽

Unit Cell Parameters ◽

Lattice Vector ◽

Cell Parameters ◽

Accuracy Of Measurements ◽

The Difference ◽

Image Position ◽

Convergent Beam

Convergent beam electron diffraction (CBED) is widely used as a microanalysis tool. By the relative position of HOLZ-lines (Higher Order Laue Zone) in CBED-patterns one can determine the unit cell parameters with a high accuracy up to 0.1%. For this purpose, maps of HOLZ-lines are simulated with the help of a computer so that the best matching of maps with experimental CBED-pattern should be reached. In maps, HOLZ-lines are approximated, as a rule, by straight lines. The actual HOLZ-lines, however, are different from the straights. If we decrease accelerating voltage, the difference is increased and, thus, the accuracy of the unit cell parameters determination by the method becomes lower.To improve the accuracy of measurements it is necessary to give up the HOLZ-lines substitution by the straights. According to the kinematical theory a HOLZ-line is merely a fragment of ellipse arc described by the parametric equationwith arc corresponding to change of β parameter from -90° to +90°, wherevector, h - the distance between Laue zones, g - the value of the reciprocal lattice vector, g‖ - the value of the reciprocal lattice vector projection on zero Laue zone.

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Structure Information From Electron Diffraction Intensities

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100136180 ◽

1990 ◽

Vol 48 (2) ◽

pp. 516-517

Author(s):

J. Gjønnes ◽

N. Bøe ◽

K. Gjønnes

Keyword(s):

Computational Effort ◽

Unit Cell ◽

Small Unit ◽

Structure Information ◽

Dispersion Surface ◽

Integrated Intensity ◽

The One ◽

Image Position ◽

Convergent Beam ◽

Beam Patterns

Structure information of high precision can be extracted from intentsity details in convergent beam patterns like the one reproduced in Fig 1. From low order reflections for small unit cell crystals,bonding charges, ionicities and atomic parameters can be derived, (Zuo, Spence and O’Keefe, 1988; Zuo, Spence and Høier 1989; Gjønnes, Matsuhata and Taftø, 1989) , but extension to larger unit cell ma seem difficult. The disks must then be reduced in order to avoid overlap calculations will become more complex and intensity features often less distinct Several avenues may be then explored: increased computational effort in order to handle the necessary many-parameter dynamical calculations; use of zone axis intensities at symmetry positions within the CBED disks, as in Figure 2 measurement of integrated intensity across K-line segments. In the last case measurable quantities which are well defined also from a theoretical viewpoint can be related to a two-beam like expression for the intensity profile:With as an effective Fourier potential equated to a gap at the dispersion surface, this intensity can be integrated across the line, with kinematical and dynamical limits proportional to and at low and high thickness respctively (Blackman, 1939).

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