HREM Profile Image Interpretation In MgO Cubes

1985 ◽  
Vol 43 ◽  
pp. 64-65
Author(s):  
M. A. O'Keefe ◽  
J. C. H. Spence ◽  
J. L. Hutchison ◽  
W. G. Waddington
Keyword(s):  
Image Interpretation ◽  
Order Structure ◽  
Specimen Thickness ◽  
Crystal Thickness ◽  
Lattice Image ◽  
Axis Orientation ◽  
Structure Factors ◽  
Experimental Image ◽  
Simulated Images ◽  
Contrast Image

For the most credible interpretation of HREM images, it is desirable to match computed and experimental images against the variation of some parameter, usually either specimen thickness or defocus (and preferably both). MgO smoke forms in perfect crystalline cubes, so that the thickness, T, of a cube in <110> zone axis orientation can accurately be determined by geometry at each co-ordinate X (normal to the wedge edge) in the image. Figure 1 shows such an experimental 90° MgO wedge lattice image recorded in an exact <110> orientation using a JEOL 200CX. Dynamical multislice calculations for the thickness dependence of the inner beams important for imaging are shown (c) scaled to the abscissa of the experimental image (the crystal thickness, T, at any point is twice the X value shown). Analysis of this image by means of simulated images (not shown) shows the thin-crystal “structure” image changing to one containing vertical (220) fringes at A (due to strong second-order (111) x (111) interference), to a re verse-contrast image at C, to one with horizontal (002) fringes at B (from strong (111) x (111) interference), and to a “structure“-like image at D. Matching with simulated images provides a rare possibility for guantitative structure analysis by HREM, since the complex low order structure factors can be adjusted in the dynamical calculation until the contrast reversals and other turning points occur at the observed thicknesses.

Electron lattice image interpretation of V2O3 wedge shaped crystal

1978 ◽  
Vol 36 (1) ◽  
pp. 286-287
Author(s):  
M. Tanaka ◽  
A. Rocher ◽  
R. Ayroles ◽  
B. Jouffrey
Keyword(s):  
Image Interpretation ◽  
Theoretical Aspect ◽  
Zone Axis ◽  
Lattice Image ◽  
Hexagonal System ◽  
Axis Orientation ◽  
Vanadium Sesquioxide ◽  
Small Lattice ◽  
Experimental Image ◽  
Unit Cell Dimensions

The thickness dependence of a two dimensional lattice image with small lattice spacings (< 5 Å) has been discussed in the theoretical aspect by some authors(1) (2),(3) in the case of zone axis orientation. Our intention is to compare the experimental image with the theoretical one for a vanadium sesquioxide (V2O3) crystal not in the zone axis orientation but in more general orientations. This crystal, one of those which show the metal-insulator transition, is metallic and belongs to the space group at room temperature. The unit cell dimensions in the hexagonal system are a = 4.951 Å and c = 14.003 Å (4). The electron microscope used is a JEM-100C equipped with the fixed specimen stage and operated at 100 kV (Cs = 1.4 mm).Figure 1 shows a lattice image from a wedge shaped crystal. Especially remarkable is the recurrence of similar contrast in three thicknesses regions : < 50 Å,∽ 250Å,and ∽ 450Å.

High-Resolution Image Simulation of a Tilted Crystal

1990 ◽  
Vol 48 (1) ◽  
pp. 68-69
Author(s):  
C. J. D. Hetherington
Keyword(s):  
High Resolution ◽  
Crystal Thickness ◽  
Thin Specimen ◽  
Atomic Structures ◽  
Kikuchi Lines ◽  
High Resolution Images ◽  
Final Estimate ◽  
Simulated Images ◽  
Habit Planes

Most high resolution images are not directly interpretable but must be compared with simulations based on model atomic structures and appropriate imaging conditions. Typically, the only parameters that are adjusted, in addition to the structure models, are crystal thickness and microscope defocus. Small tilts of the crystal away from the exact zone axis have only rarely been considered. It is shown here that, in the analysis of an image of a silicon twin intersection, the crystal tilt could be accurately estimated and satisfactorily included in the simulations.The micrograph shown in figure 1 was taken as part of an HREM study of indentation-induced hexagonal silicon. In this instance, the intersection of two twins on different habit planes has driven the silicon into hexagonal stacking. However, in order to confirm this observation, and in order to investigate other defects in the region, it has been necessary to simulate the image taking into account the very apparent crystal tilt. The inability to orientate the specimen at the exact [110] zone was influenced by i) the buckling of the specimen caused by strains at twin intersections, ii) the absence of Kikuchi lines or a clearly visible Laue circle in the diffraction pattern of the thin specimen and iii) the avoidance of radiation damage (which had marked effects on images taken a few minutes later following attempts to realign the crystal.) The direction of the crystal tilt was estimated by observing which of the {111} planes remained close to edge-on to the beam and hence strongly imaged. Further refinement of the direction and magnitude of the tilt was done by comparing simulated images to experimental images in a through-focal series. The presence of three different orientations of the silicon lattice aided the unambiguous determination of the tilt. The final estimate of a 0.8° tilt in the 200Å thick specimen gives atomic columns a projected width of about 3Å.

Experimental Studies of Bonding Related Properties in Binary Intermetallics by Convergent Beam Electron Diffraction

2011 ◽  
Vol 1295 ◽  
Author(s):  
X. H. Sang ◽  
A. Kulovits ◽  
J. Wiezorek
Keyword(s):  
Electron Diffraction ◽  
Experimental Studies ◽  
Convergent Beam Electron Diffraction ◽  
Zone Axis ◽  
Order Structure ◽  
Electron Charge Density ◽  
Beam Electron ◽  
Structure Factors ◽  
Different Temperatures ◽  
Convergent Beam

ABSTRACTAccurate Debye-Waller (DW) factors of chemically ordered β-NiAl (B2, cP2, ${\rm{Pm}}\bar 3 {\rm{m}}$) have been measured at different temperatures using an off-zone axis multi-beam convergent beam electron diffraction (CBED) method. We determined a cross over temperature below which the DW factor of Ni becomes smaller than that of Al of ~90K. Additionally, we measured for the first time DW factors and structure factors of chemically ordered γ1-FePd (L10, tP2, P4/mmm) at 120K. We were able to simultaneously determine all four anisotropic DW factors and several low order structure factors using different special off-zone axis multi-beam convergent beam electron diffraction patterns with high precision and accuracy. An electron charge density deformation map was constructed from measured X-ray diffraction structure factors for γ1-FePd.

How Accurate is Qcbed ? -- the Case of Rutile (TiO2)

2001 ◽  
Vol 7 (S2) ◽  
pp. 368-369
Author(s):  
B. Jiang ◽  
J. Friis ◽  
J.C.H. Spence
Keyword(s):  
Systematic Errors ◽  
Mean Value ◽  
Order Structure ◽  
Data Sets ◽  
X Ray ◽  
Silicon Crystals ◽  
Structure Factors ◽  
The Mean ◽  
Mott Formula ◽  
Better Than

An accuracy of better than 1% is needed to measure the changes in charge density due to bonding. Here we report an accuracy up to 0.025% (random error) obtained in rutile crystal structure factors measurement by QCBED. This error is the standard deviation in the mean value obtained from ten data sets. Systematic errors may be present. Figure 1 gives an example of the (200) refinement results. Table 1 lists several low order structure factor refinement results. The accuracy of the measured electron structure factors was 0.1-0.2% but after conversion to x-ray structure factors, the accuracy for low orders improved due to the Mott formula [1] For (110) and (101) reflections, the accuracy in x-ray structure factors became 0.025% and 0.048% respectively. This accuracy is equivalent to that of the X-ray single crystal Pendellosung method on silicon crystals [2].The experiments were done on a Leo 912 Omega TEM.

Variable Resolution Fluctuation Electron Microscopy on Cu-Zr Metallic Glass Using a Wide Range of Coherent STEM Probe Size

2010 ◽  
Vol 17 (1) ◽  
pp. 67-74 ◽  
Author(s):  
Jinwoo Hwang ◽  
P.M. Voyles
Keyword(s):  
Electron Microscopy ◽  
Metallic Glass ◽  
Dark Field ◽  
Order Structure ◽  
Specimen Thickness ◽  
Variable Resolution ◽  
Wide Range ◽  
Scanning Transmission ◽  
Annular Dark Field ◽  
Fluctuation Electron Microscopy

AbstractWe report variable resolution fluctuation electron microscopy (VRFEM) measurements on Cu64.5Zr35.5metallic glass acquired using scanning transmission electron microscopy nanodiffraction using coherent probes 0.8 to 11 nm in diameter. The VRFEM results show that medium range atomic order structure of Cu64.5Zr35.5bulk metallic glass at the ∼1 nm scale has large fluctuations, but the structure becomes almost completely homogeneous at the 11 nm scale. We show that our experimental VRFEM data are consistent with two different models, the pair persistent model and the amorphous/nanocrystal composite model. We also report a new way to filter VRFEM data to eliminate the effect of specimen thickness gradient using high-angle annular dark field images as references.

scholarly journals On the experimental determination of low-order structure factors in TiAl by energy-filtered convergent-beam electron diffraction

1993 ◽  
Vol 51 ◽  
pp. 662-663
Author(s):  
S. Swaminathan ◽  
I. P. Jones ◽  
N. J. Zaluzec ◽  
D. M. Maher ◽  
H. L. Fraser
Keyword(s):  
Electron Diffraction ◽  
Charge Density ◽  
Structure Factor ◽  
Convergent Beam Electron Diffraction ◽  
Order Structure ◽  
Electron Charge Density ◽  
Beam Electron ◽  
Structure Factors ◽  
Convergent Beam ◽  
Low Order

It has been claimed that the effective Peierls stresses and mobilities of certain dislocations in TiAl are influenced by the anisotropy of bonding charge densities. This claim is based on the angular variation of electron charge density calculated by theory. It is important to verify the results of these calculations experimentally, and the present paper describes a series of such experiments. A description of the bonding charge density distribution in materials can be obtained by utilizing the charge deformation density (Δρ (r)) defined by(1) where V is the volume of the unit cell, Fobs is the experimentally determined low order structure factor and Fcalc is the structure factor calculated using the Hartree-Fock neutral atom model. To determine the experimental low order structure factors, a technique involving a combination of convergent beam electron diffraction (CBED) and electron energy loss spectroscopy (EELS) has been used.

Determination of Crystal Thickness from Relative Transmission Measurements

1975 ◽  
Vol 33 ◽  
pp. 242-243 ◽  
Author(s):  
D. C. Joy ◽  
D. M. Maher
Keyword(s):  
Amorphous Materials ◽  
Accurate Estimate ◽  
Foil Thickness ◽  
Specimen Thickness ◽  
Crystal Thickness ◽  
Crystalline Materials ◽  
Transmission Electron ◽  
Relative Transmission ◽  
History Of ◽  
Electron Intensity

An accurate knowledge of the specimen foil thickness often is required in quantitative transmission electron microscopy. The methods used for thickness determinations of thin crystalline materials (e.g. the trace method, thickness fringe counts and stereoscopic measurements) generally are selected according to the history of the specimen and nature of the microstructure. For amorphous materials a measurement of the relative transmission of electrons I/I0, where I is the transmitted and I0 the incident electron intensity, affords an accurate estimate of the specimen thickness. In this case, for a sufficiently large specimen thickness, I/I0 varies exponentially according to the mass thickness relationship e-μt, where μ is the mass absorption coefficient and t is the specimen thickness. The purpose of this paper is to demonstrate that the thickness of a crystalline specimen also may be determined accurately from a measurement of I/I0, provided that well defined diffracting conditions are used. The results presented here are for silicon.

Extension of the “thin-crystal” condition by small crystal tilts: Why HREM images of SiC polytypes always look tilted

1992 ◽  
Vol 50 (1) ◽  
pp. 116-117
Author(s):  
Michael A. O'Keefe ◽  
Velimir Radmilovic
Keyword(s):  
Silicon Carbide ◽  
Electron Microscope ◽  
High Resolution ◽  
Group Symmetry ◽  
Crystal Thickness ◽  
Resolution Electron ◽  
Microscope Images ◽  
High Resolution Electron Microscope ◽  
Simulated Images ◽  
Thin Crystal

Both experimental and simulated high-resolution electron microscope images of silicon carbide polytypes commonly exhibit symmetry changes in thicker crystal regions compared to the perfect (projected) space group symmetry of images from thin crystals. However, the changes predicted by simulation, and those found experimentally, are quite different.High-resolution transmission electron microscope images of silicon carbide polytypes were obtained with the JEOL ARM-1000 high-resolution electron microscope in the course of an investigation into a series of metal matrix composites. Like all HRTEM images of silicon carbide, these images failed to show the correct symmetry in the thicker parts of the specimen. Changes in image symmetry as crystal thickness is increased also occur when images of silicon carbide are simulated; for example, Smith and O'Keefe simulated images of polytypes of silicon carbide for crystals oriented so that the electron beam was precisely along the <1210> direction, and found marked departure from thin-crystal symmetry at thicknesses of the order of 150Å for an electron energy of 500keV. However, the lack of symmetry in their simulated images appears to be due to the presence of many second-order terms contributing to the intensity spectra of the thick-crystal images, whereas the symmetry changes in experimental images from thicker crystals are usually of the form that preserves the thin-crystal-like contrast for one set of “twin” spots, yet smears out the contrast of the other. A typical example of this latter effect can be seen in the image of the 6H variant of SiC shown in figure 1.

Improvement of precision in refinements of structure factors using convergent-beam electron diffraction patterns taken at Bragg-excited conditions

2021 ◽  
Vol 77 (4) ◽  
Author(s):  
B. Aryal ◽  
D. Morikawa ◽  
K. Tsuda ◽  
M. Terauchi
Keyword(s):  
Electron Diffraction ◽  
Reciprocal Lattice ◽  
Correlation Coefficients ◽  
Atomic Displacement ◽  
Convergent Beam Electron Diffraction ◽  
Order Structure ◽  
Beam Electron ◽  
Structure Factors ◽  
Atomic Displacement Parameters ◽  
Convergent Beam

A local structure analysis method based on convergent-beam electron diffraction (CBED) has been used for refining isotropic atomic displacement parameters and five low-order structure factors with sin θ/λ ≤ 0.28 Å−1 of potassium tantalate (KTaO3). Comparison between structure factors determined from CBED patterns taken at the zone-axis (ZA) and Bragg-excited conditions is made in order to discuss their precision and sensitivities. Bragg-excited CBED patterns showed higher precision in the refinement of structure factors than ZA patterns. Consistency between higher precision and sensitivity of the Bragg-excited CBED patterns has been found only for structure factors of the outer zeroth-order Laue-zone reflections with larger reciprocal-lattice vectors. Correlation coefficients among the refined structure factors in the refinement of Bragg-excited patterns are smaller than those of the ZA ones. Such smaller correlation coefficients lead to higher precision in the refinement of structure factors.

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