Lomer Dislocations in Al: Deformation Field Analysis

1994 ◽  
Vol 47 (6S) ◽  
pp. S321-S325
Author(s):  
Rob Phillips
Keyword(s):  
High Resolution ◽  
Microscopic Analysis ◽  
Field Analysis ◽  
Atomistic Simulations ◽  
Image Simulation ◽  
Strain Fields ◽  
High Resolution Data ◽  
Trial Structure ◽  
Traditional Approaches

We examine two issues that arise in connection with the description of dislocations in metals. First, the experiments of Mills and Stadelmann on Lomer dislocations in Al are considered in light of atomistic simulations of their core structures. It is shown that the results of such a microscopic analysis are consistent with those obtained earlier on the basis of image simulation. Secondly, we examine these Lomer dislocations using the recently developed technique of Computational Fourier Transform Moire` (CFTM) analysis. This technique allows for a determination of the strain fields near a defect directly from a high-resolution image without the necessity of obtaining a trial structure for the defect, complementing traditional approaches to the interpretation of high-resolution data.

Combined Experimentaland Theoretical Determination of the Atomic Structure of the (310) Twin in NB

1991 ◽  
Vol 238 ◽  
Author(s):  
Geoffrey H. Campbells ◽  
Wayne E. King ◽  
Stephen M. Foiles ◽  
Peter Gumbsch ◽  
Manfred Rühle
Keyword(s):  
High Resolution ◽  
Theoretical Models ◽  
High Resolution Electron Microscopy ◽  
Interatomic Potentials ◽  
Image Simulation ◽  
Theoretical Determination ◽  
Atomic Structures ◽  
Resolution Electron ◽  
High Resolution Images

ABSTRACTA (310) twin boundary in Nb has been fabricated by diffusion bonding oriented single crystals and characterized using high resolution electron microscopy. Atomic structures for the boundary have been predicted using different interatomic potentials. Comparison of the theoretical models to the high resolution images has been performed through image simulation. On the basis of this comparison, one of the low energy structures predicted by theory can be ruled out.

The use of Mo Kα radiation in the assignment of the absolute configuration of light-atom molecules; the importance of high-resolution data

2014 ◽  
Vol 70 (4) ◽  
pp. 660-668 ◽  
Author(s):  
Eduardo C. Escudero-Adán ◽  
Jordi Benet-Buchholz ◽  
Pablo Ballester
Keyword(s):  
High Resolution ◽  
Crystal Structures ◽  
Structure Determination ◽  
Great Majority ◽  
Data Sets ◽  
Light Atom ◽  
High Resolution Data ◽  
The Absolute ◽  
Absolute Structure

Recent studies have confirmed the usefulness of the Hooft and Parsons methodologies for determination of the absolute crystal structures of enantiopure light-atom compounds using CuKα radiation. While many single-crystal diffractometers used for small-molecule structure determination are equipped with molybdenum anodes, use of data from such instruments for the absolute structure determination of light-atom crystal structures is rarely documented and has often been found to be unsuccessful. The Hooft and Parsons methodologies have been applied to 44 data sets obtained from single crystals containing light-atom molecules of known chirality using Mo Kαradiation. Several factors influencing the calculation of accurate and precise values for the Hooft and Parsons parameters obtained from these data sets have been identified, the inclusion of high-resolution diffraction data being particularly important. The correct absolute structure was obtained in all cases, with the standard uncertainties of the final absolute structure parameters below 0.1 for the great majority.

Advances in image simulation for high-resolution TEM

1995 ◽  
Vol 53 ◽  
pp. 38-39
Author(s):  
Michael A. O'Keefe
Keyword(s):  
High Resolution ◽  
Structure Determination ◽  
Incident Beam ◽  
Hrtem Image ◽  
Image Simulation ◽  
Lattice Image ◽  
Transmission Electron ◽  
High Resolution Tem ◽  
Projection Approximation

The original high-resolution transmission electron microscope (HRTEM) image simulation program was written as a tool to confirm interpretation of HRTEM images of niobium oxides. Thorough testing on known structures showed that image simulation could reliably duplicate the imaging process occurring in the HRTEM, and could thus be confidently used to interpret images of unknown structures. Mainstream application of image simulation to routine structure determination by HRTEM was ushered in by the establishment of the wide applicability of the SHRLI (simulated high-resolution lattice image) programs. Structure determination of the mineral takéuchiite by HRTEM and image simulation was the first such determination accepted by the KJCr without x-ray data. Of course, once the reliability of image simulation had been established, it was realized that the technique could be put to work for applications other than structure determination. Early on, simulations were used to explore various HRTEM imaging parameters, including specimen ionicity, validity of the projection approximation, and the resolutionlimiting effects of incident-beam convergence. Since the inception of HRTEM image simulation, its range of uses has continued to expand, and so has the number of programs available; distribution of the SHRLI code spawned improved versions as well as some new programs.

scholarly journals PAIREF: paired refinement also for Phenix users

2021 ◽  
Vol 77 (7) ◽  
Author(s):  
Martin Malý ◽  
Kay Diederichs ◽  
Jan Dohnálek ◽  
Petr Kolenko
Keyword(s):  
High Resolution ◽  
Cross Validation ◽  
Software Tool ◽  
Published Data ◽  
High Resolution Data ◽  
Validation Procedure ◽  
Optimal Approach ◽  
Insight Into

In macromolecular crystallography, paired refinement is generally accepted to be the optimal approach for the determination of the high-resolution cutoff. The software tool PAIREF provides automation of the protocol and associated analysis. Support for phenix.refine as a refinement engine has recently been implemented in the program. This feature is presented here using previously published data for thermolysin. The results demonstrate the importance of the complete cross-validation procedure to obtain a thorough and unbiased insight into the quality of high-resolution data.

Spot Scan Imaging and the Future of High Resolution

1990 ◽  
Vol 48 (1) ◽  
pp. 88-89
Author(s):  
K.H. Downing
Keyword(s):  
High Resolution ◽  
Transfer Function ◽  
Good Deal ◽  
Photographic Film ◽  
Modulation Transfer ◽  
Contrast Transfer Function ◽  
Electron Dose ◽  
High Resolution Data ◽  
Resolution Data

Electron crystallographers who have been working on determination of protein structure have set a goal of obtaining image information to a resolution of about 3.5 Å, from specimens tilted up to 60 degrees. This information would allow the construction of a three-dimensional density map within which the path of the peptide chain could be followed and locations of side chains defined. The recent determination of an atomic model of the membrane protein bacteriorhodopsin (bR) from EM data (1) which was not as complete as we would like, used a good deal of other biochemical and biophysical data to constrain the model. In cases where this type of information is not as extensive as with bR, isotropic high-resolution data would be required. Significant advances in several different areas have brought us tantalizingly close to reaching our goal, but there are still improvements to be made.The essential limitations in obtaining high resolution data from proteins arise from the radiation sensitivity of the specimen, which severely limits the electron exposure that can be used in recording an image and thus limits the signal-to-noise ratio (SNR). Increasing both the electron dose, which is possible with cold specimens, and the area processed, which required implementation of significant computer software, have each given about a factor of three improvement in SNR. Still, with conventional imaging, a study by Henderson and Glaeser (2) revealed that the best images contained only a small fraction of the signal that would be present in a perfect image. Factors such as the envelope of the contrast transfer function and the modulation transfer function of the photographic film account for some loss of contrast, but the factor causing the most loss was found to be beam-induced specimen motion. This motion results from the stress which is produced by changes in bond structure during the course of radiation damage.

Effect of strain on ADF-STEM high-resolution images

1991 ◽  
Vol 49 ◽  
pp. 666-667
Author(s):  
M. Kelly ◽  
D.M. Bird
Keyword(s):  
High Resolution ◽  
Strong Influence ◽  
Intensity Variation ◽  
Dark Field ◽  
Atomic Resolution ◽  
Strain Fields ◽  
High Resolution Image ◽  
Annular Dark Field ◽  
High Resolution Images ◽  
Interesting Alternative

It is well known that strain fields can have a strong influence on the details of HREM images. This, for example, can cause problems in the analysis of edge-on interfaces between lattice mismatched materials. An interesting alternative to conventional HREM imaging has recently been advanced by Pennycook and co-workers where the intensity variation in the annular dark field (ADF) detector is monitored as a STEM probe is scanned across the specimen. It is believed that the observed atomic-resolution contrast is correlated with the intensity of the STEM probe at the atomic sites and the way in which this varies as the probe moves from cell to cell. As well as providing a directly interpretable high-resolution image, there are reasons for believing that ADF-STEM images may be less suseptible to strain than conventional HREM. This is because HREM images arise from the interference of several diffracted beams, each of which is governed by all the excited Bloch waves in the crystal.

Determination of the Burgers vector of a dislocation in a Fe-Mn alloy by weak-beam image in HVEM

1978 ◽  
Vol 36 (1) ◽  
pp. 330-331
Author(s):  
Y. Ishida ◽  
H. Ishida ◽  
K. Kohra ◽  
H. Ichinose
Keyword(s):  
High Order ◽  
Simulation Technique ◽  
The Other ◽  
Image Simulation ◽  
Diffraction Vector ◽  
Burgers Vector ◽  
Weak Beam ◽  
Beam Image ◽  
Edge Component

IntroductionA simple and accurate technique to determine the Burgers vector of a dislocation has become feasible with the advent of HVEM. The conventional image vanishing technique(1) using Bragg conditions with the diffraction vector perpendicular to the Burgers vector suffers from various drawbacks; The dislocation image appears even when the g.b = 0 criterion is satisfied, if the edge component of the dislocation is large. On the other hand, the image disappears for certain high order diffractions even when g.b ≠ 0. Furthermore, the determination of the magnitude of the Burgers vector is not easy with the criterion. Recent image simulation technique is free from the ambiguities but require too many parameters for the computation. The weak-beam “fringe counting” technique investigated in the present study is immune from the problems. Even the magnitude of the Burgers vector is determined from the number of the terminating thickness fringes at the exit of the dislocation in wedge shaped foil surfaces.

The quantitative comparison of experimental and simulated diffraction contrast images

1995 ◽  
Vol 53 ◽  
pp. 102-103
Author(s):  
Stuart McKernan
Keyword(s):  
Image Processing ◽  
Personal Computer ◽  
Close Agreement ◽  
Quantitative Comparison ◽  
Image Simulation ◽  
Computing Power ◽  
Diffraction Contrast ◽  
Simulated Images ◽  
Made In

For many years the concept of quantitative diffraction contrast experiments might have consisted of the determination of dislocation Burgers vectors using a g.b = 0 criterion from several different 2-beam images. Since the advent of the personal computer revolution, the available computing power for performing image-processing and image-simulation calculations is enormous and ubiquitous. Several programs now exist to perform simulations of diffraction contrast images using various approximations. The most common approximations are the use of only 2-beams or a single systematic row to calculate the image contrast, or calculating the image using a column approximation. The increasing amount of literature showing comparisons of experimental and simulated images shows that it is possible to obtain very close agreement between the two images; although the choice of parameters used, and the assumptions made, in performing the calculation must be properly dealt with. The simulation of the images of defects in materials has, in many cases, therefore become a tractable problem.

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