Analysis and Interpretation of Diffraction Data from Amorphous Materials

1980 ◽  
Vol 24 ◽  
pp. 63-72
Author(s):  
J. H. Konnert ◽  
P. D'Antonio ◽  
J. Karle
Keyword(s):  
Distribution Function ◽  
Data Collection ◽  
Data Reduction ◽  
Interatomic Distance ◽  
Amorphous Materials ◽  
Structural Information ◽  
Structural Features ◽  
Polycrystalline Materials ◽  
Reduction Procedure ◽  
Diffraction Patterns

Amorphous materials give rise to rather diffuse diffraction patterns. In contrast to diffraction patterns from polycrystalline materials which are characterized by a large number of sharp diffraction rings, patterns from amorphous materials are composed of relatively few broadened features. Such a diffuse pattern, however, contains much structural information in the form of an interatomic distance distribution that may be computed directly from the measured diffraction pattern. This radial distribution function (RDF) provides information concerning bonded distances, the types of atomic groupings and the extent of ordering in the sample. Special care, however, must be taken during the analysis to avoid introducing spurious details into the RDF that may be confused with, or mask real structural features. Such false detail may arise both from data collection and data reduction procedures. The availability of modern computers has greatly facilitated the calculation of accurate RDF's by readily permitting the introduction of physical criteria into the data reduction procedure that must be satisfied by the RDF.

Structure of pure metallic and semiconductor glasses

1990 ◽  
Vol 48 (4) ◽  
pp. 142-143
Author(s):  
Louis M. Holzman ◽  
Yeon-Wook Kim ◽  
Thomas F. Kelly
Keyword(s):  
Distribution Function ◽  
Radial Distribution Function ◽  
Amorphous Alloys ◽  
Amorphous Materials ◽  
Room Temperature ◽  
Amorphous State ◽  
Amorphous Structure ◽  
Pure Element ◽  
Diffraction Patterns ◽  
Semiconductor Glasses

There has been a great deal of interest in amorphous materials as the advance of technology has enabled a greater variety of alloys and elements to be quenched into the amorphous state and as new uses for amorphous materials have been developed. Because the analysis of the structure of amorphous alloys is quite complicated, it is highly desirable to have specimens available of pure elements in the amorphous state in order to more easily study amorphous structure and for comparison with theory. However, very few pure elements have been quenched into the amorphous state and most of those that have been are only stable at temperatures close to absolute zero. This has limited the methods available for the study of their structure. We have produced room-temperature-stable amorphous samples of pure elements (V,Nb,Ta,Mo,W,Fe,Co,Ni,Si,Ge) from the melt using electrohydrodynamic (EHD) atomization. Diffraction patterns of these samples were obtained using a Vacuum Generators HB501 STEM and these patterns were analyzed to obtain the radial distribution function for the pure element specimens.

scholarly journals Low-resolution refinement tools in REFMAC5

2012 ◽  
Vol 68 (4) ◽  
pp. 404-417 ◽  
Author(s):  
Robert A. Nicholls ◽  
Fei Long ◽  
Garib N. Murshudov
Keyword(s):  
Crystal Structure ◽  
Interatomic Distance ◽  
Prior Information ◽  
Structural Information ◽  
Structural Features ◽  
Crystal Structure Analysis ◽  
Low Resolution ◽  
Structural Units ◽  
Distance Restraints ◽  
Subsequent Application

Two aspects of low-resolution macromolecular crystal structure analysis are considered: (i) the use of reference structures and structural units for provision of structural prior information and (ii) map sharpening in the presence of noise and the effects of Fourier series termination. The generation of interatomic distance restraints by ProSMART and their subsequent application in REFMAC5 is described. It is shown that the use of such external structural information can enhance the reliability of derived atomic models and stabilize refinement. The problem of map sharpening is considered as an inverse deblurring problem and is solved using Tikhonov regularizers. It is demonstrated that this type of map sharpening can automatically produce a map with more structural features whilst maintaining connectivity. Tests show that both of these directions are promising, although more work needs to be performed in order to further exploit structural information and to address the problem of reliable electron-density calculation.

Evaluation of nano- and mesoscale structural features in composite materials through hierarchical decomposition of the radial distribution function

2018 ◽  
Vol 51 (1) ◽  
pp. 76-86 ◽  
Author(s):  
Valerie García-Negrón ◽  
Akinola D. Oyedele ◽  
Eduardo Ponce ◽  
Orlando Rios ◽  
David P. Harper ◽  
...  
Keyword(s):  
Distribution Function ◽  
Composite Materials ◽  
Radial Distribution Function ◽  
Radial Distribution ◽  
Structural Features ◽  
Computational Effort ◽  
Scattering Data ◽  
Hierarchical Decomposition ◽  
Structural Refinement ◽  
Diffraction Patterns

Composite materials possessing both crystalline and amorphous domains, when subjected to X-ray and neutron scattering, generate diffraction patterns that are often difficult to interpret. One approach is to perform atomistic simulations of a proposed structure, from which the analogous diffraction pattern can be obtained for validation. The structure can be iteratively refined until simulation and experiment agree. The practical drawback to this approach is the significant computational resources required for the simulations. In this work, an alternative approach based on a hierarchical decomposition of the radial distribution function is used to generate a physics-based model allowing rapid interpretation of scattering data. In order to demonstrate the breadth of this approach, it is applied to a series of carbon composites. The model is compared with atomistic simulation results in order to demonstrate that the contributions of the crystalline and amorphous domains, as well as their interfaces, are correctly captured. Because the model is more efficient, additional structural refinement is performed to increase the agreement of the simulation result with the experimental data. The model achieves a reduction in computational effort of six orders of magnitude relative to simulation. The model can be generally extended to other composite materials.

Electron microscopy and diffraction studies of polyimides

1983 ◽  
Vol 41 ◽  
pp. 28-29
Author(s):  
O.C. de Hodgins ◽  
K. R. Lawless ◽  
R. Anderson
Keyword(s):  
Electron Microscopy ◽  
Electron Microscope ◽  
Structural Features ◽  
Inert Atmosphere ◽  
Polyimide Films ◽  
Resolution Electron ◽  
The Matrix ◽  
High Resolution Electron Microscope ◽  
Diffraction Patterns ◽  
Polymeric Samples

Commercial polyimide films have shown to be homogeneous on a scale of 5 to 200 nm. The observation of Skybond (SKB) 705 and PI5878 was carried out by using a Philips 400, 120 KeV STEM. The objective was to elucidate the structural features of the polymeric samples. The specimens were spun and cured at stepped temperatures in an inert atmosphere and cooled slowly for eight hours. TEM micrographs showed heterogeneities (or nodular structures) generally on a scale of 100 nm for PI5878 and approximately 40 nm for SKB 705, present in large volume fractions of both specimens. See Figures 1 and 2. It is possible that the nodulus observed may be associated with surface effects and the structure of the polymers be regarded as random amorphous arrays. Diffraction patterns of the matrix and the nodular areas showed different amorphous ring patterns in both materials. The specimens were viewed in both bright and dark fields using a high resolution electron microscope which provided magnifications of 100,000X or more on the photographic plates if desired.

Measurement and Reduction of Radiation Damage in Frozen Hydrated Crystalline Specimens

1978 ◽  
Vol 36 (3) ◽  
pp. 70-77 ◽  
Author(s):  
R.M. Glaeser ◽  
S.B. Hayward
Keyword(s):  
Diffraction Pattern ◽  
Structural Information ◽  
Structural Disorder ◽  
Structural Features ◽  
Biological Macromolecules ◽  
Diffraction Intensity ◽  
Object Structure ◽  
Specimen Material ◽  
Unit Cells ◽  
Identical Unit

Highly ordered or crystalline biological macromolecules become severely damaged and structurally disordered after a brief electron exposure. Evidence that damage and structural disorder are occurring is clearly given by the fading and eventual disappearance of the specimen's electron diffraction pattern. The fading and disappearance of sharp diffraction spots implies a corresponding disappearance of periodic structural features in the specimen. By the same token, there is a oneto- one correspondence between the disappearance of the crystalline diffraction pattern and the disappearance of reproducible structural information that can be observed in the images of identical unit cells of the object structure. The electron exposures that result in a significant decrease in the diffraction intensity will depend somewhat upon the resolution (Bragg spacing) involved, and can vary considerably with the chemical makeup and composition of the specimen material.

Analysis of Dark-Field Images of Disordered Materials

1972 ◽  
Vol 30 ◽  
pp. 560-561
Author(s):  
J. M. Cowley
Keyword(s):  
Amorphous Materials ◽  
Careful Analysis ◽  
Dark Field ◽  
Minimum Diameter ◽  
Statistical Fluctuations ◽  
Bright Spots ◽  
Sufficient Detail ◽  
Random Arrangement ◽  
Diffraction Patterns ◽  
Imaging Conditions

Recently a number of authors have reported detail in dark-field images obtained from diffuse-scattering regions of electron diffraction patterns. Bright spots in images from short-range order diffuse peaks of disordered binary alloys have been interpreted as evidence for the existence of microdomains of ordered lattice or of segragated clusters of one component. Spotty contrast in dark field images of near-amorphous materials has been interpreted as evidence for the existense of microcrystals. Without a careful analysis of the imaging conditions such conclusions may be invalid. Usually the conditions of the experiment have not been specified in sufficient detail to allow evaluation of the conclusions.Elementary considerations show that even for a completely random arrangement of atoms the statistical fluctuations of density will give a spotty contrast with spots of minimum diameter determined by the dark field aperture size and other factors influencing the minimum resolvable distance under darkfield imaging conditions, including fluctuations and drift over long exposure times (resolution usually 10Å or more).

Observations on the Structure of Ta2O5

1972 ◽  
Vol 30 ◽  
pp. 540-541
Author(s):  
P. S. Kotval ◽  
C. J. Dewit
Keyword(s):  
Electron Microscope ◽  
Structural Features ◽  
Amorphous Structure ◽  
Polished Surface ◽  
Distilled Water ◽  
Anodic Films ◽  
Amorphous Films ◽  
Beam Heating ◽  
Diffraction Patterns ◽  
Hexagonal Arrays

The structure of Ta2O5 has been described in the literature in several different crystallographic forms with varying unit cell lattice parameters. Earlier studies on films of Ta2O5 produced by anodization of tantalum have revealed structural features which are not consistent with the parameters of “bulk” Ta2O5 crystalsFilms of Ta2O5 were prepared by anodizing a well-polished surface of pure tantalum sheet. The anodic films were floated off in distilled water, collected on grids, dried and directly examined in the electron microscope. In all cases the films were found to exhibit diffraction patterns representative of an amorphous structure. Using beam heating in the electron microscope, recrystallization of the amorphous films can be accomplished as shown in Fig. 1. As suggested by earlier work, the recrystallized regions exhibit diffraction patterns which consist of hexagonal arrays of main spots together with subsidiary rows of super lattice spots which develop as recrystallization progresses (Figs. 2a and b).

Automated electron tomography: from data collection to image processing

1995 ◽  
Vol 53 ◽  
pp. 26-27
Author(s):  
Weiping Liu ◽  
Jennifer Fung ◽  
W.J. de Ruijter ◽  
Hans Chen ◽  
John W. Sedat ◽  
...  
Keyword(s):  
Image Processing ◽  
Data Collection ◽  
Structural Information ◽  
Imaging Technique ◽  
Electron Tomography ◽  
Ccd Camera ◽  
Automated Data Collection ◽  
Full Control ◽  
Transmission Electron ◽  
Amorphous Structures

Electron tomography is a technique where many projections of an object are collected from the transmission electron microscope (TEM), and are then used to reconstruct the object in its entirety, allowing internal structure to be viewed. As vital as is the 3-D structural information and with no other 3-D imaging technique to compete in its resolution range, electron tomography of amorphous structures has been exercised only sporadically over the last ten years. Its general lack of popularity can be attributed to the tediousness of the entire process starting from the data collection, image processing for reconstruction, and extending to the 3-D image analysis. We have been investing effort to automate all aspects of electron tomography. Our systems of data collection and tomographic image processing will be briefly described.To date, we have developed a second generation automated data collection system based on an SGI workstation (Fig. 1) (The previous version used a micro VAX). The computer takes full control of the microscope operations with its graphical menu driven environment. This is made possible by the direct digital recording of images using the CCD camera.

Multislice calculations for quasi-periodic and non-periodic objects

1990 ◽  
Vol 48 (4) ◽  
pp. 138-139
Author(s):  
R. Herrera ◽  
A. Gómez
Keyword(s):  
Electron Diffraction ◽  
Computer Simulations ◽  
Periodic Table ◽  
Amorphous Materials ◽  
Space Group ◽  
Essential Step ◽  
Shape Effects ◽  
Atomic Scattering ◽  
Diffraction Patterns ◽  
Periodic Continuation

Computer simulations of electron diffraction patterns and images are an essential step in the process of structure and/or defect elucidation. So far most programs are designed to deal specifically with crystals, requiring frequently the space group as imput parameter. In such programs the deviations from perfect periodicity are dealt with by means of “periodic continuation”.However, for many applications involving amorphous materials, quasiperiodic materials or simply crystals with defects (including finite shape effects) it is convenient to have an algorithm capable of handling non-periodicity. Our program “HeGo” is an implementation of the well known multislice equations in which no periodicity assumption is made whatsoever. The salient features of our implementation are: 1) We made Gaussian fits to the atomic scattering factors for electrons covering the whole periodic table and the ranges [0-2]Å−1 and [2-6]Å−1.

Structural studies of small amorphous volumes by electron diffraction

1990 ◽  
Vol 48 (4) ◽  
pp. 122-123
Author(s):  
Pierre Moine
Keyword(s):  
Electron Diffraction ◽  
Born Approximation ◽  
Structural Information ◽  
Fundamental Relation ◽  
X Rays ◽  
Structural Studies ◽  
Diffraction Experiment ◽  
Transmission Electron ◽  
Amorphous Structures ◽  
Diffraction Patterns

Qualitatively, amorphous structures can be easily revealed and differentiated from crystalline phases by their Transmission Electron Microscopy (TEM) images and their diffraction patterns (fig.1 and 2) but, for quantitative structural information, electron diffraction pattern intensity analyses are necessary. The parameters describing the structure of an amorphous specimen have been introduced in the context of scattering experiments which have been, so far, the most used techniques to obtain structural information in the form of statistical averages. When only small amorphous volumes (< 1/μm in size or thickness) are available, the much higher scattering of electrons (compared to neutrons or x rays) makes, despite its drawbacks, electron diffraction extremely valuable and often the only feasible technique.In a diffraction experiment, the intensity IN (Q) of a radiation, elastically scattered by N atoms of a sample, is measured and related to the atomic structure, using the fundamental relation (Born approximation) : IN(Q) = |FT[U(r)]|.

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