Practical Examples of Point and Space Group Determination in Convergent Beam Diffraction

1980 ◽  
Vol 38 ◽  
pp. 188-191 ◽  
Author(s):  
J.W. Steeds ◽  
N.S. Evans
Keyword(s):  
Stacking Faults ◽  
Objective Lens ◽  
High Symmetry ◽  
Space Group ◽  
Thermal Disorder ◽  
Order Of Magnitude ◽  
Convergent Beam ◽  
Beam Diffraction ◽  
Unambiguous Assignment

The high quality of diffraction information generated by convergent beam electron microscopy provides many ways of arriving at an unambiguous assignment of a space group to a crystal under investigation. One method of space group determination involves tilting the crystal to a number of different zone axes and combining the information derived from each. However, it is often possible to obtain the same information more efficiently by concentrating on just one high symmetry zone axis and it is this approach which will be illustrated here. The prerequisites for the application of the technique are a good crystal and a microscope which offers a large angular view of the back focal plane of the objective lens. Specimen cooling is generally advantageous but has not been used in most of our work and, in particular, was not used for the results used as illustrations here. By the term 'good' crystal is meant a homogeneous and strain-free crystal without planar disorder (stacking faults, antiphase boundaries) linear disorder (dislocations) or point disorder. The requirements are certainly stringent but they need only operate over a cube of side approximately 1OO nm. Point disorder which produces an effect of the same order of magnitude as thermal disorder can be tolerated and strains less than 10-4are not detected. An angular view of 20° or so in the diffraction plane is generally acceptable although a larger view would be helpful for certain crystal structures and for low-specimen temperatures.

Transmission Scanning Electron Microscopy and Energy Analysis with the Siemens ELMISKOP 101

1972 ◽  
Vol 30 ◽  
pp. 450-451
Author(s):  
H. M. Thieringer
Keyword(s):  
Objective Lens ◽  
Transmission Microscopy ◽  
Image Recording ◽  
Mode Of Operation ◽  
Scanning Transmission ◽  
Electron Microscopes ◽  
And Performance ◽  
Convergent Beam ◽  
Ray Path ◽  
Beam Diffraction

It has repeatedly been show that with conventional electron microscopes very fine electron probes can be produced, therefore allowing various micro-techniques such as micro recording, X-ray microanalysis and convergent beam diffraction. In this paper the function and performance of an SIEMENS ELMISKOP 101 used as a scanning transmission microscope (STEM) is described. This mode of operation has some advantages over the conventional transmission microscopy (CTEM) especially for the observation of thick specimen, in spite of somewhat longer image recording times.Fig.1 shows schematically the ray path and the additional electronics of an ELMISKOP 101 working as a STEM. With a point-cathode, and using condensor I and the objective lens as a demagnifying system, an electron probe with a half-width ob about 25 Å and a typical current of 5.10-11 amp at 100 kV can be obtained in the back focal plane of the objective lens.

Using crystallographic symmetry in diffraction and contrast analysis

1989 ◽  
Vol 47 ◽  
pp. 504-505
Author(s):  
U. Dahmen ◽  
K.H. Westmacott
Keyword(s):  
Electron Microscopy ◽  
High Symmetry ◽  
Two Phase ◽  
Tem Analysis ◽  
Crystallographic Symmetry ◽  
The Matrix ◽  
Contrast Analysis ◽  
Practical Tool ◽  
Convergent Beam ◽  
Beam Diffraction

Despite the increased use of convergent beam diffraction, symmetry concepts in their more general form are not commonly applied as a practical tool in electron microscopy. Crystal symmetry provides an abundance of information that can be used to facilitate and improve the TEM analysis of crystalline solids. This paper draws attention to some aspects of symmetry that can be put to practical use in the analysis of structures and morphologies of two-phase materials.It has been shown that the symmetry of the matrix that relates different variants of a precipitate can be used to determine the axis of needle- or lath-shaped precipitates or the habit plane of plate-shaped precipitates. By tilting to a special high symmetry orientation of the matrix and by measuring angles between symmetry-related variants of the precipitate it is possible to find their habit from a single micrograph.

scholarly journals Combing electron diffraction techniques for structure solution

2014 ◽  
Vol 70 (a1) ◽  
pp. C369-C369
Author(s):  
Andrew Stewart
Keyword(s):  
Electron Diffraction ◽  
Electron Diffraction Data ◽  
Data Sets ◽  
Data Set ◽  
Space Group ◽  
X Ray ◽  
Structure Solution ◽  
Convergent Beam ◽  
Beam Diffraction ◽  
Convergent Beam Diffraction

The last few years have seen a revolution in the field of 3D electron diffraction or diffraction tomography. We have moved from only acquiring a few low index zone axis patterns to full tomographic data sets recording all accessible areas of reciprocal space. These new larger data sets have made it easier for structure solution techniques such as direct methods from the x-ray world to be applied to the electron diffraction data for structure solution. While structure solution with tomographic electron diffraction is non trivial when compared to the x-ray case it is significantly easier than it was a few years ago. Mugnaioli et al. We are now in a situation where the most difficult and time consuming step can be the assignment of the space group to a data set. Electron diffraction has many advantages over the x-ray case in terms of the manner in which we can manipulate the electron beam. This allows the collection to convergent beam diffraction (CBD) or large angle convergent beam diffraction (LACBED) patterns, via the recently developed technique by Beanland et al. These techniques can make the assignment of space group significantly easier affair, and the path to structure solution a lot smoother. We will present the combination of data from tomographic, selected area (SA) and nano-beam (NBD) datasets, with diffraction from tomographic LACBED experiments where using the strengths of each technique can be leveraged for a much quicker route to structure solution.

scholarly journals Particle size and convergent electron diffraction patterns of triangular prismatic gold nanoparticles

2021 ◽  
Vol 67 (4 Jul-Aug) ◽  
Author(s):  
Clemente Fernando-Marquez ◽  
Gilberto Mondragón-Galicia ◽  
Lourdes Bazán-Díaz ◽  
José Reyes-Gasga
Keyword(s):  
Particle Size ◽  
Gold Nanoparticles ◽  
Electron Diffraction ◽  
Diffraction Pattern ◽  
Stacking Faults ◽  
Au Nanoparticle ◽  
Diffraction Patterns ◽  
Convergent Beam ◽  
Beam Diffraction ◽  
Fcc Structure

Convergent beam diffraction (CBED) patterns of nanoparticles are possible. CBED of triangular prismatic shaped Au nanoparticle with focus on diffraction pattern symmetry and forbidden reflections observed along [111] and [112] zone axes are reported in this work. It is well known that the CBED patterns of nanoparticles of 30 nm or less in size only show bright kinematical discs. The dynamic contrast with Kikuchi and sharp HOLZ lines within the bright discs, as observed in CBED of volumetric materials, is well observed in particles larger of 500 nm in size. In addition, it is shown that the 1/3[422] and 1/2[311] weak forbidden reflections observed in the [111] and [112] electron diffraction patterns of these particles do not modify the symmetry of the CBED patterns, but they disappear as the size of the particle increases. The symmetry of the CBED patterns are always observed in concordance with the space group Fm3m (No. 225) of the Au unit cell. The possible explanations for observing forbidden reflections are the incomplete ABC stacking due to surface termination and the stacking faults in the fcc structure.

Improving the accuracy of convergent-beam thickness measurements

1988 ◽  
Vol 46 ◽  
pp. 520-521
Author(s):  
K. K. Christenson
Keyword(s):  
Objective Lens ◽  
Fringe Spacing ◽  
Underlying Assumption ◽  
Angular Scale ◽  
Beam Thickness ◽  
Extinction Length ◽  
Diffraction Patterns ◽  
Convergent Beam ◽  
Thickness Measurements ◽  
Beam Diffraction

Convergent-beam diffraction patterns taken at appropriate “two-beam” conditions allow simple, rapid determinations of a specimen's thickness, extinction length and even its anomalous absorption coefficient (1,2). We here note three points to consider when obtaining the pattern.First, in thickness measurements the ratio of the fringe spacing to the spacing between the disks is utilized; there is an underlying assumption that the two distances are on the same angular scale. This assumption is incorrect if the illumination crossover is not in the plane of the specimen and simultaneously, the diffraction lens is focused incorrectly. If the crossover is at the specimen (Fig. 1a), varying the focus of the diffraction lens (changing w) varies the distance between the disks and the sizes of features within the disks in the same way, only the magnification of the pattern is changed. Likewise, if the diffraction lens is focused correctly, on the back focal plane of the objective lens, the angular scale within the disks matches that between the disks and neither scale is affected by variations in the illumination.

Convergent beam electron diffraction

1987 ◽  
Vol 51 (359) ◽  
pp. 33-48 ◽  
Author(s):  
P. E. Champness
Keyword(s):  
Electron Beam ◽  
Electron Diffraction ◽  
Point Group ◽  
Convergent Beam Electron Diffraction ◽  
Objective Lens ◽  
Specimen Thickness ◽  
High Symmetry ◽  
Beam Electron ◽  
Cell Parameters ◽  
Convergent Beam

AbstractIn convergent-beam electron diffraction (CBED) a highly convergent electron beam is focussed on to a small (⩽50 nm) area of the sample. Instead of the diffraction spots that are obtained in the back focal plane of the objective lens with parallel illumination in conventional selected-area electron diffraction, CBED produces discs of intensity. The point group can be determined uniquely from the symmetry within the individual discs and the overall pattern. In order to determine the point group, it is usually necessary to record a number of CBED patterns with the electron beam aligned along different zone axes, but sometimes only one, high-symmetry pattern is required. The positions of reflections in higher-order Laue zones can be used to identify the crystal system and lattice type and to detect the presence of certain glide planes. The repeat along the zone axis that is parallel to the beam can be calculated from the diameters of the Laue zones. Hence the presence ofpolymorphs can be detected. Doubly-diffracted discs in CBED often contain a ‘line of dynamic absence’, the orientation of this line with respect to the symmetry seen in the bright field disc allows the symmetry element responsible for it (glide plane or screw diad) to be identified. This allows 191 of the 230 space groups to be uniquely identified. The measurement of specimen thickness, extinction distance and cell parameters are also briefly discussed.

Synthesis of BaAl2Si2O8 from solid Ba–Al–Al2O3–SiO2 precursors: Part III. The structure of BaAl2Si2O8 formed by annealing at ≤650 °C and at 1650 °C

1998 ◽  
Vol 13 (11) ◽  
pp. 3122-3134 ◽  
Author(s):  
Xiao-Dong Zhang ◽  
Kenneth H. Sandhage ◽  
Hamish L. Fraser
Keyword(s):  
Heat Treatment ◽  
Stacking Faults ◽  
Antiphase Boundaries ◽  
Analytical Tem ◽  
Diffraction Patterns ◽  
Convergent Beam ◽  
Beam Diffraction ◽  
Convergent Beam Diffraction

Analytical TEM and HREM have been used to examine the structure of BaAl2Si2O8 crystals produced within oxidized Ba–Al–Al2O3–SiO2 precursors upon annealing: (i) at ≤650 °C and (ii) up to 1650 °C. A BaAl2Si2O8 polymorph with a c-axis parameter of 15.6 Å was detected after annealing at ≤650 °C. Stacking faults and antiphase boundaries were detected within this polymorph after the 650 °C treatment. After a 15 h heat treatment at 1650 °C, convergent beam diffraction patterns and HREM confirmed that the predominant phase was β–hexacelsian. Although antiphase boundaries were absent in the β–hexacelsian crystals, dislocations and stacking faults were detected after the 1650 °C anneal. The generation of defects in BaAl2Si2O8 crystals within specimens annealed at ≤650 °C and at 1650 °C is discussed in light of prior structural analyses.

Observability of Very Thick Specimen by Using Transmission Scanning Electron Microscope

1971 ◽  
Vol 29 ◽  
pp. 26-27
Author(s):  
K. Shibatomi ◽  
T. Yamanoto ◽  
H. Koike
Keyword(s):  
Electron Microscope ◽  
Image Quality ◽  
Scanning Electron Microscope ◽  
Chromatic Aberration ◽  
Image Contrast ◽  
Objective Lens ◽  
Thick Specimen ◽  
Transmission Electron ◽  
Scanning Electron

In the observation of a thick specimen by means of a transmission electron microscope, the intensity of electrons passing through the objective lens aperture is greatly reduced. So that the image is almost invisible. In addition to this fact, it have been reported that a chromatic aberration causes the deterioration of the image contrast rather than that of the resolution. The scanning electron microscope is, however, capable of electrically amplifying the signal of the decreasing intensity, and also free from a chromatic aberration so that the deterioration of the image contrast due to the aberration can be prevented. The electrical improvement of the image quality can be carried out by using the fascionating features of the SEM, that is, the amplification of a weak in-put signal forming the image and the descriminating action of the heigh level signal of the background. This paper reports some of the experimental results about the thickness dependence of the observability and quality of the image in the case of the transmission SEM.

Relative Intensities in ED Patterns From Thin Gold Films

1972 ◽  
Vol 30 ◽  
pp. 636-637
Author(s):  
R. H. Morriss ◽  
J. D. C. Peng ◽  
C. D. Melvin
Keyword(s):  
Theoretical Calculations ◽  
Diffraction Theory ◽  
Gold Films ◽  
Reasonable Accuracy ◽  
Experimental Conditions ◽  
Crystal Perfection ◽  
Heavy Atoms ◽  
Fine Focus ◽  
Convergent Beam ◽  
Beam Diffraction

Although dynamical diffraction theory was modified for electrons by Bethe in 1928, relatively few calculations have been carried out because of computational difficulties. Even fewer attempts have been made to correlate experimental data with theoretical calculations. The experimental conditions are indeed stringent - not only is a knowledge of crystal perfection, morphology, and orientation necessary, but other factors such as specimen contamination are important and must be carefully controlled. The experimental method of fine-focus convergent-beam electron diffraction has been successfully applied by Goodman and Lehmpfuhl to single crystals of MgO containing light atoms and more recently by Lynch to single crystalline (111) gold films which contain heavy atoms. In both experiments intensity distributions were calculated using the multislice method of n-beam diffraction theory. In order to obtain reasonable accuracy Lynch found it necessary to include 139 beams in the calculations for gold with all but 43 corresponding to beams out of the [111] zone.

Performance and applications of a field-emission gun TEM/STEM

1992 ◽  
Vol 50 (2) ◽  
pp. 942-943
Author(s):  
Judith M. Brock ◽  
Max T. Otten ◽  
Marc. J.C. de Jong
Keyword(s):  
Field Emission ◽  
High Resolution Imaging ◽  
High Quality ◽  
Small Energy ◽  
High Brightness ◽  
Small Probe ◽  
Resolution Imaging ◽  
Convergent Beam ◽  
Beam Patterns ◽  
Beam Diffraction

A Field Emission Gun (FEG) on a TEM/STEM instrument provides a major improvement in performance relative to one equipped with a LaB6 emitter. The improvement is particularly notable for small-probe techniques: EDX and EELS microanalysis, convergent beam diffraction and scanning. The high brightness of the FEG (108 to 109 A/cm2srad), compared with that of LaB6 (∼106), makes it possible to achieve high probe currents (∼1 nA) in probes of about 1 nm, whilst the currents for similar probes with LaB6 are about 100 to 500x lower. Accordingly the small, high-intensity FEG probes make it possible, e.g., to analyse precipitates and monolayer amounts of segregation on grain boundaries in metals or ceramics (Fig. 1); obtain high-quality convergent beam patterns from heavily dislocated materials; reliably detect 1 nm immuno-gold labels in biological specimens; and perform EDX mapping at nm-scale resolution even in difficult specimens like biological tissue.The high brightness and small energy spread of the FEG also bring an advantage in high-resolution imaging by significantly improving both spatial and temporal coherence.

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