The Symmetry of Ordered Cubic γ-Fe2O3 Investigated by TEM

2006 ◽  
Vol 61 (6) ◽  
pp. 665-671 ◽  
Author(s):  
Klemens Kelm ◽  
Werner Mader
Keyword(s):  
Electron Diffraction ◽  
Spinel Structure ◽  
Intermediate Phase ◽  
Dark Field ◽  
Continuous Group ◽  
Cubic Phase ◽  
Bright Field ◽  
Space Groups ◽  
Octahedral Sites ◽  
Diffraction Patterns

Well-crystallized particles of cubic and tetragonal γ -Fe2O3 embedded in a Pd matrix were produced besides other oxides by internal oxidation of a Pd-Fe alloy in air. Particles of tetragonal γ -Fe2O3 consist of orientation domains with the c axes normal to each other. Particles of the ordered cubic γ -Fe2O3 appear single crystalline in bright field and in dark field images with reflections of the basic spinel structure. In dark field images enantiomorphous domains were observed using reflections of the ordered phase. From the analysis of electron diffraction patterns in the principal zone axes the description of ordered cubic γ -Fe2O3 in the enantiomorphous space groups P4132/P4332 follows without further presumptions. In the sequence from space group Fd3m of disordered cubic γ -Fe2O3 via P4132/P4332 of the ordered cubic phase to the pair P41212/P43212 of tetragonal γ -Fe2O3 a continuous group-subgroup relation can be derived. This relation shows that ordered cubic γ -Fe2O3 is an intermediate phase upon ordering of vacant octahedral sites towards tetragonal γ -Fe2O3

Two Investigations into Grain Boundary Structure

1977 ◽  
Vol 35 ◽  
pp. 688-691
Author(s):  
P. Humble
Keyword(s):  
Grain Boundary ◽  
Dark Field ◽  
Boundary Structure ◽  
Boundary Dislocation ◽  
Bright Field ◽  
Intrinsic Structure ◽  
Weak Beam ◽  
Grain Boundary Structure ◽  
Independent Information ◽  
Diffraction Patterns

There has been sustained interest over the last few years into both the intrinsic (primary and secondary) structure of grain boundaries and the extrinsic structure e.g. the interaction of matrix dislocations with the boundary. Most of the investigations carried out by electron microscopy have involved only the use of information contained in the transmitted image (bright field, dark field, weak beam etc.). Whilst these imaging modes are appropriate to the cases of relatively coarse intrinsic or extrinsic grain boundary dislocation structures, it is apparent that in principle (and indeed in practice, e.g. (1)-(3)) the diffraction patterns from the boundary can give extra independent information about the fine scale periodic intrinsic structure of the boundary.In this paper I shall describe one investigation into each type of structure using the appropriate method of obtaining the necessary information which has been carried out recently at Tribophysics.

Removal of inelastically scattered electrons substantially increases phase contrast on frozen-hydrated molecules

1987 ◽  
Vol 45 ◽  
pp. 652-653
Author(s):  
John P. Langmore ◽  
Brian D. Athey
Keyword(s):  
Electron Diffraction ◽  
Phase Contrast ◽  
Low Dose ◽  
Dark Field ◽  
Biological Structure ◽  
Bright Field ◽  
Low Contrast ◽  
Negative Stain ◽  
The Difference ◽  
Better Than

Although electron diffraction indicates better than 0.3nm preservation of biological structure in vitreous ice, the imaging of molecules in ice is limited by low contrast. Thus, low-dose images of frozen-hydrated molecules have significantly more noise than images of air-dried or negatively-stained molecules. We have addressed the question of the origins of this loss of contrast. One unavoidable effect is the reduction in scattering contrast between a molecule and the background. In effect, the difference in scattering power between a molecule and its background is 2-5 times less in a layer of ice than in vacuum or negative stain. A second, previously unrecognized, effect is the large, incoherent background of inelastic scattering from the ice. This background reduces both scattering and phase contrast by an additional factor of about 3, as shown in this paper. We have used energy filtration on the Zeiss EM902 in order to eliminate this second effect, and also increase scattering contrast in bright-field and dark-field.

A New Detector System For The HB5 Stem Instrument

1985 ◽  
Vol 43 ◽  
pp. 134-135
Author(s):  
J.M. Cowley
Keyword(s):  
Dark Field ◽  
Detector System ◽  
Electron Holography ◽  
Small Specimen ◽  
Bright Field ◽  
Multiple Images ◽  
Practical Applications ◽  
Wide Range ◽  
Diffraction Patterns ◽  
Operational Modes

The HB5 STEM instrument at ASU has been modified previously to include an efficient two-dimensional detector incorporating an optical analyser device and also a digital system for the recording of multiple images. The detector system was built to explore a wide range of possibilities including in-line electron holography, the observation and recording of diffraction patterns from very small specimen regions (having diameters as small as 3Å) and the formation of both bright field and dark field images by detection of various portions of the diffraction pattern. Experience in the use of this system has shown that sane of its capabilities are unique and valuable. For other purposes it appears that, while the principles of the operational modes may be verified, the practical applications are limited by the details of the initial design.

scholarly journals Digital Imaging for TEM Part 1

1994 ◽  
Vol 2 (5) ◽  
pp. 24-25
Author(s):  
Anthony D. Buonaquisit
Keyword(s):  
Thin Section ◽  
Digital Imaging ◽  
Dark Field ◽  
Bright Field ◽  
Photographic Record ◽  
Physical Mechanisms ◽  
Viewing Screen ◽  
Diffraction Patterns

We are all familiar with digital imaging for SEM instruments. Digital Imaging for TEM applications is not as well established. Nevertheless, it seems clear that it will not be long before digital imaging for TEM becomes common place. Systems are improving and costs are plummeting. With this in mind it is timely to review what digital imaging for TEM involves.In normal TEM operation an electron bream is scattered through a thin section of a sample. Physical mechanisms cause the electrons of the beam to scatter, producing bright-field images, dark-field images and diffraction patterns. The operator adjusts the instrument to display one of these images on the instrument's viewing screen. A photographic record is collected by flipping the viewing screen and exposing a sheet of film held in the TEM's camera. Exposed negatives can be removed for developing and printing in batches, using standard darkroom techniques.

New detection system for HAADE and holography in STEM

1992 ◽  
Vol 50 (1) ◽  
pp. 102-103
Author(s):  
Marian Mankos ◽  
Shi Yao Wang ◽  
J.K. Weiss ◽  
J.M. Cowley
Keyword(s):  
High Efficiency ◽  
Detection System ◽  
Dark Field ◽  
Light Signal ◽  
Bright Field ◽  
Transmitted Beam ◽  
Wide Range ◽  
Annular Dark Field ◽  
Resolution Imaging ◽  
Diffraction Patterns

A novel detection system has been designed and realized experimentally on the HB5 STEM instrument. Shadow images, diffraction patterns as well as high-angle annular dark field and bright field images are observed simultaneously with high efficiency using CCD and TV cameras. The microscope can be operated in a wide range of instrument modes which includes the implementation of new techniques for high resolution imaging.As shown in Fig. 1, the detection system has three triple choice stages. Diffracted beams can be collected by three P47 fast phosphor annular detectors inclined at 45 degree to the axis and having different inner and outer acceptance angles, which can be adjusted by the postspecimen lenses. The detector is observed through a window by a photomultiplier. The annular detectors have been used also for a new bright field STEM technique which utilizes the inner rim of the detectors to collect only the outermost annular part of the central beam and promises an improvement in resolution by a factor of about 1.6. Initial results show some promise (Fig. 2). The transmitted beam is then converted into a light signal in YAG and P47 detectors; optionally the central part of the beam can be detected in the EELS spectrometer. The generated light signal is reflected through a system of mirrors, exits the vacuum chamber and is collected with high efficiency by high aperture optical lenses.

EELS of colloids in Mg+ implanted MgAl2O4 spinel

1994 ◽  
Vol 52 ◽  
pp. 980-981
Author(s):  
N. D. Evans ◽  
S. J. Zinkle
Keyword(s):  
Electron Diffraction ◽  
Elevated Temperatures ◽  
Lattice Parameter ◽  
Dark Field ◽  
Phase Identification ◽  
Dislocation Loops ◽  
Void Swelling ◽  
Electron Energy Loss Spectrometry ◽  
Diffraction Patterns ◽  
Electron Energy Loss

Because magnesium aluminate spinel (MgAl2O4) shows a strong resistance to void swelling during neutron irradiation at elevated temperatures, it is a candidate material for specialized applications in proposed fusion reactors. During implantation at 25°C with 2 MeV Mg+ ions to ∼2.8 × 1021 Mg+/m2, dislocation loops are formed at midrange depths (∼0.5 - 1.0 μm) on {110} and {111}. The microstructurc in the implanted ion region (∼1.5 - 2.0 μm) is shown in cross-section in Fig. 1. Within this implanted ion region, small features (4 - 10 nm diam.) were observed in dark field (DF) images using a spinel 222 reflection (Fig. 2). No evidence was found in electron diffraction patterns to suggest these features are (hexagonal) metallic Mg. However, in an earlier study, similar features in Al+ implanted spinel were identified by parallel electron energy loss spectrometry (PEELS) as metallic Al colloids. Phase identification of metallic Al within this spinel by electron diffraction is complicated because the lattice parameter of spinel (0.8083 nm) is almost exactly twice that of aluminum (0.4049 nm) and the phases are oriented cube-on-cube.

Identification of the kinematical forbidden reflections from precession electron diffraction

2009 ◽  
Vol 1184 ◽  
Author(s):  
Jean-Paul Morniroli ◽  
Gang Ji
Keyword(s):  
Electron Diffraction ◽  
Dark Field ◽  
Double Diffraction ◽  
Precession Angle ◽  
Space Groups ◽  
Ewald Sphere ◽  
Precession Electron Diffraction

AbstractThe visibility of the kinematical forbidden reflections due to glide planes, screw axes and Wyckoff positions is considered both on experimental and theoretical electron precession patterns as a function of the precession angle. The forbidden reflections due to glide planes and screw axes become very weak and disappear at large precession angle so that they can be distinguished from the allowed reflections and used to deduce the space groups. Contrarily, those due to Wyckoff positions remain visible and strong provided they are located on a major systematic row. This difference of behavior between the forbidden reflections is confirmed by observation of the corresponding dark-field LACBED patterns and is interpreted using the Ewald sphere and the Laue circles from the availability of double diffraction paths. This study also proves that dynamical interactions remain strong along the main systematic rows present on precession patterns.

Distortion of the oxygen sublattice in pure cubic-ZrO2

2004 ◽  
Vol 19 (5) ◽  
pp. 1315-1319 ◽  
Author(s):  
C.M. Wang ◽  
S. Azad ◽  
S. Thevuthasan ◽  
V. Shutthanandan ◽  
D.E. McCready ◽  
...  
Keyword(s):  
Electron Diffraction ◽  
Molecular Beam ◽  
Film Growth ◽  
Dark Field ◽  
Oxygen Sublattice ◽  
Dark Field Imaging ◽  
Selected Area Electron Diffraction ◽  
Diffraction Patterns ◽  
Field Imaging ◽  
The Ideal

Multilayer films of pure ZrO2 and CeO2 were grown using molecular beam epitaxy on a yttria-stabilized zirconia (YSZ) substrate. Distinctive forbidden diffraction spots of (odd, odd, even) type were observed on the selected-area electron-diffraction patterns of the film. Dark-field imaging clearly revealed that these forbidden diffraction spots were solely due to the ZrO2 layers. Comparison of the electron diffraction pattern with that simulated by dynamical calculations suggest that the pure ZrO2 layers possess a cubic structure of space with the group P4 3m oxygen sublattice being displaced diagonally, rather than along the c axis as suggested for YSZ. Our results further suggest that the displacement of the oxygen from the ideal (¼, ¼, ¼) position might have been introduced during the film growth process.

Experimental High Resolution Dark Field Electron Microscopy

1977 ◽  
Vol 35 ◽  
pp. 142-143
Author(s):  
Larry Pierce ◽  
Peter R. Buseck
Keyword(s):  
Electron Microscopy ◽  
High Resolution ◽  
Dark Field ◽  
Objective Lens ◽  
Aperture Size ◽  
Bright Field ◽  
Chromatic Aberrations ◽  
Field Electron Microscopy ◽  
Dark Field Electron ◽  
Diffraction Patterns

High resolution dark field (DF) images of the superstructures of the pyrrhotite (Fe1-xS) and bornite-digenite (Cu5FeS4-Cu9S5) series can be related to structure. Further, they provide more detail than bright field (BF) images. The same objective aperture size and stigmater settings were used for DF as for BF imaging; symmetrical arrangements of diffracted beams in the objective aperture were used. Images that can be related to structure were obtained at the defocus value giving the greatest image contrast, thereby enabling proper defocusing without requiring extensive through-focus series.For the minerals of interest, diffraction patterns consist of many superstructure reflections and a few subcell reflections. BF images contain primarily features of the superstructure, presumably because the subcell reflections fall far from the axis of the objective lens and thus are affected by spherical and chromatic aberrations and beam divergence. Likewise, DF images formed with a similar arrangement of beams as that in BF contain only features of superstructure, but with reverse contrast to BF.

Sign in / Sign up

Export Citation Format

Share Document