The interpretation of the zone axis critical voltage effect

1978 ◽  
Vol 36 (1) ◽  
pp. 194-195
Author(s):  
B. F. Buxton
Keyword(s):  
Structural Information ◽  
Incident Beam ◽  
Beam Theory ◽  
High Energy ◽  
Zone Axis ◽  
Critical Voltage ◽  
High Symmetry ◽  
Brillouin Zone Boundary ◽  
Extinction Length ◽  
Thickness Parameter

In cross-grating high energy electron diffraction, the scattering of the incident beam by the atomic string potentials is often so strong that there are many zone axis critical voltages below 1 MV. Steeds et al. (1976, 1977) have therefore explored the possibility of obtaining structural information from these critical voltages. In particular, for simple zone axes of high symmetry with only one string of atoms in each unit cell of the projected potential, they were able to characterize zone axis patterns by the critical voltage Ec and a thickness parameter ξ24 defined as the (2)-(4) extinction length at an orientation approximately midway between the zone axis and the first Brillouin zone boundary. Here a simple model atomic string potential will be used to investigate the information which can be gleaned from these parameters.The atomic string approximation (ASA) developed by Buxton and Tremewan (1977) will be used in order to avoid the large matrices encountered in the conventional many-beam theory.

Bloch wave degeneracies in systematic high energy electron diffraction

1976 ◽  
Vol 282 (1308) ◽  
pp. 485-525 ◽  
Keyword(s):  
Band Structure ◽  
Incident Beam ◽  
High Energy ◽  
Real Space ◽  
Bloch Wave ◽  
Critical Voltage ◽  
Bloch Waves ◽  
One Dimensional ◽  
Symmetry Properties ◽  
Reflexion Coefficient

For the systematic diffraction of high energy electrons by a thin crystalline slab, we study the accidental degeneracies of the Bloch waves excited in the specimen by the incident beam. It is shown that these Bloch waves are the eigenstates of a one dimensional band structure problem, and this is solved by wave matching methods. For a symmetric potential, the symmetry properties of the Bloch waves are discussed, and it is shown how accidental degeneracies of these waves can occur when the reflexion coefficient for waves incident on one unit cell of the one dimensional periodic potential vanishes. The form of the band structure and the Bloch waves in the neighbourhood of a degeneracy are derived by expanding the Kramers function in a Taylor series. It is then shown analytically how the degeneracy affects the diffracted waves emerging from the crystalline specimen (in particular, the Kikuchi pattern). To understand these effects fully, W. K. B. approximations for the Bloch waves are used to derive the Bloch wave excitations and the absorption coefficients. However, to predict the degeneracies themselves, it is shown that a different formula for the reflexion coefficient, due to Landauer, must be used. This formula shows how the critical voltage at which the Bloch waves degenerate depends on the form of the potential, and allows quick, accurate, computations of the critical voltages to be made. Also, a new higher order degeneracy is predicted for some of the systematic potentials of cadmium, lead and gold. Finally, to infer the potential in real space from measurements of critical voltages and several other quantities, we suggest an inversion scheme based on the Landauer formula for the reflexion coefficient. To a close approximation this potential is proportional to V 2 of the crystal charge density.

The usefulness of high index low symmetry zone axes for holz line analysis

1987 ◽  
Vol 45 ◽  
pp. 384-385
Author(s):  
C. M. Sung ◽  
D. B. Williams
Keyword(s):  
Lattice Parameter ◽  
High Order ◽  
Zone Axis ◽  
High Index ◽  
High Symmetry ◽  
Second Phases ◽  
Line Patterns ◽  
Low Symmetry ◽  
Line Analysis

Researchers have tended to use high symmetry zone axes (e.g. <111> <114>) for High Order Laue Zone (HOLZ) line analysis since Jones et al reported the origin of HOLZ lines and described some of their applications. But it is not always easy to find HOLZ lines from a specific high symmetry zone axis during microscope operation, especially from second phases on a scale of tens of nanometers. Therefore it would be very convenient if we can use HOLZ lines from low symmetry zone axes and simulate these patterns in order to measure lattice parameter changes through HOLZ line shifts. HOLZ patterns of high index low symmetry zone axes are shown in Fig. 1, which were obtained from pure Al at -186°C using a double tilt cooling holder. Their corresponding simulated HOLZ line patterns are shown along with ten other low symmetry orientations in Fig. 2. The simulations were based upon kinematical diffraction conditions.

Kinematic Approximations in TED and RHEED Analysis of Reconstructed Surfaces

1990 ◽  
Vol 48 (2) ◽  
pp. 396-397
Author(s):  
L. -M. Peng ◽  
M. J. Whelan
Keyword(s):  
Electron Diffraction ◽  
Kinematic Analysis ◽  
Incident Beam ◽  
High Energy ◽  
Energy Electron ◽  
Great Success ◽  
High Energy Electron ◽  
Kinematic Approximation ◽  
Reconstructed Surfaces

In recent years there has been a trend in the structure determination of reconstructed surfaces to use high energy electron diffraction techniques, and to employ a kinematic approximation in analyzing the intensities of surface superlattice reflections. Experimentally this is motivated by the great success of the determination of the dimer adatom stacking fault (DAS) structure of the Si(111) 7 × 7 reconstructed surface.While in the case of transmission electron diffraction (TED) the validity of the kinematic approximation has been examined by using multislice calculations for Si and certain incident beam directions, far less has been done in the reflection high energy electron diffraction (RHEED) case. In this paper we aim to provide a thorough Bloch wave analysis of the various diffraction processes involved, and to set criteria on the validity for the kinematic analysis of the intensities of the surface superlattice reflections.The validity of the kinematic analysis, being common to both the TED and RHEED case, relies primarily on two underlying observations, namely (l)the surface superlattice scattering in the selvedge is kinematically dominating, and (2)the superlattice diffracted beams are uncoupled from the fundamental diffracted beams within the bulk.

Systematic zone-Axis orientation of NM-scale particles for microdiffraction

1990 ◽  
Vol 48 (4) ◽  
pp. 262-263
Author(s):  
E D Boyes ◽  
L Hanna
Keyword(s):  
Real Space ◽  
Dark Field ◽  
Zone Axis ◽  
High Symmetry ◽  
Lattice Imaging ◽  
Cell Level ◽  
Bright Field ◽  
Axis Orientation ◽  
Annular Dark Field ◽  
Target Designation

A VG HB501 FEG STEM has been modified to provide track whilst tilt [TWIT] facilities for controllably tilting selected and initially randomly aligned nanometer-sized particles into the high symmetry zone-axis orientations required for microdiffraction, lattice imaging and chemical microanalysis at the unit cell level. New electronics display in alternate TV fields and effectively in parallel on split [+VTR] or adjacent externally synchronized screens, the micro-diffraction pattern from a selected area down to <1nm2 in size, together with the bright field and high angle annular dark field [HADF] STEM images of a much wider [˜1μm] area centered on the same spot. The new system makes it possible to tilt each selected and initially randomly aligned small particle into a zone axis orientation for microdiffraction, or away from it to minimize orientation effects in chemical microanalysis. Tracking of the inevitable specimen movement with tilt is controlled by the operator, with realtime [60Hz] update of the target designation in real space and the diffraction data in reciprocal space. The spot mode micro-DP and images of the surrounding area are displayed continuously. The regular motorized goniometer stage for the HB501STEM is a top entry design but the new control facilities are almost equivalent to having a stage which is eucentric with nanometric precision about both tilt axes.

Calculation of many-beam dynamic electron diffraction without high-energy approximation

1996 ◽  
Vol 54 ◽  
pp. 128-129
Author(s):  
B. R. Ahn ◽  
N. J. Kim
Keyword(s):  
Electron Diffraction ◽  
Flat Surface ◽  
Dynamic Theory ◽  
High Energy ◽  
Perfect Crystal ◽  
Zone Axis ◽  
Beam Dynamic ◽  
Diffraction Angle ◽  
Energy Approximation ◽  
The Many

High energy approximation in dynamic theory of electron diffraction involves some intrinsic problems. First, the loss of theoretical strictness makes it difficult to comprehend the phenomena of electron diffraction. Secondly, it is difficult to believe that the approximation is reasonable especially in the following cases: 1) when accelerating voltage is not sufficiently high, 2) when the specimen is thick, 3) when the angle between the surface normal of the specimen and zone axis is large, and 4) when diffracted beam with large diffraction angle is included in the calculation. However, until now the method to calculate the many beam dynamic electron diffraction without the high energy approximation has not been proposed. For this reason, the authors propose a method to eliminate the high energy approximation in the calculation of many beam dynamic electron diffraction. In this method, a perfect crystal with flat surface was assumed. The method was applied to the calculation of [111] zone axis CBED patterns of Si.

Characterizations of Synthetic 8mol% YSZ with Comparison to 3mol %YSZ for HT-SOFC

2020 ◽  
Vol 38 (4A) ◽  
pp. 491-500
Author(s):  
Abeer F. Al-Attar ◽  
Saad B. H. Farid ◽  
Fadhil A. Hashim
Keyword(s):  
Ionic Conductivity ◽  
Electrochemical Impedance ◽  
Structural Information ◽  
Impedance Analysis ◽  
High Energy ◽  
Yttria Stabilized Zirconia ◽  
X Ray Diffraction ◽  
Stabilized Zirconia ◽  
Powder Morphology ◽  
X Ray

In this work, Yttria (Y2O3) was successfully doped into tetragonal 3mol% yttria stabilized Zirconia (3YSZ) by high energy-mechanical milling to synthesize 8mol% yttria stabilized Zirconia (8YSZ) used as an electrolyte for high temperature solid oxide fuel cells (HT-SOFC). This work aims to evaluate the densification and ionic conductivity of the sintered electrolytes at 1650°C. The bulk density was measured according to ASTM C373-17. The powder morphology and the microstructure of the sintered electrolytes were analyzed via Field Emission Scanning Electron Microscopy (FESEM). The chemical analysis was obtained with Energy-dispersive X-ray spectroscopy (EDS). Also, X-ray diffraction (XRD) was used to obtain structural information of the starting materials and the sintered electrolytes. The ionic conductivity was obtained through electrochemical impedance spectroscopy (EIS) in the air as a function of temperatures at a frequency range of 100(mHz)-100(kHz). It is found that the 3YSZ has a higher density than the 8YSZ. The impedance analysis showed that the ionic conductivity of the prepared 8YSZ at 800°C is0.906 (S.cm) and it was 0.214(S.cm) of the 3YSZ. Besides, 8YSZ has a lower activation energy 0.774(eV) than that of the 3YSZ 0.901(eV). Thus, the prepared 8YSZ can be nominated as an electrolyte for the HT-SOFC.

Algorithms for determining the phase of RHEED oscillations

2015 ◽  
Vol 48 (6) ◽  
pp. 1927-1934 ◽  
Author(s):  
Zbigniew Mitura ◽  
Sergei L. Dudarev
Keyword(s):  
Experimental Data ◽  
Direct Method ◽  
Incident Beam ◽  
Diffraction Theory ◽  
High Energy ◽  
Angle Of Incidence ◽  
High Energy Electron ◽  
Dynamical Diffraction ◽  
Glancing Angle ◽  
Low Symmetry

Oscillations of reflection high-energy electron diffraction (RHEED) intensities are computed using dynamical diffraction theory. The phase of the oscillations is determined using two different approaches. In the first, direct, approach, the phase is determined by identifying the time needed to reach the second oscillation minimum. In the second approach, the phase is found using harmonic analysis. The two approaches are tested by applying them to oscillations simulated using dynamical diffraction theory. The phase of RHEED oscillations observed experimentally is also analysed. Experimental data on the variation of the phase as a function of the glancing angle of incidence, derived using the direct method, are compared with the values computed using both the direct and harmonic methods. For incident-beam azimuths corresponding to low-symmetry directions, both approaches produce similar results.

scholarly journals Improved Phebus laser performances required for precision laser–target experiments

1998 ◽  
Vol 16 (2) ◽  
pp. 253-265 ◽  
Author(s):  
G. Thiell ◽  
R. Bailly-Salins ◽  
J.L. Bruneau ◽  
G. Coulaud ◽  
P. Estraillier ◽  
...  
Keyword(s):  
Laser Energy ◽  
Incident Beam ◽  
High Energy ◽  
Target Chamber ◽  
Laser Target ◽  
Time Resolved ◽  
X Ray ◽  
Target Experiment ◽  
Pointing Precision ◽  
Energy Shots

The Precision Phebus program, started in 1993, emphasizes a series of laser and target experiment objectives on the two-beam Phebus Nd-phosphate glass laser. Recently, three major objectives that are also very important issues for megajoule-class lasers have been met: First, the balance of the incident beam-to-beam 3ω power is shown to be in the range from 5 to 12% for 3-ns, 3ω-shaped pulses of reproducible high-energy shots; second, the smoothing uniformity of the laser energy deposited on the target, that is, the contrast of the spatial beam modulations, can be kept lower than 5%; and, finally, the tight control of the beam targeting leads to a pointing precision of less than 10 μrd on the target at the target chamber center (TCC) and of 80 μrd on X-ray sources located up to 3 cm from the TCC to improve the space- and time-resolved X-ray shadowgraphy techniques performed for target physics experiments such as implosion and hydrodynamical instability studies.

Sign in / Sign up

Export Citation Format

Share Document