Optimized Structure Parameter Determination From CBED

1990 ◽  
Vol 48 (2) ◽  
pp. 492-493
Author(s):  
Knut Marthinsen ◽  
Ragnvald Høier ◽  
Lars Nils Bakken
Keyword(s):  
Structure Factor ◽  
Intensity Variation ◽  
Computer Time ◽  
Parameter Determination ◽  
Unknown Parameters ◽  
Experimental Conditions ◽  
Line Intensities ◽  
The Many ◽  
Convergent Beam

The application of convergent beam electron diffraction (CBED) for structure symmetry determination is now well established. Application of quantitative CBED in crystal structure studies is so far much less developed. However, the 2-dimensional dynamical intensity distributions in the disks depend strongly on the structure factor magnitudes and phases for the reflections involved. This is particularly the case in the general non-systematic many-beam case where a quantitative determination of structure factor magnitudes and phases from CBED in general has to be based on full dynamical many-beam calculations. It has been shown however that for carefully chosen experimental conditions it is possible to find line intensities which can be analysed on a kinematic basis. The 1-dimensional intensity variation along a systematic row has also been utilized to determine a structure factor phase with high accuracy. However, an optimal use of the CBED patterns should utilize the general 2-dimensional intensity distribution. The number of unknown parameters in a full many-beam simulation will in principle be large, the computer time large even on supercomputers and an effective algorithm for searching in the many-parameter room is therefore highly needed.

Crystal Polarity Determination by Electron Channeling

2001 ◽  
Vol 7 (S2) ◽  
pp. 334-335
Author(s):  
J. Tafto
Keyword(s):  
Experimental Conditions ◽  
Electron Channeling ◽  
Structure Factors ◽  
Small Crystals ◽  
Polarity Determination ◽  
Convergent Beam ◽  
Interface Structures ◽  
Diffraction Effects ◽  
Beam Diffraction

Multilayers, heterostructures, nanostructures and composites are of great interest to the materials scientists, and frequently we encounter crystals lacking centrosymmetry. Thus crystal polarity determination on a microscopic scale is becoming increasingly important in describing interface structures and the internal defects in small crystals. in many cases the polarity of a crystallite can be determined by convergent beam electron diffraction, CBED. Powerful alternatives are to monitor the electron induced x-ray emission, EDS, or electron energy losses, EELS, under channeling conditions. While the determination of the phase of the structure factors, and thus the determination of the crystal polarity, relies on many beam diffraction effects when the CBED technique is used, two-beam experiments provide information about the phase of the structure factor when localized EDS or EELS signals are detected under channeling conditions.The experimental conditions used to determine the polarity and absolute orientation from electron channeling are similar to those used in ALCHEMI experiments to locate small amounts of atoms by electron channeling.

Determination of structure factor phase invariants from non-systematic many-beam effects in convergent-beam patterns

1988 ◽  
Vol 26 (1-2) ◽  
pp. 25-30 ◽  
Author(s):  
R. Høier ◽  
J.-M. Zuo ◽  
K. Marthinsen ◽  
J.C.H. Spence
Keyword(s):  
Structure Factor ◽  
Convergent Beam ◽  
Beam Patterns

On the determination of enanttomorphs from non-systematic many-beam effects in CBED patterns

1989 ◽  
Vol 47 ◽  
pp. 484-485
Author(s):  
K. Marthinsen ◽  
R. Høier
Keyword(s):  
Structure Factor ◽  
High Accuracy ◽  
Convergent Beam Electron Diffraction ◽  
Bragg Condition ◽  
Sign Problem ◽  
Three Phase ◽  
Convergent Beam ◽  
Diffraction Effects ◽  
Beam Diffraction

A convergent beam electron diffraction (CBED) method which makes it possible to determine structure factor magnitudes and phases with high accuracy has recently been suggested. It is based on detailed simulations of non-systematic many-beam diffraction effects in the disks. Basis for the phase determination is an asymmetry which may appear in a line h with respect to the Bragg condition of the coupled reflection g near a three-beam condition. Approximate analytical three-beam solutions show that the sign and size of this asymmetry depends on the structure factor phases Θh of the reflections h involved through a term cos(Φ) where Φ is the three phase structure invariant, Φ = Θh + Θg + Θh-g. The magnitude of the phase invariant is thus in principle available, but not the sign. The aim of the present work has been to discuss the origin of the sign problem and the possibilities of distinguishing +/−Φ.

Determination of Positional Parameters by CBED

1990 ◽  
Vol 48 (2) ◽  
pp. 518-519
Author(s):  
Michiyoshi Tanaka ◽  
Kenji Tsuda
Keyword(s):  
Low Temperature ◽  
Waller Factor ◽  
Parameter Determination ◽  
Temperature Phase ◽  
Crystal Imperfections ◽  
Temperature Form ◽  
Debye Waller Factor ◽  
Slight Rotation ◽  
Convergent Beam

Convergent-beam electron diffraction (CBED) has been successfully applied to the determination of symmetries of perfect crystals and of characteristics of crystal imperfections. CBED is now entering a stage of quantitative studies. Vincent et al. first demonstrated for AuGeAs a method to determine atomic positions with use of CBED patterns, in which the positional parameters were determined by fitting the intensities of higher-order Laue-zone (HOLZ) reflections calculated under the (quasi) kinematical approximation with the experimental intensities.We report a simple case of the positional parameter determination for the low temperature phase of SrTiO3. This material undergoes a second order phase transformation at 110 K from the high temperature form of Pm3m to the low temperature form of I4/mcm upon the slight rotation of the oxygen octahedra or the condensation of the R25 mode (Fig. 1). The structure analysis of the phase I4/mcm means to determine the rotation angle ϕ and the isotropic Debye-Waller factor B of the oxygen ions.

ON THE EFFECT OF OVERBURDEN ON ELECTROMAGNETIC ANOMALIES—A REVIEW

Geophysics ◽  
1970 ◽  
Vol 35 (4) ◽  
pp. 646-659 ◽  
Author(s):  
Amalendu Roy
Keyword(s):  
Theoretical Work ◽  
Unknown Parameters ◽  
Model Studies ◽  
Theoretical Investigations ◽  
Fundamental Objection ◽  
The Many ◽  
Major Objection ◽  
Theoretical Analyses ◽  
Made In

The effect of a conducting overburden in EM prospecting intuitively is considered to be one of degeneration of anomalies, in the sense that the detection of a target and the determination of its unknown parameters become more difficult or ambiguous when an overburden is present than when it is absent. Recently, however, Negi (1967) and Negi and Raval (1969) have suggested on the basis of theoretical work that, if a certain combination of the parameters involved occurs, a conducting overburden can make a target more detectable than it would be without any overburden. These theoretical results and the existing experimental evidence are examined in this paper for the possible existence of “negative screening,” as this effect has been called. Due to a number of incorrect assumptions made in the theoretical analyses by the authors who predicted negative screening, their conclusions do not seem to be valid. A fundamental objection in the case of the stratified sphere, for instance, pertains to the assumption that, in presence of the annulus, the contribution of the inner sphere alone to the total external measurable magnetic field can be obtained by simply subtracting the response of the larger uniform isolated sphere from that of the double sphere. Another major objection concerns the notion in the theoretical analyses that detectability is determined by the contribution from the target alone—a quantity which one can never measure—without regard to the simultaneous contribution from the conducting overburden. Defined on the basis of the measurable total response from the system as a whole, detectability falls progressively with overburden conductivity. Although the existing results of model EM experiments are generally indicative of the absence of a phenomenon like negative screening, no clearcut and indisputable conclusion can be arrived at on the basis of model studies. In some recently published experimental results, there is one solitary instance, unnoticed by the experimenters themselves, which could suggest the existence of negative screening. We believe, however, that, due to the many inherent uncertainties in model EM work, conclusions based on theoretical investigations have to be accepted as more reliable until carefully planned model work is carried out with this specific problem in view.

scholarly journals Non-systematic Three-beam Effects in Dynamical Electron Diffraction and Their Use in Determination of Amplitude and Phase of Structure Factors

1988 ◽  
Vol 41 (3) ◽  
pp. 449 ◽  
Author(s):  
K Marthinsen ◽  
H Matsuhata ◽  
R Hfier ◽  
J Gjfnnes
Keyword(s):  
Electron Diffraction ◽  
Structure Factor ◽  
Bloch Wave ◽  
Critical Voltage ◽  
Dispersion Surface ◽  
Structure Factors ◽  
Three Phase ◽  
Convergent Beam ◽  
Diffraction Condition

The treatment of non-systematic multiple-beam effects in dynamical diffraction is extended. Expressions for Bloch wave degeneracies are given in the centrosymmetrical four-beam case and for some symmetrical directions. These degeneracies can be determined experimentally either as critical voltages or by locating the exact diffraction condition at a fixed voltage. The accuracy when applied to structure factor determination is comparable with the systematical critical voltage, namely 1% in UfT The three-beam case 0, g, h is treated as well in the non-centrosymmetrical case, where it can be used for determination of phases. It is shown that the contrast features can be represented .by an effective structure factor defined by the gap at the dispersion surface. From the variation in the gap with diffraction condition, a method to determine the three-phase structure invariant I\J = 9 + _ h + h _ 9 is given. The method is based upon the contrast asymmetry in the weaker diffracted beam and can be applied in Kikuchi, convergent beam or channelling patterns. Calculations relating to channelling in backscattering are also presented.

Sign in / Sign up

Export Citation Format

Share Document