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.

Space Group Determination of the Room-Temperature Phase ofα '-NaV2O5Using Convergent-Beam Electron Diffraction

2000 ◽  
Vol 69 (7) ◽  
pp. 1939-1941 ◽  
Author(s):  
Kenji Tsuda ◽  
Shuichi Amamiya ◽  
Michiyoshi Tanaka ◽  
Yukio Noda ◽  
Masahiko Isobe ◽  
...  
Keyword(s):  
Electron Diffraction ◽  
Room Temperature ◽  
Convergent Beam Electron Diffraction ◽  
Temperature Phase ◽  
Beam Electron ◽  
Space Group ◽  
Convergent Beam

Debye-Waller Factor Measurements by Quantitative Convergent Beam Electron Diffraction (CBED)

1997 ◽  
Vol 3 (S2) ◽  
pp. 1011-1012
Author(s):  
M. Saunders ◽  
A. G. Fox ◽  
P. A. Midgley
Keyword(s):  
Electron Diffraction ◽  
Waller Factor ◽  
Order Structure ◽  
Crystalline Materials ◽  
Thermal Diffuse Scattering ◽  
X Ray ◽  
Debye Waller Factor ◽  
Convergent Beam ◽  
Thermal Diffuse ◽  
Limited Accuracy

Quantitative CBED techniques are now capable of making low-order structure factor measure-ments with sufficient accuracy to study bonding effects in crystalline materials. The main limitation of these techniques has been identified as the accuracy with which one knows the Debye-Waller factor(s) (DWF(s)). Even where X-ray measurements exist, values have usually been determined at room temperature whereas we often want to perform our electron diffraction experiments at liquid nitrogen temperatures to reduce the effects of thermal diffuse scattering (TDS). Attempts to calculate theoretical DWF values have been shown to have limited accuracy when compared to experimental measurements.This has led to a search for new electron diffraction methods for DWF determination such as the use of HOLZ line segments, high-index systematic rows and electron precession patterns. The aim should be to measure the DWF(s) under identical conditions to those used for the charge density studies, e.g. the same sample thickness, temperature and microscope settings.

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.

Order and disorder in acenaphthylene

1973 ◽  
Vol 334 (1596) ◽  
pp. 19-48 ◽  
Keyword(s):  
Low Temperature ◽  
Room Temperature ◽  
Temperature Structure ◽  
Intermediate Form ◽  
X Ray Diffraction ◽  
Space Group ◽  
Order And Disorder ◽  
Temperature Form ◽  
Diffraction Effects

Acenaphthylene, C 12 H 8 , occurs in space group Pbam (or Pba2) at room temperatures (23 °C) with a = 7.705 (5), b = 7.865 (5), c = 14.071 (5) Å and Z = 4, and is disordered. At about 130 K it undergoes a reversible transition to space group P2 1 nm with a = 7.588 (13), b = 7.549 (10), c = 27.822 (2) Å and Z = 8 (85 K) with an ordered structure. A general study of the system has revealed that the structure of both forms consists of layers of closely packed molecules stacked in the c direction. The room temperature structure has a two-layer repeat and the low temperature form a four-layer repeat. Observation of diffuse X-ray diffraction effects at temperatures close to the transition indicates that an intermediate form having a six-layer repeat is formed. A preliminary structure determination of the low-temperature form reveals that the four layers though having a similar packing scheme differ in the orientation of the constituent molecules relative to c . It is proposed that the almost circular shape of the molecules allows each layer to change its identity under thermal agitation by a rotation of its constituent molecules in their own planes. The transition can be explained in terms of changes of the correlations between neighbouring layers. A simple model based on short-range order parameters is described, which explains the occurrence of the six-layer intermediate and the observed sequence of diffuse diffraction phenomena. The nature of the structure of the disordered room temperature form, which is predicted by this model, is confirmed as far as possible with the data available which are limited because of the dearth of high-angle diffraction maxima.

Determination of Composition and Linear Lattice Expansion Coefficient in Si1−x Gex/Si Thin Films by Simulation of X-Ray Rocking Curves

1990 ◽  
Vol 208 ◽  
Author(s):  
F. Cembali ◽  
R. Fabbri ◽  
M. Servidori ◽  
A. Zani ◽  
S. Iyer
Keyword(s):  
Thin Films ◽  
Expansion Coefficient ◽  
Molecular Beam ◽  
Depth Distribution ◽  
Waller Factor ◽  
Lattice Expansion ◽  
Linear Lattice ◽  
X Ray ◽  
Debye Waller Factor

ABSTRACTBy simulation of X-ray rocking curves of Si-Ge alloys grown on Si by molecular beam epitaxy and of Ge implanted samples, the Ge composition, the linear lattice expansion coefficient, the strain depth-distribution and the static Debye-Waller factor in the MBE alloys have been determined.

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