An X-ray diffraction study of AgNbO3 and comparison with NaNbO3

1999 ◽  
Vol 14 (4) ◽  
pp. 253-257 ◽  
Author(s):  
C. N. W. Darlington
Keyword(s):  
Diffraction Pattern ◽  
Figure Of Merit ◽  
Parent Phase ◽  
Incident Beam ◽  
Large Error ◽  
High Symmetry ◽  
X Ray Diffraction ◽  
Beam Focusing ◽  
Low Symmetry

The powder diffraction pattern of the perovskite AgNbO3 has been measured using CuKα1 radiation with an incident beam focusing monochromator to eliminate the Kα2 component. Indexing the pattern shows that the multipartite cell is 2×2×4 times that of the pseudocubic subcell. Comparison is made with the diffraction pattern of NaNbO3, which has a similar multipartite unit cell. There are strong similarities, but close inspection shows that the structures are not isomorphous. The paper concludes with a discussion of the figure of merit FN for pseudosymmetric structures. It is suggested that two figures of merit be reported. The first should be the standard one using either all measured reflections or just the first 30. The proposed second figure of merit does not include any superlattice reflections. These superlattice reflections tend to be very weak, resulting in a low completeness factor and relatively large error in the measurement of their position. This effect produces an unrealistically low value of the standard figure of merit. By including only “main” reflections, i.e., those reflections that are common to both the low-symmetry and high-symmetry parent phase (if it exists), a much better estimate of the quality of the fitting of the measured diffraction pattern is obtained.

High-resolution X-ray diffraction study of the chromite (Mg0.60Fe0.402+)(Al0.39Cr1.50Fe0.093+)O4

1998 ◽  
Vol 13 (2) ◽  
pp. 96-99 ◽  
Author(s):  
T. R. C. Fernandes ◽  
J. I. Langford
Keyword(s):  
High Resolution ◽  
Diffraction Study ◽  
Diffraction Data ◽  
Cell Parameter ◽  
Incident Beam ◽  
X Ray Diffraction ◽  
Calculated Density ◽  
X Ray ◽  
Figures Of Merit ◽  
Beam Focusing

As part of a study by high-resolution X-ray diffraction of chrome ores from the Great Dyke, Zimbabwe, powder data are reported for a well-crystallized ferroan magnesiochromite spinel, in which some Cr had been replaced by Al. Data were obtained by using CuKα1 radiation, with an incident beam focusing monochromator to eliminate the Kα2 component. The cell parameter is a0=8.3123(2) Å, the figures of merit are M17=383 and F17=182 (0.0055, 17) and the calculated density is 4.50(5) Mgm−3. A small amount of sample broadening was observed and this was attributed to a mean crystallite size of 259(1) nm. © 1997 International Center for Diffraction Data.

Multiple scattering and displacive modulation in the YBa2Cu3O7-δ system

1991 ◽  
Vol 49 ◽  
pp. 1082-1083
Author(s):  
Yimei Zhu ◽  
M. Suenaga ◽  
A.R. Moodenbaugh
Keyword(s):  
Diffraction Pattern ◽  
Multiple Scattering ◽  
Diffuse Scattering ◽  
Incident Beam ◽  
High Symmetry ◽  
Diffuse Intensity ◽  
Modulated Structure ◽  
Transmission Electron ◽  
Scattering Effects ◽  
Structure Modulation

Using transmission electron microscopy, structure modulation associated with electron multiple scattering and diffuse scattering in oxygen reduced, and Fe and Co doped YBa2Cu3O7_δ has been studied. The modulated structure (tweed) exhibits [110] and roughly-periodic lenticular domain images (see Fig.l) and cross-shaped diffuse intensity in diffraction (see Fig.2). The overall diffraction pattern in the [001] zone has a 4-fold symmetry with diffraction intensity which falls off monotonically from one Brillouin zone to the next. Superimposed over each Bragg spot seem to be two cross-streaks of equal length in the [110] and directions.However, for a diffraction pattern in a high symmetry Laue case, one must consider the possibility of multiple scattering effects, whereby a diffracted electron beam acts as an incident beam. This can complicate diffuse scattering phenomena, even in a thin crystal. Careful inspection of Fig.2 reveals that the shapes of the diffuse scattering are not all identical at all diffraction spots.

scholarly journals Matching X-ray source, optics and detectors to protein crystallography requirements

1999 ◽  
Vol 55 (10) ◽  
pp. 1663-1668 ◽  
Author(s):  
Colin Nave
Keyword(s):  
Radiation Damage ◽  
Diffraction Pattern ◽  
Spatial Resolution ◽  
Diffraction Data ◽  
Incident Beam ◽  
Unit Cell ◽  
X Ray Diffraction ◽  
X Ray ◽  
Cell Dimensions ◽  
Unit Cell Dimensions

A review of the requirements for collecting X-ray diffraction data from protein crystals is given, with an emphasis on the properties of the crystal and its diffraction pattern. The size, unit-cell dimensions and perfection of the crystals can all be related to the required size and divergence of the incident X-ray beam, together with the size and spatial resolution of the detector. The X-ray beam causes primary radiation damage, even in frozen crystals. If the incident beam is very intense, temperature rises and gradients could occur in the crystal. The extent to which these problems can be overcome is also discussed.

Modeling of energy-dispersive X-ray diffraction for high-symmetry crystal orientation

2019 ◽  
Vol 75 (1) ◽  
pp. 63-70
Author(s):  
Danut Dragoi ◽  
Alexandru Dragoi
Keyword(s):  
Crystal Orientation ◽  
Incident Beam ◽  
Energy Dispersive ◽  
High Symmetry ◽  
X Ray Diffraction ◽  
X Ray ◽  
Reduced Gradient Method ◽  
Reduced Gradient ◽  
Generalized Reduced Gradient ◽  
Crystallographic Planes

The methods for X-ray crystal orientation are rapidly evolving towards versatility, fewer goniometry measurements, automation, high accuracy and precision. One method that attracts a lot of attention is energy-dispersive X-ray diffraction (EDXRD) which is based on detecting reflections from crystallographic planes in a crystal at fixed angles of a parallel polychromatic X-ray incident beam. In theory, an EDXRD peak can move in a diffraction pattern as a function of a crystallographic plane d-spacing and its orientation relative to a fixed direction in space can change also. This is equivalent to the possibility of measuring the orientation of single crystals. The article provides a modeling for the EDXRD method whose main feature is the nonmoving crystal in the sense of traditional goniometry where the angle measurements of diffracting planes are a must. The article defines the equation of orientation for the method and shows the derivation in great detail. It is shown that the exact solutions of the equations can be obtained using the generalized reduced gradient method, a mathematical subroutine that is implemented in Excel software. The significance and scientific impact of the work are discussed along with the validated tested results.

A protocol for searching the most probable phase-retrieved maps in coherent X-ray diffraction imaging by exploiting the relationship between convergence of the retrieved phase and success of calculation

2017 ◽  
Vol 24 (5) ◽  
pp. 1024-1038 ◽  
Author(s):  
Yuki Sekiguchi ◽  
Saki Hashimoto ◽  
Amane Kobayashi ◽  
Tomotaka Oroguchi ◽  
Masayoshi Nakasako
Keyword(s):  
Diffraction Pattern ◽  
Electron Density ◽  
Figure Of Merit ◽  
X Ray Diffraction ◽  
X Ray ◽  
Density Maps ◽  
Diffraction Imaging ◽  
Density Map ◽  
Diffraction Patterns ◽  
Electron Density Map

Coherent X-ray diffraction imaging (CXDI) is a technique for visualizing the structures of non-crystalline particles with size in the submicrometer to micrometer range in material sciences and biology. In the structural analysis of CXDI, the electron density map of a specimen particle projected along the direction of the incident X-rays can be reconstructed only from the diffraction pattern by using phase-retrieval (PR) algorithms. However, in practice, the reconstruction, relying entirely on the computational procedure, sometimes fails because diffraction patterns miss the data in small-angle regions owing to the beam stop and saturation of the detector pixels, and are modified by Poisson noise in X-ray detection. To date, X-ray free-electron lasers have allowed us to collect a large number of diffraction patterns within a short period of time. Therefore, the reconstruction of correct electron density maps is the bottleneck for efficiently conducting structure analyses of non-crystalline particles. To automatically address the correctness of retrieved electron density maps, a data analysis protocol to extract the most probable electron density maps from a set of maps retrieved from 1000 different random seeds for a single diffraction pattern is proposed. Through monitoring the variations of the phase values during PR calculations, the tendency for the PR calculations to succeed when the retrieved phase sets converged on a certain value was found. On the other hand, if the phase set was in persistent variation, the PR calculation tended to fail to yield the correct electron density map. To quantify this tendency, here a figure of merit for the variation of the phase values during PR calculation is introduced. In addition, a PR protocol to evaluate the similarity between a map of the highest figure of merit and other independently reconstructed maps is proposed. The protocol is implemented and practically examined in the structure analyses for diffraction patterns from aggregates of gold colloidal particles. Furthermore, the feasibility of the protocol in the structure analysis of organelles from biological cells is examined.

Molecular Dynamics Study of Commensurate-Incommensurate Phases in Hexamethylenetetramine Suberate

2002 ◽  
Vol 731 ◽  
Author(s):  
Yuansheng Pan ◽  
David Brown ◽  
Gervais Chapuis
Keyword(s):  
Molecular Dynamics ◽  
Dynamics Simulation ◽  
High Symmetry ◽  
X Ray Diffraction ◽  
X Ray ◽  
Incommensurate Structure ◽  
Incommensurate Modulation ◽  
Long Range Ordering ◽  
Lock In ◽  
Low Symmetry

AbstractIncommensurate structure of Hexamethylenetetramine suberate (C6H12N4)(HOOC-(CH2)6-COOH) has been solved from single crystal x-ray diffraction data. A molecular dynamics simulation of this system was carried out from 15 K to 580K. A second-generation consistent forcefield (CFF) and a compensating pressure tensor field were used to describe the interactions between atoms and to account for deficiencies in the forcefield. Starting from the experimental 298K structure, the phase transitions were investigated over an extended temperature range. A high symmetry commensurate structure exists at temperatures between 410K and 290K. For temperatures lower than 290K, a new periodicity appears in the structure. The system reaches a low symmetry lock-in phase at about 150K. An incommensurate structure appears between the high and low symmetry phases between 290K and 150K. The new periodicity associated with the incommensurate modulation is due to the appearance of additional long range ordering of the carbon chains. The present simulation not only reproduces well the experimental x-ray diffraction results but also gives new insight into the origin of the incommensurate behavior.

scholarly journals Custom Hardware Processor to Compute a Figure of Merit for the Fit of X-Ray Diffraction Peaks

2008 ◽  
Vol 2008 ◽  
pp. 1-7 ◽  
Author(s):  
Juan A. Gomez-Pulido ◽  
Florentino Sanchez-Bajo ◽  
Sidolina Pereira dos Santos ◽  
Miguel A. Vega-Rodriguez ◽  
Juan M. Sanchez-Perez
Keyword(s):  
Figure Of Merit ◽  
Optimization Problem ◽  
Large Set ◽  
X Ray Diffraction ◽  
X Ray ◽  
Parallel Increase ◽  
Custom Hardware ◽  
Diffraction Peaks ◽  
Computationally Intensive

A custom processor based on reconfigurable hardware technology is proposed in order to compute the figure of merit used to measure the quality of the fit of X-ray diffraction peaks. As the experimental X-ray profiles can present many peaks severely overlapped, it is necessary to select the best model among a large set of reasonably good solutions. Determining the best solution is computationally intensive, because this is a hard combinatorial optimization problem. The proposed processors, working in parallel, increase the performance relative to a software implementation.

scholarly journals Hybrid multiple diffraction in semipolar wurtzite materials: (\bf 01\overline{1}2)-oriented ZnMgO/ZnO heterostructures as an illustration

2017 ◽  
Vol 50 (4) ◽  
pp. 1165-1173 ◽  
Author(s):  
Esther de Prado ◽  
M. Carmen Martínez-Tomás ◽  
Christiane Deparis ◽  
Vicente Muñoz-Sanjosé ◽  
Jesús Zúñiga-Pérez
Keyword(s):  
A Priori ◽  
Structural Quality ◽  
Material System ◽  
X Ray Diffraction ◽  
Single Measurement ◽  
Multiple Diffraction ◽  
Out Of Plane ◽  
Low Symmetry ◽  
Symmetric Reflection

X-ray diffraction has been widely used to characterize the structural properties (strain and structural quality) of semiconductor heterostructures. This work employs hybrid multiple diffraction to analyzer-oriented Zn1−xMgxO layers grown by molecular beam epitaxy on ZnO substrates. In such a low-symmetry material system, additional features appear in symmetric reflection scans, which are described as arising from hybrid multiple diffraction. First, the Bragg conditions necessary for these high-order processes to occur are introduced and applied to explain all the observed satellite reflections, identify the planes that contribute and computea priorithe angles at which they are observed. Furthermore, thanks to this hybrid multiple-diffraction technique, it is possible to determine the layer lattice parameters (in-plane and out-of-plane) in an easy and accurate way by using one single measurement in standard symmetric conditions. The achieved precision is at least as high as that obtained from the combination of symmetric and asymmetric reciprocal space map measurements.

Effect of Mixing Methods on the Precipitation of Basic Copper Acetate

2014 ◽  
Vol 625 ◽  
pp. 201-204 ◽  
Author(s):  
Ken Ichiro Sotowa ◽  
Takahiro Togawa ◽  
Yuika Shimizu ◽  
Masashi Kurashina ◽  
Toshihide Horikawa ◽  
...  
Keyword(s):  
Sodium Hydroxide ◽  
Diffraction Pattern ◽  
Strong Influence ◽  
Copper Acetate ◽  
X Ray Diffraction ◽  
Mixing Rate ◽  
X Ray ◽  
Batch System ◽  
Unknown Peak

Basic copper acetate can be obtained by mixing copper acetate with sodium hydroxide solutions. However, the quality of the resulting crystals is usually poor. An unknown peak appears in its X-ray diffraction pattern, and moreover, the intensity of the peak varies from sample to sample. In this study, effect of mixing methods on the quality of the basic copper acetate crystals was studied. The crystals were synthesized by employing three different mixing methods, which were batch, semi-batch system, and micromixer. The intensity of the unknown peak weakened when the semi-batch system was employed. The results implied that the mixing rate gave strong influence on the quality of the resulting basic copper acetate crystals.

Improved crystallographic data for graphite

2003 ◽  
Vol 18 (2) ◽  
pp. 150-154 ◽  
Author(s):  
J. Y. Howe ◽  
C. J. Rawn ◽  
L. E. Jones ◽  
H. Ow
Keyword(s):  
Powder Diffraction ◽  
Figure Of Merit ◽  
Interlayer Spacing ◽  
X Ray Diffraction ◽  
Hexagonal Cell ◽  
Data Set ◽  
X Ray ◽  
Cell Dimensions ◽  
Unit Cell Dimensions

Powder diffraction pattern of SP-1 graphite has been obtained using synchrotron X-ray diffraction. Unit cell dimensions were calculated using a least-squares analysis that refined to a |Δ2θ°| of no more than 0.007. A hexagonal cell was determined with a space group of P63/mmc (194), a=2.4617(2) and c=6.7106 (4) Å. The Smith/Synder figure of merit is 167 based upon 11 peaks, which indicates that the quality of this data set is superior to the existing PDF card for graphite, 41-1487. It is also emphasized that the interlayer spacing of graphite should be 3.355(1) Å. Using GAS and EXPGUI codes, a new set of calculated powder diffraction data based upon the interlayer spacing of 3.555 Å is generated. A comparison with the current calculated card, 75-1621, has also been made.

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