Structure solution of the Al69.2Cu20Cr10.8 ϕ phase

2022 ◽  
Vol 55 (1) ◽  
Author(s):  
Shmuel Samuha ◽  
Rimon Tamari ◽  
Benjamin Grushko ◽  
Louisa Meshi
Keyword(s):  
Rietveld Refinement ◽  
Structural Model ◽  
Layered Structures ◽  
Lattice Parameter ◽  
Structural Relationship ◽  
Golden Mean ◽  
Common Features ◽  
Structure Solution

The stable ϕ phase that forms below ∼923 K around the Al69.2Cu20.0Cr10.8 composition was found to be hexagonal [P63, a = 11.045 (2), c = 12.688 (2) Å] and isostructural to the earlier reported Al6.2Cu2Re X phase [Samuha, Grushko & Meshi (2016). J. Alloys Compd. 670, 18–24]. Using the structural model of the latter, a successful Rietveld refinement of the XRD data for Al69.5Cu20.0Cr10.5 was performed. Both ϕ and X were found to be structurally related to the Al72.6Cu11.0Cr16.4 ζ phase [P63/m, a = 17.714, c = 12.591 Å; Sugiyama, Saito & Hiraga (2002). J. Alloys Compd. 342, 148–152], with a close lattice parameter c and a τ-times-larger lattice parameter a (τ is the golden mean). The structural relationship between ζ and ϕ was established on the basis of the similarity of their layered structures and common features. Additionally, the strong-reflections approach was successfully applied for the modeling of the ϕ phase based on the structural model of the ζ phase. The latter and the experimental structural model (retrieved following Rietveld refinement) were found to be essentially identical.

scholarly journals Structural Relationship of Key Factors for Student Satisfaction and Achievement in Asynchronous Online Learning

2021 ◽  
Vol 13 (12) ◽  
pp. 6734
Author(s):  
Sohee Kim ◽  
Dae-Jin Kim
Keyword(s):  
Online Learning ◽  
Student Engagement ◽  
Student Satisfaction ◽  
Survey Data ◽  
Structural Model ◽  
Student Interaction ◽  
Structural Relationship ◽  
Key Factors ◽  
Course Structure ◽  
Instructor Presence

This study examines the structural relationship among key factors influencing student satisfaction and achievement in online learning. A structural model was developed by considering course structure, student–student interaction, instructor presence, student engagement, student satisfaction and achievement as key factors. In order to verify the effectiveness of the developed structural model, we utilized the survey data collected from a total of 250 students enrolled in two asynchronous online courses offered at Kyung Hee University in Korea in the fall semester of 2020. Then, the collected survey data were analyzed using the structural equation model. The verification of the statistical analysis results indicates that the course structure has a more significant effect on the student satisfaction and achievement than the other key factors such as the student–student interaction, instructor presence and student engagement. It also reveals that the student engagement affects only the student satisfaction and has a mediated effect between student–student interaction and student satisfaction.

scholarly journals The Rietveld Refinement in the EXPO Software: A Powerful Tool at the End of the Elaborate Crystal Structure Solution Pathway

Crystals ◽  
2018 ◽  
Vol 8 (5) ◽  
pp. 203 ◽  
Author(s):  
Angela Altomare ◽  
Francesco Capitelli ◽  
Nicola Corriero ◽  
Corrado Cuocci ◽  
Aurelia Falcicchio ◽  
...  
Keyword(s):  
Crystal Structure ◽  
Rietveld Refinement ◽  
Structure Solution

Chemical reasonableness in Rietveld analysis; organics

2007 ◽  
Vol 22 (1) ◽  
pp. 74-82 ◽  
Author(s):  
James A. Kaduk
Keyword(s):  
Rietveld Refinement ◽  
Normal Values ◽  
Solution Process ◽  
Molecular Geometry ◽  
Rietveld Analysis ◽  
Rigid Bodies ◽  
Torsion Angles ◽  
Structure Solution ◽  
Calcium Tartrate

We know a lot about normal values of bond distances, bond angles, torsion angles, and other molecular parameters. This knowledge can be incorporated into the structure solution process and into Rietveld refinement through the use of restraints and rigid bodies. An important measure of the quality of the refined model is the chemical reasonableness of molecular geometry. Refinement of the structures of calcium tartrate tetrahydrate and guaifenesin is used to illustrate the importance of chemical reasonableness in determining the quality of a Rietveld refinement.

Rietveld Refinement of Sintered Magnesium Substituted Calcium Apatite

2008 ◽  
Vol 396-398 ◽  
pp. 277-280 ◽  
Author(s):  
Marcia S. Sader ◽  
Elizabeth L. Moreira ◽  
Valéria C.A. Moraes ◽  
Jorge C. Araújo ◽  
Racquel Z. LeGeros ◽  
...  
Keyword(s):  
Rietveld Refinement ◽  
Tricalcium Phosphate ◽  
Rietveld Method ◽  
Lattice Parameter ◽  
Commercial Product ◽  
Precipitation Method ◽  
Osteoporosis Therapy ◽  
Biomineralization Process ◽  
Apatite Lattice

The incorporation of magnesium in the synthetic apatite has been associated with biomineralization process and osteoporosis therapy in human and animals. Magnesium easily replaces calcium in the apatite lattice and influences or controls the hydroxyapatite crystallization processes. In this work, Mg-substituted calcium deficient apatite, with Mg/Ca ratio = 0.1, 0.15 and 0.2 were synthesized by precipitation method. Then, sintered at 1000 oC and compared with a commercial product labeled as tricalcium phosphate sintered at the 1000 oC. The sintered products showed tricalcium phosphate (β-TCP) structure. The Mg2+ substitution in the Ca(4) and Ca(5) sites of β-TCP and the lattice parameter changes were estimated using the Rietveld method. Using this method, the formulas Ca2.73(Mg0.27)(PO4)2, Ca2.71(Mg0.29)(PO4)2 and Ca2.70(Mg0.23Mg0.07)(PO4)2 were calculated for the samples with Mg/Ca ratio = 0.1, 0.15 and 0.2 respectively.

RES2INS: a graphical interface for theSHELXprogram suite

1998 ◽  
Vol 31 (6) ◽  
pp. 963-964
Author(s):  
Leonard J. Barbour ◽  
Jerry L. Atwood
Keyword(s):  
Operating System ◽  
Structural Model ◽  
Graphical Interface ◽  
Structure Solution ◽  
User Friendly

RES2INSruns under the MS-DOS operating system and allows the user to view graphically the results of successiveSHELXstructure solution and refinement runs. In addition, the structural model can be edited in a user-friendly manner and these changes can be carried through to a newSHELXinstruction file. The program is menu driven and extensive use is made of the mouse for the facilitation of operations on individual atoms.

scholarly journals FIDEL: A new method for SDPD of nanocrystalline organic compounds

2014 ◽  
Vol 70 (a1) ◽  
pp. C134-C134
Author(s):  
Martin Schmidt ◽  
Stefan Habermehl ◽  
Philipp Moerschel ◽  
Pierre Eisenbrandt
Keyword(s):  
Crystal Structures ◽  
Rietveld Refinement ◽  
Organic Compounds ◽  
Structure Data ◽  
Structural Model ◽  
Lattice Energy ◽  
Lattice Parameters ◽  
Low Energy ◽  
Powder Pattern ◽  
Field Methods

Rietveld refinements generally fail, if the lattice parameters of the structural model differ more than slightly from the correct lattice parameters and the simulated reflections do not overlap with the experimental ones. For molecular crystals, we have developed a more robust fitting algorithm, which uses the cross-correlation function of calculated and experimental powder patterns, and allows a FIt with DEviating Lattice parameters (FIDEL). The method is also successful for nanocrystalline organic compounds showing only 10-20 peaks in their powder diagrams. The FIDEL method has proven to be useful for various applications, including refinements starting from (1) structure data of an isostructural chemical derivative; (2) structure data of an isostructural hydrate or solvate; (3) structure data from measurements at another temperature (e.g. for fitting a room-temperature powder diagram starting with a structure determined from a single-crystal measurement at 100K). FIDEL is also used for determining crystal structures from non-indexed powder diagrams of nanocrystalline organic compounds. Three steps are performed: (1) Prediction of possible crystal structures in various space groups using global lattice-energy minimizations by force-field methods. (2) FIDEL fit of 100 to 600 low-energy structures to the experimental powder pattern. The structure candidate leading to the correct structure results in a significantly better fit than all other structures. (3) Rietveld refinement. The FIDEL method was used to determine the hitherto unknown crystal structure of the nanocrystalline alpha-phase of 2,9-dichloroquinacridone (C20H10Cl2N2O2). The upper part of the figure shows the experimental powder pattern and the simulated powder diagram of one of the predicted low-energy structures before any fitting. The lower part displays the result of the FIDEL fit, before the Rietveld refinement.

Real-space technique applied to crystal structure determination from powder data

2002 ◽  
Vol 35 (2) ◽  
pp. 182-184 ◽  
Author(s):  
Angela Altomare ◽  
Corrado Cuocci ◽  
Carmelo Giacovazzo ◽  
Antonietta Guagliardi ◽  
Anna Grazia Giuseppina Moliterni ◽  
...  
Keyword(s):  
Crystal Structure ◽  
Powder Diffraction ◽  
Structural Model ◽  
Real Space ◽  
Structure Parameters ◽  
Powder Diffraction Data ◽  
Structure Solution ◽  
Phase Extension ◽  
Space Technique ◽  
New Process

Real-space techniques used for phase extension and refinement in the modern direct procedures forab initiocrystal structure solution of proteins have been optimized for application to powder diffraction data. The new process has been implemented in a modified version ofEXPO[Altomareet al.(1999).J.Appl.Cryst.32, 339–340]. The method is able to supply a structural model that is more complete than that provided by the standardEXPOprogram. The model is then refinedviaa diagonal least-squares procedure, but only when the ratio of the number of observations to the number of structure parameters to be refined is larger than a given threshold.

In situ structure solution of helical sulphur at 3GPa and 400ºC

2001 ◽  
Vol 216 (8) ◽  
Author(s):  
W. A. Crichton ◽  
G. B. M. Vaughan ◽  
M. Mezouar
Keyword(s):  
Global Optimization ◽  
Simulated Annealing ◽  
Rietveld Refinement ◽  
Diffraction Data ◽  
Unit Cell ◽  
X Ray Diffraction ◽  
Space Group ◽  
X Ray ◽  
Structure Solution

AbstractThe structure of a 2-chain helical form of sulphur with 9 atoms per unit-cell has been determined from powder synchrotron x-ray diffraction data obtained at 3 GPa and 400ºC, using a combination of global optimization, simulated annealing and Rietveld refinement techniques. Final refinement of the structure in trigonal space group

A NOTE ON THE RATIONALITY TEST

2016 ◽  
Vol 21 (8) ◽  
pp. 2121-2137
Author(s):  
Alex Ilek
Keyword(s):  
Low Power ◽  
High Probability ◽  
Structural Model ◽  
Structural Relationship ◽  
Expectations Formation

The rationality test is based on regressing the actual series of variable in period t on its forecast formed in period t − 1. The nonrejection of the rationality hypothesis requires expectations' unbiasedness and efficiency. We show that in a model with t − 1 dating, where expectations strongly and positively affect the economy, the rationality test suffers from low power. It provides a high probability of nonrejection of the rationality hypothesis against numerous alternatives for expectations formation. This result is attained because the realization of the economy is driven by the public's expectations, irrespective of how well they are formed, via the structural relationship between them. The parameters in this test are predetermined by the parameters of the structural model, supporting expectations' unbiasedness under reasonable assumptions. Thus, successfully passing the rationality test could be misleading in terms of interpretation of the quality of the expectations and could lead to questionable accuracy of many applications.

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