TEMPERATURE DEPENDENCE OF THE STRUCTURAL PARAMETERS IN THE TRANSFORMATION OF ARAGONITE TO CALCITE, AS DETERMINED FROM IN SITU SYNCHROTRON POWDER X-RAY-DIFFRACTION DATA

2010 ◽  
Vol 48 (5) ◽  
pp. 1225-1236 ◽  
Author(s):  
S. M. Antao ◽  
I. Hassan
Keyword(s):  
Temperature Dependence ◽  
Diffraction Data ◽  
Structural Parameters ◽  
X Ray Diffraction ◽  
X Ray

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

Accounting for unknown systematic errors in Rietveld refinements: a Bayesian statistics approach

2015 ◽  
Vol 48 (4) ◽  
pp. 1201-1211 ◽  
Author(s):  
Anton Gagin ◽  
Igor Levin
Keyword(s):  
Bayesian Statistics ◽  
Diffraction Data ◽  
Structural Parameters ◽  
Simulated Data ◽  
Systematic Errors ◽  
Data Sets ◽  
X Ray Diffraction ◽  
Rietveld Refinements ◽  
X Ray ◽  
Minimization Algorithms

A method has been developed to address the effects of systematic errors in Rietveld refinements using powder diffraction data. Relevant errors were categorized into multiplicative, additive and peak-shape types. Corrections for these errors were incorporated into structural refinements using a Bayesian statistics approach, with the corrections themselves treated as nuisance parameters and marginalized out of the analysis. Structural parameters refined using the proposed method represent probability-weighted averages over all possible error corrections. The developed formalism has been adapted to least-squares minimization algorithms and implemented as an extension to the Rietveld software packageGSAS-II. The technique was first tested using neutron and X-ray diffraction data simulated for PbSO4and then applied to the equivalent experimental data sets for the same compound. The results obtained using the simulated data confirmed that the proposed method yields significantly more accurate estimates of structural parameters and their uncertainties than standard refinements. The benefits were particularly significant for joint refinements using neutron and X-ray diffraction data because accounting for systematic errors enabled more adequate weighting of the individual data sets.

Synthesis and Structures of Simple (Silylmethyl)(methyl)ethers

2003 ◽  
Vol 58 (8) ◽  
pp. 759-763 ◽  
Author(s):  
Norbert W. Mitzel
Keyword(s):  
Crystal Structures ◽  
Structural Parameters ◽  
Structural Basis ◽  
X Ray Diffraction ◽  
X Ray ◽  
Donor Acceptor

The compound Cl3SiCH2OCH3 was prepared by reacting ClCH2OCH3 with the Cl3SiH/NEt3 reagent. H3SiCH2OCH3 and F3SiCH2OCH3 were synthesized from Cl3SiCH2OCH3 by reduction with LiAlH4 and by fluorination with SbF3, respectively. The crystal structures of the low-melting compounds H3SiCH2OCH3 and F3SiCH2OCH3 were determined by X-ray diffraction of in situ grown crystals. Both compounds do not show any observable β -donor-acceptor interactions, but behave structurally like usual dialkylethers or silanes, as is obvious from the structural parameters in H3SiCH2OCH3 (<SiCO 108.4(3)-109.4(3)°, <COC 111.0(4)-111.6(4)°) and in F3SiCH2OCH3 (<SiCO 107.1(1), <COC 111.2(2)°). Earlier postulates of Si· · ·O interactions in compounds with SiCO units could thus not be confirmed on a structural basis.

The Crystal Structures of Chlorodimethyl( dimethylamino)silane and Dimethyl-bis-(dimethylamino)silane

2003 ◽  
Vol 58 (7) ◽  
pp. 708-710 ◽  
Author(s):  
Norbert W. Mitzel ◽  
Krunoslav Vojinović
Keyword(s):  
Single Crystals ◽  
Crystal Structures ◽  
Low Temperatures ◽  
Structural Parameters ◽  
X Ray Diffraction ◽  
X Ray

Single crystals of chlorodimethyl(dimethylamino)silane, Me2NSiMe2Cl, and dimethyl-bis-(dimethylamino)silane, (Me2N)2SiMe2, have been grown in situ from the melt at low temperatures and their structures determined by X-ray diffraction. Important structural parameters (Å / °): Me2NSiMe2Cl (C2/m) Si-N 1.686(2), Si-C 1.851(1), Si- Cl 2.109(1), N-Si-Cl 111.7(1), C-Si-Cl 105.1(1), C-N-C 112.8(2), Si-N-C 123.4(1); (Me2N)2SiMe2 (P21/c) Si-N(1) 1.725(1), Si-C(1) 1.868(1), N(1)-Si-N(2) 105.7(1), C(3)- N(1)-C(4) 111.6(1), Si-N(1)-C(3) 122.4(1), Si-N(1)-C(4) 120.0(1).

Temperature dependence of thermal expansion tensors of energetic materials

2015 ◽  
Vol 48 (5) ◽  
pp. 1364-1380 ◽  
Author(s):  
Nadezhda B. Bolotina ◽  
A. Alan Pinkerton
Keyword(s):  
Thermal Expansion ◽  
Temperature Dependence ◽  
Diffraction Data ◽  
Energetic Materials ◽  
Variable Temperature ◽  
Unit Cell ◽  
X Ray Diffraction ◽  
X Ray

Unit-cell values as well as thermal expansion tensors for 13 energetic materials are calculated from variable-temperature X-ray diffraction data. The thermal expansion tensors and their temperature dependence are reported numerically, algebraically and graphically.

Thermal behaviour of alum-(K) KAl(SO4)2·12H2O from in situ laboratory high-temperature powder X-ray diffraction data: thermal expansion and modelling of the sulfate orientational disorder

2015 ◽  
Vol 79 (1) ◽  
pp. 157-170 ◽  
Author(s):  
Paolo Ballirano
Keyword(s):  
Thermal Expansion ◽  
High Temperature ◽  
Diffraction Data ◽  
Bond Valence ◽  
X Ray Diffraction ◽  
Orientational Disorder ◽  
X Ray ◽  
Disorder Parameter ◽  
Bond Valence Sum

AbstractThe present work analyses the thermal behaviour of alum-(K), KAl(SO4)2·12H2O, by in situ laboratory high-temperature powder X-ray diffraction data from 303 K to melting, which starts at 355 K and is completed, due to kinetics, at 359 K. The calculated a0 linear thermal expansion coefficient is of 14.68(11) × 10–6 K–1 within the investigated thermal range. The k disorder parameter, describing the extension of the orientational disorder of the sulfate group, has been found to decrease from ∼0.70 to ∼0.65 just before melting. It has been demonstrated that the occurrence of the disorder implies the coexistence of K+ ions in both six- and seven-fold coordination. This is necessary for assigning a reasonable bond-valence sum of 0.81 valence units (vu) to the 'average' K+ ion a instead of 0.66 vu, which is obtained in the case of six-fold coordination alone. We can describe the temperature dependence of k from 93–355 K by means of the empirical equation k = 0.798(12) + 2.5(11) × 10–4 T – 1.9(2) × 10–6T2, which includes reference low-temperature data. Bond-valence analysis has shown that, on cooling, an increase of the k disorder parameter and shortening of the K–O2 bond distance act together to maintain constancy in the bond-valence sum at the K site, stabilizing the structure. Therefore, the need for keeping the 'average' K+ ion at a reasonable bond-valence sum appears to be the driving force for the ordering process involving the sulfate group.

Atomic displacement parameters for Ni(ND3)4(NO2)2 from 9 K X-ray and 13 K time-of-flight neutron diffraction data

1996 ◽  
Vol 52 (6) ◽  
pp. 923-931 ◽  
Author(s):  
B. B. Iversen ◽  
F. K. Larsen ◽  
B. N. Figgis ◽  
P. A. Reynolds ◽  
A. J. Schultz
Keyword(s):  
Neutron Diffraction ◽  
Diffraction Data ◽  
Structural Information ◽  
Structural Parameters ◽  
Structure Refinement ◽  
Time Of Flight ◽  
Thermal Parameters ◽  
Neutron Diffraction Data ◽  
X Ray Diffraction ◽  
X Ray

Structural parameters derived from 9 1) K X-ray diffraction data and 13 (1) K time-of-flight neutron diffraction data on perdeuterated tetraamminedinitronickel(II), Ni(ND3)4(NO2)2, are compared. It is shown that excellent agreement can be obtained for both positional and thermal parameters derived separately from the two experiments, provided that great care is taken in all steps of the process, including data collection, data reduction, and nuclear and electronic structure refinement. The mean difference in the thermal parameters, <|ΔUij |>, is as low as 0.00034 Å2 and <(ΔUij/σ)2>1/2 = 1.92, showing that, even without any form of scaling between the parameters, the same values can be obtained. This, compared with other such studies, indicates that time-of-flight neutron diffraction data can give structural information of a quality comparable to monochromatic neutron diffraction. The excellent correspondence between the thermal parameters derived separately from X-ray and neutron diffraction data gives confidence in the deconvolution of the thermal motion from the X-ray diffraction data, which is necessary for any study of a static electron density distribution.

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