scholarly journals “Thought experiments” as dry-runs for “tough experiments”: novel approaches to the hydration behavior of oxyanions

2016 ◽  
Vol 88 (3) ◽  
pp. 163-176 ◽  
Author(s):  
Ariel A. Chialvo ◽  
Lukas Vlcek
Keyword(s):  
Neutron Diffraction ◽  
Pair Correlation ◽  
Local Environment ◽  
Pair Formation ◽  
Distribution Functions ◽  
Ambient Conditions ◽  
Coordination Numbers ◽  
Anion Coordination ◽  
The Individual

AbstractWe explore the deconvolution of correlations for the interpretation of the microstructural behavior of aqueous electrolytes according to the neutron diffraction with isotopic substitution (NDIS) approach toward the experimental determination of ion coordination numbers of systems involving oxyanions, in particular, sulfate anions. We discuss the alluded interplay in the title of this presentation, emphasized the expectations, and highlight the significance of tackling the challenging NDIS experiments. Specifically, we focus on the potential occurrence of $N{i^{2 + }} \cdots SO_4^{2 - }$ pair formation, identify its signature, suggest novel ways either for the direct probe of the contact ion pair (CIP) strength and the subsequent correction of its effects on the measured coordination numbers, or for the determination of anion coordination numbers free of CIP contributions through the implementation of null-cation environments. For that purpose we perform simulations of NiSO4 aqueous solutions at ambient conditions to generate the distribution functions required in the analysis (a) to identify the individual partial contributions to the total neutron-weighted distribution function, (b) to isolate and assess the contribution of $N{i^{2 + }} \cdots SO_4^{2 - }$ pair formation, (c) to test the accuracy of the neutron diffraction with isotope substitution based coordination calculations and X-ray diffraction based assumptions, and (d) to describe the water coordination around both the sulfur and oxygen sites of the sulfate anion. We finally discuss the strength of this interplay on the basis of the inherent molecular simulation ability to provide all pair correlation functions that fully characterize the system microstructure and allows us to “reconstruct” the eventual NDIS output, i.e., to take an atomistic “peek” (e.g., see Figure 1) at the local environment around the isotopically-labeled species before any experiment is ever attempted, and ultimately, to test the accuracy of the “measured” NDIS-based coordination numbers against the actual values by the “direct” counting.

scholarly journals X-Ray and Neutron Diffraction of Amorphous Nickel-Zirconium-Alloys as Prepared in Different Ways

1991 ◽  
Vol 46 (6) ◽  
pp. 491-498 ◽  
Author(s):  
L. Schultz ◽  
P. Lamparter ◽  
S. Steeb
Keyword(s):  
Neutron Diffraction ◽  
Short Range ◽  
Melt Spinning ◽  
Short Range Order ◽  
Pair Correlation ◽  
Zirconium Alloys ◽  
X Ray ◽  
Coordination Numbers ◽  
Structure Factors ◽  
The One

AbstractThe structure of amorphous NiχZr100-χ-alloys (Χ= 30, 31, 34, 63.7, and 65), which were produced by melt spinning (MS), mechanical alloying (MA), and sputtering (SP) was studied by X-ray- and neutron diffraction yielding structure factors, pair correlation functions, coordination numbers, atomic distances, and Warren-Cowley chemical short range order parameters. The atomic arrangement within the first coordination sphere is independent of the method of preparation while in the second and higher spheres it differs between the MS- and the MA-alloys on the one side and the SP-specimens on the other side. Thus one understands that some physical properties of the different specimens differ

scholarly journals Crystalline perfection of an aluminium single crystal determined by neutron diffraction

2011 ◽  
Vol 44 (4) ◽  
pp. 681-687
Author(s):  
Sabrina Metairon ◽  
Carlos Benedicto Ramos Parente ◽  
Vera Lucia Mazzocchi ◽  
Thierry Jacques Lemaire
Keyword(s):  
Neutron Diffraction ◽  
Single Crystal ◽  
Relative Intensity ◽  
Three Dimensional ◽  
Point Of View ◽  
Crystalline Perfection ◽  
Contour Map ◽  
The Individual ◽  
Crystalline Domains

In this work, a study of the crystalline perfection of an aluminium single crystal is presented. The study shows that, from three-dimensional neutron diffraction rocking curves, it is possible to characterize the individual crystalline domains of a multidomain crystal. From a macroscopic point of view, the determination of the domains allows an evaluation of the crystalline perfection of the crystal under study. Three-dimensional rocking curves have been obtained by neutron diffraction from a large mosaic aluminium crystal. Construction of a contour map of individual domains made it easier to determine the breadth and relative intensity of each domain. The angular distances between domains were also determined.

scholarly journals Local Environment and Dynamic Behavior of Fluoride Anions in Silicogermanate Zeolites: A Computational Study of the AST Framework

2018 ◽  
Author(s):  
Michael Fischer
Keyword(s):  
Density Functional ◽  
Computational Study ◽  
Local Environment ◽  
Distribution Functions ◽  
Suitable Model ◽  
General Behaviour ◽  
Functional Theory ◽  
Molecular Dynamics Calculations ◽  
Equilibrium Structures

<p>In silicogermanate zeolites containing double four-ring (<i>d4r</i>) building units, the germanium atoms preferentially occupy the corners of these cube-like units. While this general behaviour is well known, the absence of long-range order precludes a determination of the preferred arrangements of Si and Ge atoms at the corners of <i>d4r</i> cages by means of crystallographic methods. If fluoride anions are present during the synthesis, these are incorporated into the <i>d4r</i> cages. Due to the sensitivity of the <sup>19</sup>F chemical shift to the local environment, NMR experiments can provide indirect insights into the predominant (Si,Ge) arrangements. However, conflicting interpretations have been reported, both with regard to the preference for, or avoidance of, Ge-O-Ge linkages, and concerning the equilibrium position of fluorine inside the cage, where fluorine might either occupy the cage centre or participate in a partly covalent Ge-F bond. In order to shed light on the energetically preferred local arrangements, periodic electronic structure calculations in the framework of dispersion-corrected density functional theory (DFT) were performed. The AST framework was used as a suitable model system, as this zeolite is synthetically accessible across the range of (Si<sub>1-n</sub>,Ge<sub>n</sub>)O<sub>2</sub> compositions (0 ≤ <i>n</i> ≤ 1). DFT structure optimisations for (Si,Ge)-AST systems containing fluoride anions and organic cations revealed that arrangements of Si and Ge at the cage vertices which maximise the number of Ge-O-Ge linkages are energetically preferred, and that fluorine tends to form relatively short (~2.2 to 2.4 Å) bonds to Ge atoms that are surrounded by Ge-O-Ge linkages. The preference for Ge-O-Ge linkages disappears in the absence of fluorine, pointing to a “templating” effect of the anions. In addition to the prediction of equilibrium structures, DFT-based Molecular Dynamics calculations were performed for selected AST models in order to analyse the dynamics of fluoride anions confined to <i>d4r</i> cages. These calculations showed that the freedom of movement of fluorine varies markedly depending on the local environment, and that it correlates with the average Ge-F distance (short Ge-F bonds = restricted motion). An analysis of the Ge-F radial distribution functions provided no evidence for a coexistence of separate local energy minima at the cage centre and in the proximity of a germanium atom for any of the systems considered. The computational approach pursued in this work provides important new insights into the local structure of silicogermanate zeolites with <i>d4r</i> units, enhancing the atomic-level understanding of these materials. In particular, the findings presented here constitute valuable complementary information that can aid the interpretation of experimental data.<i></i></p>

Local Environment and Dynamic Behavior of Fluoride Anions in Silicogermanate Zeolites: A Computational Study of the AST Framework

2018 ◽  
Author(s):  
Michael Fischer
Keyword(s):  
Density Functional ◽  
Computational Study ◽  
Local Environment ◽  
Distribution Functions ◽  
Suitable Model ◽  
General Behaviour ◽  
Functional Theory ◽  
Molecular Dynamics Calculations ◽  
Equilibrium Structures

<p>In silicogermanate zeolites containing double four-ring (<i>d4r</i>) building units, the germanium atoms preferentially occupy the corners of these cube-like units. While this general behaviour is well known, the absence of long-range order precludes a determination of the preferred arrangements of Si and Ge atoms at the corners of <i>d4r</i> cages by means of crystallographic methods. If fluoride anions are present during the synthesis, these are incorporated into the <i>d4r</i> cages. Due to the sensitivity of the <sup>19</sup>F chemical shift to the local environment, NMR experiments can provide indirect insights into the predominant (Si,Ge) arrangements. However, conflicting interpretations have been reported, both with regard to the preference for, or avoidance of, Ge-O-Ge linkages, and concerning the equilibrium position of fluorine inside the cage, where fluorine might either occupy the cage centre or participate in a partly covalent Ge-F bond. In order to shed light on the energetically preferred local arrangements, periodic electronic structure calculations in the framework of dispersion-corrected density functional theory (DFT) were performed. The AST framework was used as a suitable model system, as this zeolite is synthetically accessible across the range of (Si<sub>1-n</sub>,Ge<sub>n</sub>)O<sub>2</sub> compositions (0 ≤ <i>n</i> ≤ 1). DFT structure optimisations for (Si,Ge)-AST systems containing fluoride anions and organic cations revealed that arrangements of Si and Ge at the cage vertices which maximise the number of Ge-O-Ge linkages are energetically preferred, and that fluorine tends to form relatively short (~2.2 to 2.4 Å) bonds to Ge atoms that are surrounded by Ge-O-Ge linkages. The preference for Ge-O-Ge linkages disappears in the absence of fluorine, pointing to a “templating” effect of the anions. In addition to the prediction of equilibrium structures, DFT-based Molecular Dynamics calculations were performed for selected AST models in order to analyse the dynamics of fluoride anions confined to <i>d4r</i> cages. These calculations showed that the freedom of movement of fluorine varies markedly depending on the local environment, and that it correlates with the average Ge-F distance (short Ge-F bonds = restricted motion). An analysis of the Ge-F radial distribution functions provided no evidence for a coexistence of separate local energy minima at the cage centre and in the proximity of a germanium atom for any of the systems considered. The computational approach pursued in this work provides important new insights into the local structure of silicogermanate zeolites with <i>d4r</i> units, enhancing the atomic-level understanding of these materials. In particular, the findings presented here constitute valuable complementary information that can aid the interpretation of experimental data.<i></i></p>

Sum rules for the pair-correlation functions of inhomogeneous fluids: results for the hard-sphere–hard-wall system

1987 ◽  
Vol 410 (1839) ◽  
pp. 409-420 ◽  
Keyword(s):  
Sum Rules ◽  
Hard Spheres ◽  
Pair Correlation ◽  
Distribution Functions ◽  
Hard Wall ◽  
Chain Equation ◽  
Hypernetted Chain ◽  
Chain Approximation ◽  
Derivatives Of

Starting from well-known relations for the derivatives of the radial distribution functions of a mixture of fluids, and allowing the diameter of one particle to become exceedingly large, three sum rules for a fluid with density inhomogeneities are obtained. None of these sum rules are new. However, the relation between the Lovett–Mou–Buff–Wertheim and the Born–Green hierarchy of equations seems not well known. The accuracy of a recent parametrization of the pair correlation of hard spheres near a hard wall and of the solutions of the Percus–Yevick and hypernetted-chain equation for this same function are examined by determination of how well these functions satisfy these sum rules and the accuracy of their surface tension, calculated from the sum rule of Triezenberg and Zwanzig. Generally speaking, the Percus–Yevick theory gives the best results and the hypernetted-chain approximation gives the worst results with the parametrization being intermediate.

Structure of Molten Bi-Zn-Alloys by Means of Neutron Diffraction

1983 ◽  
Vol 38 (4) ◽  
pp. 395-398 ◽  
Author(s):  
W. Knoll ◽  
P. Lamparter ◽  
S. Steeb
Keyword(s):  
Neutron Diffraction ◽  
Concentration Dependence ◽  
Correlation Functions ◽  
Pair Correlation ◽  
Nearest Neighbour ◽  
Pair Correlation Functions ◽  
Coordination Numbers ◽  
Structure Factors ◽  
Total Structure ◽  
Zn Alloys

Neutron diffraction experiments were performed with five molten Bi-Zn-alloys and the two molten elements Bi and Zn. The total structure factors and the pair correlation functions are given. The concentration dependence of the experimental nearest neighbour distances and the experimental coordination numbers is in accordance with the segregation behaviour prevailing in these melts.

Atomic Structure of Rare Earth Si-Al-O-N Glasses

1998 ◽  
Vol 53 (5) ◽  
pp. 259-264 ◽  
Author(s):  
H. Uhlig ◽  
M.-J. Hoffmann ◽  
S. Steeb
Keyword(s):  
Rare Earth ◽  
Neutron Diffraction ◽  
Atomic Structure ◽  
Pair Correlation ◽  
X Ray Diffraction ◽  
Ionic Radii ◽  
X Ray ◽  
Structure Factors ◽  
Total Structure

Abstract In this paper the results of X-ray diffraction experiments of Ln-Si-Al-O-N (Ln = La, Gd, Yb) glasses are presented. Total structure factors and pair correlation functions allow the determination of the first coordination sphere of Ln atoms. The bond lengths observed correspond to the ionic radii of the Ln-ions surrounded by oxygen and nitrogen atoms. The presence of non-bridging nitrogen is discussed together with results of neutron diffraction, NMR-experiments and XPS-studies of other authors.

Local Environment and Dynamic Behavior of Fluoride Anions in Silicogermanate Zeolites: A Computational Study of the AST Framework

2018 ◽  
Author(s):  
Michael Fischer
Keyword(s):  
Density Functional ◽  
Computational Study ◽  
Local Environment ◽  
Distribution Functions ◽  
Suitable Model ◽  
General Behaviour ◽  
Functional Theory ◽  
Molecular Dynamics Calculations ◽  
Equilibrium Structures

<p>In silicogermanate zeolites containing double four-ring (<i>d4r</i>) building units, the germanium atoms preferentially occupy the corners of these cube-like units. While this general behaviour is well known, the absence of long-range order precludes a determination of the preferred arrangements of Si and Ge atoms at the corners of <i>d4r</i> cages by means of crystallographic methods. If fluoride anions are present during the synthesis, these are incorporated into the <i>d4r</i> cages. Due to the sensitivity of the <sup>19</sup>F chemical shift to the local environment, NMR experiments can provide indirect insights into the predominant (Si,Ge) arrangements. However, conflicting interpretations have been reported, both with regard to the preference for, or avoidance of, Ge-O-Ge linkages, and concerning the equilibrium position of fluorine inside the cage, where fluorine might either occupy the cage centre or participate in a partly covalent Ge-F bond. In order to shed light on the energetically preferred local arrangements, periodic electronic structure calculations in the framework of dispersion-corrected density functional theory (DFT) were performed. The AST framework was used as a suitable model system, as this zeolite is synthetically accessible across the range of (Si<sub>1-n</sub>,Ge<sub>n</sub>)O<sub>2</sub> compositions (0 ≤ <i>n</i> ≤ 1). DFT structure optimisations for (Si,Ge)-AST systems containing fluoride anions and organic cations revealed that arrangements of Si and Ge at the cage vertices which maximise the number of Ge-O-Ge linkages are energetically preferred, and that fluorine tends to form relatively short (~2.2 to 2.4 Å) bonds to Ge atoms that are surrounded by Ge-O-Ge linkages. The preference for Ge-O-Ge linkages disappears in the absence of fluorine, pointing to a “templating” effect of the anions. In addition to the prediction of equilibrium structures, DFT-based Molecular Dynamics calculations were performed for selected AST models in order to analyse the dynamics of fluoride anions confined to <i>d4r</i> cages. These calculations showed that the freedom of movement of fluorine varies markedly depending on the local environment, and that it correlates with the average Ge-F distance (short Ge-F bonds = restricted motion). An analysis of the Ge-F radial distribution functions provided no evidence for a coexistence of separate local energy minima at the cage centre and in the proximity of a germanium atom for any of the systems considered. The computational approach pursued in this work provides important new insights into the local structure of silicogermanate zeolites with <i>d4r</i> units, enhancing the atomic-level understanding of these materials. In particular, the findings presented here constitute valuable complementary information that can aid the interpretation of experimental data.<i></i></p>

Sign in / Sign up

Export Citation Format

Share Document