scholarly journals Origin and structure of polar domains in doped molecular crystals

2016 ◽  
Vol 7 (1) ◽  
Author(s):  
E. Meirzadeh ◽  
I. Azuri ◽  
Y. Qi ◽  
D. Ehre ◽  
A. M. Rappe ◽  
...  
Keyword(s):  
Crystal Engineering ◽  
Density Functional ◽  
Molecular Crystals ◽  
Strong Support ◽  
Functional Theory ◽  
Guest Molecules ◽  
Molecular Dynamics Calculations ◽  
Different Temperatures ◽  
Global Understanding

Abstract Doping is a primary tool for the modification of the properties of materials. Occlusion of guest molecules in crystals generally reduces their symmetry by the creation of polar domains, which engender polarization and pyroelectricity in the doped crystals. Here we describe a molecular-level determination of the structure of such polar domains, as created by low dopant concentrations (<0.5%). The approach comprises crystal engineering and pyroelectric measurements, together with dispersion-corrected density functional theory and classical molecular dynamics calculations of the doped crystals, using neutron diffraction data of the host at different temperatures. This approach is illustrated using centrosymmetric α-glycine crystals doped with minute amounts of different L-amino acids. The experimentally determined pyroelectric coefficients are explained by the structure and polarization calculations, thus providing strong support for the local and global understanding of how different dopants influence the properties of molecular crystals.

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>

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>

Theoretical determination of the structures of CaSiO3 perovskites

2006 ◽  
Vol 62 (6) ◽  
pp. 1025-1030 ◽  
Author(s):  
Razvan Caracas ◽  
Renata M. Wentzcovitch
Keyword(s):  
Crystal Structure ◽  
Experimental Data ◽  
Density Functional Theory ◽  
Density Functional ◽  
Theoretical Determination ◽  
Functional Theory ◽  
Starting Point ◽  
Atomic Coordinates ◽  
The Ideal

Density functional theory is used to determine the possible crystal structure of the CaSiO3 perovskites and their evolution under pressure. The ideal cubic perovskite is considered as a starting point for studying several possible lower-symmetry distorted structures. The theoretical lattice parameters and the atomic coordinates for all the structures are determined, and the results are discussed with respect to experimental data.

scholarly journals Theoretical Investigation of the Fusion Process of Mono-Cages to Tri-Cages with CH4/C2H6 Guest Molecules in sI Hydrates

Molecules ◽  
2021 ◽  
Vol 26 (23) ◽  
pp. 7071
Author(s):  
Shuxian Wei ◽  
Siyuan Liu ◽  
Shoufu Cao ◽  
Sainan Zhou ◽  
Yong Chen ◽  
...  
Keyword(s):  
Natural Gas ◽  
Density Functional ◽  
Fusion Energy ◽  
Fusion Process ◽  
Large Cage ◽  
Cage Structure ◽  
Microscopic Mechanism ◽  
Functional Theory ◽  
Guest Molecules ◽  
Further Development

Owing to a stable and porous cage structure, natural gas hydrates can store abundant methane and serve as a potentially natural gas resource. However, the microscopic mechanism of how hydrate crystalline grows has not been fully explored, especially for the structure containing different guest molecules. Hence, we adopt density functional theory (DFT) to investigate the fusion process of structure I hydrates with CH4/C2H6 guest molecules from mono-cages to triple-cages. We find that the volume of guest molecules affects the stabilities of large (51262, L) and small (512, s) cages, which are prone to capture C2H6 and CH4, respectively. Mixed double cages (small cage and large cage) with the mixed guest molecules have the highest stability and fusion energy. The triangular triple cages exhibit superior stability because of the three shared faces, and the triangular mixed triple cages (large-small-large) structure with the mixed guest molecules shows the highest stability and fusion energy in the triple-cage fusion process. These results can provide theoretical insights into the growth mechanism of hydrates with other mono/mixed guest molecules for further development and application of these substances.

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