pair distribution functions
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2022 ◽  
Vol 8 ◽  
Author(s):  
Huanqing Zhang ◽  
Honggang Sun ◽  
Qiutong Li ◽  
Li Wang

The local structures of U-Co melts have been studied by first-principle calculations. Two sub-peaks are observed in the first peaks of U-U pair distribution functions. The Voronoi polyhedral analyses also show two separate core-shell U-U distances. Therefore, the calculated results propose that U atoms will play dual roles, “chemical” and “topological”, in the local structures of U-Co melts. In addition, the chemical effect of U atoms will be strengthened when containing more U atoms. The interaction of Co and U atoms is slightly affected by the compositions. The Co-centered clusters are mostly prism-like or antiprism-like polyhedral, which can be predicted by the solute-solvent model. The distribution of the coordinated numbers of Co atoms is much narrower than that of U atoms, showing relatively stable Co-centered clusters. The chemical and topological roles of U atoms are intuitively observed in the electron density of U-Co melts, which presents both metallic and covalent bonding characteristics for U-U bonds. In the end, we conclude that the partial localization of U 5f-electron is responsible for the dual roles of U atoms. The present results provide a theoretical understanding of the origin of the local structures of U-Co melts.


2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Ryota Jono ◽  
Syogo Tejima ◽  
Jun-ichi Fujita

AbstractWe studied the shear-thickening behavior of systems containing rigid spherical bodies immersed in smaller particles using non-equilibrium molecular dynamics simulations. We generated shear-thickening states through particle mass modulation of the systems. From the microstructures, i.e., two-dimensional pair distribution functions, we found anisotropic structures resulting from shear thickening, that are explained by the difference between the velocities of rigid bodies and fluid particles. The increasing viscosity in our system originated from collisions between fluid particles and rigid bodies. The lubrication forces defined in macroscale physics are then briefly discussed.


2021 ◽  
Author(s):  
Adam Sapnik ◽  
Irene Bechis ◽  
Alice Bumstead ◽  
Timothy Johnson ◽  
Philip Chater ◽  
...  

The rational design of disordered frameworks is an appealing route to target functional materials. However, intentional realisation of such materials relies on our ability to readily characterise and quantify structural disorder. Here, we use multivariate analysis of pair distribution functions to fingerprint and quantify the disorder within a series of compositionally identical metal–organic frameworks, possessing different crystalline, disordered, and amorphous structures. We find this approach can provide powerful insight into the kinetics and mechanism of structural collapse that links these materials. Our methodology is also extended to a very different system, namely the melting of a zeolitic imidazolate framework, to demonstrate the potential generality of this approach across many areas of disordered structural chemistry.


2021 ◽  
Vol 54 (2) ◽  
pp. 612-623
Author(s):  
Stefan Habermehl ◽  
Carina Schlesinger ◽  
Dragica Prill

An approach for the comparison of pair distribution functions (PDFs) has been developed using a similarity measure based on cross-correlation functions. The PDF is very sensitive to changes in the local structure, i.e. small deviations in the structure can cause large signal shifts and significant discrepancies between the PDFs. Therefore, a comparison based on pointwise differences (e.g. R values and difference curves) may lead to the assumption that the investigated PDFs as well as the corresponding structural models are not in agreement at all, whereas a careful visual inspection of the investigated structural models and corresponding PDFs may reveal a relatively good match. To quantify the agreement of different PDFs for those cases an alternative approach is introduced: the similarity measure based on cross-correlation functions. In this paper, the power of this application of the similarity measure to the analysis of PDFs is highlighted. The similarity measure is compared with the classical R wp values as representative of the comparison based on pointwise differences as well as with the Pearson product-moment correlation coefficient, using polymorph IV of barbituric acid as an example.


2021 ◽  
Vol 4 (1) ◽  
Author(s):  
Bernadette R. Cladek ◽  
S. Michelle Everett ◽  
Marshall T. McDonnell ◽  
Matthew G. Tucker ◽  
David J. Keffer ◽  
...  

AbstractA vast source of methane is found in gas hydrate deposits, which form naturally dispersed throughout ocean sediments and arctic permafrost. Methane may be obtained from hydrates by exchange with hydrocarbon byproduct carbon dioxide. It is imperative for the development of safe methane extraction and carbon dioxide sequestration to understand how methane and carbon dioxide co-occupy the same hydrate structure. Pair distribution functions (PDFs) provide atomic-scale structural insight into intermolecular interactions in methane and carbon dioxide hydrates. We present experimental neutron PDFs of methane, carbon dioxide and mixed methane-carbon dioxide hydrates at 10 K analyzed with complementing classical molecular dynamics simulations and Reverse Monte Carlo fitting. Mixed hydrate, which forms during the exchange process, is more locally disordered than methane or carbon dioxide hydrates. The behavior of mixed gas species cannot be interpolated from properties of pure compounds, and PDF measurements provide important understanding of how the guest composition impacts overall order in the hydrate structure.


2021 ◽  
Vol 24 (3) ◽  
pp. 33605
Author(s):  
M. F. Holovko ◽  
M. Ya. Korvatska

Enskog theory is extended for the description of the self-diffusion coefficient of patchy colloidal fluid in disordered porous media. The theory includes the contact values of fluid-fluid and fluid-matrix pair distribution functions that are modified to include the dependence from the so-called probe particle porosity, φ, in order to correctly describe the effects of trapping the fluid particles by a matrix. The proposed expressions for the modified contact values of fluid-fluid and fluid-matrix pair distribution functions include three terms. Namely, a hard sphere contribution obtained by us in the previous work [Holovko M. F., Korvatska M. Ya., Condens. Matter Phys., 2020, 23, 23605], the depletion contribution connected with the cluster-cluster and cluster-matrix repulsion and the intramolecular correlation inside the cluster. It is shown that the last term leads to a remarkable decrease of the self-diffusion coefficient at a low fluid density. With a decreasing matrix porosity, this effect becomes weaker. For intermediate fluid densities, the depletion contribution leads to an increase of the self-diffusion coefficient in comparison with the hard sphere fluid. For a sufficiently dense fluid, the self-diffusion coefficient strongly decreases due to a hard sphere effect. The influence of the cluster size and the type of clusters as well as of the parameters of porous media is investigated and discussed in detail.


Author(s):  
C.J. Benmore ◽  
O.L.G. Alderman ◽  
D. Robinson ◽  
G. Jennings ◽  
A. Tamalonis ◽  
...  

The Analyst ◽  
2020 ◽  
Vol 145 (6) ◽  
pp. 2245-2255
Author(s):  
Fayan Zhu ◽  
Toshio Yamaguchi ◽  
Koji Yoshida ◽  
Wenqian Zhang ◽  
Hongyan Liu ◽  
...  

The overlapping peaks are clearly investigated in the form of pair distribution functions using the EPSR program.


2020 ◽  
Vol 76 (1) ◽  
pp. 24-31 ◽  
Author(s):  
Soham Banerjee ◽  
Chia-Hao Liu ◽  
Kirsten M. Ø. Jensen ◽  
Pavol Juhás ◽  
Jennifer D. Lee ◽  
...  

A novel approach for finding and evaluating structural models of small metallic nanoparticles is presented. Rather than fitting a single model with many degrees of freedom, libraries of clusters from multiple structural motifs are built algorithmically and individually refined against experimental pair distribution functions. Each cluster fit is highly constrained. The approach, called cluster-mining, returns all candidate structure models that are consistent with the data as measured by a goodness of fit. It is highly automated, easy to use, and yields models that are more physically realistic and result in better agreement to the data than models based on cubic close-packed crystallographic cores, often reported in the literature for metallic nanoparticles.


Nanoscale ◽  
2020 ◽  
Vol 12 (24) ◽  
pp. 13103-13112
Author(s):  
Ann-Christin Dippel ◽  
Olof Gutowski ◽  
Lars Klemeyer ◽  
Ulrich Boettger ◽  
Fenja Berg ◽  
...  

The local atomic structure of two stacked thin films is probed by applying grazing incidence x-ray total scattering at variable incidence angle and resolving pair distribution functions for each individual layer.


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