Local Coordination Number Research Articles

Unveiling the Janus-Like Properties of OH(-).

Using ab initio simulations, we explore the glassy landscape of the OH(-)(H2O)20 cluster and its infrared spectrum. We show that the OH(-) has an amphiphilic Janus-type behavior like the hydronium ion induced by the ability of its O-H bond to be buried inside of the cluster or exposed at the surface with different coordination numbers. Recent infrared experiments of aqueous NaOH have found two pronounced peaks at 2000 and 2850 cm(-1) [Mandal, A.; J. Chem. Phys. 2014, 140, 1-12]. The microscopic origins of these spectral features remain elusive. Herein, we disentangle the contribution of the spectra between 1700 and 3000 cm(-1) in terms of the microscopic solvation structure of OH(-) and dub this as the amphiphilic band. The delocalized nature of OH(-) results in a red shift to the O-H stretch, which mixes with bend-vibrations, the extent to which is tuned by the local coordination number. These results have important bearing on understanding the spectroscopic signatures of OH(-) in environments like the air-water interface.

Local energy landscape in a simple liquid.

It is difficult to relate the properties of liquids and glasses directly to their structure because of complexity in the structure that defies precise definition. The potential energy landscape (PEL) approach is a very insightful way to conceptualize the structure-property relationship in liquids and glasses, particularly the effect of temperature and history. However, because of the highly multidimensional nature of the PEL it is hard to determine, or even visualize, the actual details of the energy landscape. In this article we introduce a modified concept of the local energy landscape (LEL), which is limited in phase space, and demonstrate its usefulness using molecular dynamics simulation on a simple liquid at high temperatures. The local energy landscape is given as a function of the local coordination number, the number of the nearest-neighbor atoms. The excitation in the LEL corresponds to the so-called β-relaxation process. The LEL offers a simple but useful starting point to discuss complex phenomena in liquids and glasses.

Multi-particle FEM modeling on microscopic behavior of 2D particle compaction

In this paper, the discrete random packing and various ordered packings such as tetragonal and hexagonal close packed structures generated by discrete element method and honeycomb, which is manually generated were input as the initial packing structures into the multi-particle finite element model (FEM) to study their densification during compaction, where each particle is discretized as a FEM mesh. The macro-property such as relative density and micro-properties such as local morphology, stress, coordination number and densification mechanism obtained from various initial packings are characterized and analyzed. The results show that the coupling of discrete feature in particle scale with the continuous FEM in macro-scale can effectively conquer the difficulties in traditional FEM modeling, which provides a reasonable way to reproduce the compaction process and identify the densification mechanism more accurately and realistically.

Donor-Bridge-Acceptor Proton Transfer in Aqueous Solution.

We use ab initio molecular dynamics to study proton transfer in a donor-bridge-acceptor system in which the bridge is a single water molecule and the entire system is embedded in aqueous solution. The results, based on a large number of proton transfer trajectories, demonstrate that the dominant charge-transfer pathway is a subpicosecond "through bridge" event in which the bridge adopts an Eigen-like (hydronium) structure. We also identify another state in which the bridge forms a Zundel-like configuration with the acceptor that appears to be a dead end for the charge transfer. The reaction coordinate is inherently multidimensional and, as we demonstrate, cannot be given in terms of either local structural parameters of the donor-bridge-acceptor system or local solvent coordination numbers.

Recent developments in the structure of high temperature oxide melts

Aerodynamic levitation with laser heating has now become a standard technique for studying the structure of oxide melts on synchrotron and neutron beamlines. Here we summarize the results of a growing number of findings that show a distinct decrease in the local cation-oxygen coordination number, for liquid state single and binary oxides, compared to their crystalline forms. This phenomenon is often correlated with a significant decrease in density upon melting and diffraction measurements show a distribution of lower coordinated polyhedra. The diffraction data allow us to refine interatomic potential parameters within molecular dynamics simulations to obtain very good agreement between the structural models and experiments. The feasibility of very high temperature experiments around and above 3000 degrees Celsius, as well as the safety aspects associated with measuring radioactive samples will be discussed. Future prospects on changing the oxidation state of high temperature oxide melts through reduction-oxidation reactions will also be considered. The photograph below shows a Uranium dioxide pellet being loaded into the aerodynamic levitator on a high energy x-ray beam line at the APS prior to melting.

An investigation of the electronic structure and structural stability of RE2Ti2O7 by glancing angle and total electron yield XANES

Pyrochlore-type oxides (A2B2O7) have been investigated for nuclear waste sequestration applications due to their overall resistance to radiation-induced structural damage. This damage can be simulated by implanting the materials with high-energy ions to aid in understanding how the structure of a crystalline material responds to radioactive decay of incorporated nuclear waste elements. RE2Ti2O7 (RE=La–Lu), Yb1.85Ca0.15Ti2O7−δ, and Yb2Ti1.85Fe0.15O7−δ were synthesized by the ceramic method and investigated by Ti K-edge X-ray absorption near edge spectroscopy (XANES) and powder X-ray diffraction before and after being implanted by 2MeV Au− ions. Surface sensitive total electron yield (TEY) XANES spectra and glancing-angle (GA) XANES spectra were collected to study the ion-implanted materials. It was necessary to use these surface sensitive detection methods because the 2MeV Au− ions only implant to ∼450nm into the surface of the materials. An effective sample mounting procedure has been developed that uses conductive silver paint to collect high quality TEY-XANES spectra from the damaged surface layer of the implanted pellets so as to minimize charging effects. This investigation has shown that the local coordination number of Ti decreased after implantation, and that the degree of damage observed decreases with decreasing radius of the RE cation. The study has also shown that the oxygen vacancies generated by a slight substitution of Yb3+ for Ca2+ or Ti4+ for Fe3+ does not considerably affect the resistance of Yb2Ti2O7 to radiation-induced damage.

Reverse Monte Carlo and Voronoi analysis of the local atomic structure of metallic glasses Fe63Er2Mo14C15B6 and Fe75Y5B20

Using neutron diffraction data the analysis of local atomic structure of two metallic glasses has been performed. The analysis is based on the reverse Monte Carlo method in combination with the Voronoi tessellation technique. For the system Fe63Er2Mo14C15B6 the results are compared with the ab initio molecular dynamic modeling. The local structure and coordination numbers of Fe75Y5B20 to our knowledge are obtained for the first time and the validity of the results obtained is discussed.

NATURAL ORBITALS IN RELATION TO QUANTUM INFORMATION THEORY: FROM MODEL LIGHT ATOMS THROUGH TO EMERGENT METALLIC PROPERTIES

The review begins with a consideration of three forms of quantum information entropy associated with Shannon and Jaynes. For model two-electron spin compensated systems, some analytic progress is first reported. The Jaynes entropy is clearly related to correlation kinetic energy. A way of testing the usefulness of a known uncertainty principle inequality is proposed for a whole class of model two-electron atoms with harmonic confinement but variable electron-electron interaction. Emerging properties are then studied by reference to bcc Na at ambient pressure and its modeling by "jellium". Jellium itself has collective behavior with changes of the density, especially noteworthy being the discontinuity of the momentum distribution at the Fermi surface. This has almost reduced to zero at rs = 100 a.u., the neighborhood in which the quantal Wigner electron solid transition is known to occur. However, various workers have studied crystalline Na under pressure and their results are compared and contrasted. Work by DFT on K, Rb and Cs is discussed, but now with reduced density from the ambient pressure value. The crystalline results for the cohesive energy of these metals as a function of lattice parameters and local coordination number are shown to be closely reproduced by means of ground and excited states for dimer potential energy curves. Then, pair potentials for liquid Na and Be are reviewed, and compared with the results of computer simulations from the experimental structure factor for Na . Finally, magnetic field effects are discussed. First a phenomenological model of the metal-to-insulator transition is presented with an order parameter which is the discontinuity in the Fermi momentum distribution. Lastly, experiments on a two-dimensional electron assembly in a GaAs / AlGaAs heterojunction in a perpendicular magnetic field are briefly reviewed and then interpreted.

On the Pr3+ interconfigurational optical transitions in glasses and an comparative study of the Photon Cascade Emission in crystals and glasses

This paper offers a study that compares the nature of Pr3+ luminescence in crystalline and glass modifications of LaB3O6, LiLaP4O12 and SrB4O7. In the crystalline derivative of these materials, the crystal-field split Pr3+ 4f15d1 state is higher in energy than the 1S0 state. The resulting energy level structure permits the observation of the Pr3+ 1S0→1I6 emission transition and the production of two photons for every absorbed UV photon, a process which is referred to as Photon Cascade Emission. In the corresponding glass derivatives, the Pr3+ 4f15d1 lies below the 1S0 state with the consequence that the emission is dominated by the interconfigurational (broad-band) Pr3+ 4f15d1→4f2 transition. The reason for this is traced to increased strength of the crystalline field at the Pr3+ site in the glass phase. The increased crystal-field strength is traceable to decreased local coordination number and to shorter Pr3+–O2− bond distances. In this paper we also analyze the results of the Pr3+ ion luminescence in fluoride, oxyfluoride, borate and phosphate glasses. With the exception of one pure fluoride glass (ZBLAN), the Pr3+ luminescence in all these glasses is dominated by the interconfigurational Pr3+ 4f15d1→4f2 emission transition. It is pointed out that under broad-band excitation, emission from the Pr3+ 1S0 state has yet to be observed in an oxide based glass (phosphates and borates).

Reconstruction of three-dimensional porous media using a single thin section

The purpose of any reconstruction method is to generate realizations of two- or multiphase disordered media that honor limited data for them, with the hope that the realizations provide accurate predictions for those properties of the media for which there are no data available, or their measurement is difficult. An important example of such stochastic systems is porous media for which the reconstruction technique must accurately represent their morphology--the connectivity and geometry--as well as their flow and transport properties. Many of the current reconstruction methods are based on low-order statistical descriptors that fail to provide accurate information on the properties of heterogeneous porous media. On the other hand, due to the availability of high resolution two-dimensional (2D) images of thin sections of a porous medium, and at the same time, the high cost, computational difficulties, and even unavailability of complete 3D images, the problem of reconstructing porous media from 2D thin sections remains an outstanding unsolved problem. We present a method based on multiple-point statistics in which a single 2D thin section of a porous medium, represented by a digitized image, is used to reconstruct the 3D porous medium to which the thin section belongs. The method utilizes a 1D raster path for inspecting the digitized image, and combines it with a cross-correlation function, a grid splitting technique for deciding the resolution of the computational grid used in the reconstruction, and the Shannon entropy as a measure of the heterogeneity of the porous sample, in order to reconstruct the 3D medium. It also utilizes an adaptive technique for identifying the locations and optimal number of hard (quantitative) data points that one can use in the reconstruction process. The method is tested on high resolution images for Berea sandstone and a carbonate rock sample, and the results are compared with the data. To make the comparison quantitative, two sets of statistical tests consisting of the autocorrelation function, histogram matching of the local coordination numbers, the pore and throat size distributions, multiple-points connectivity, and single- and two-phase flow permeabilities are used. The comparison indicates that the proposed method reproduces the long-range connectivity of the porous media, with the computed properties being in good agreement with the data for both porous samples. The computational efficiency of the method is also demonstrated.

Gemischte Alkalimetall-Oxidosulfidomolybdate A2[MoOxS4– x] (x = 1, 2, 3; A = K, Rb, Cs, NH4). Synthesen, Kristallstrukturen und Eigenschaften / Mixed Alkali Oxidosulfidomolybdates A2[MoOxS4−x] (x = 1, 2, 3; A = K, Rb, Cs, NH4).Synthesis, Crystal Structure and Properties

Mixed sulfido/oxidomolybdate anions [MoOxS4−x]2− (x = 1, 2, 3) have been prepared by passing H2S gas through a solution of oxidomolybdates. The alkali salts of K+, Rb+, Cs+, and NH+ 4 precipitate as crystalline salts from these solutions depending on the pH, the polarity of the solvent, the educt concentrations and the temperature. Their structures have been determined by means of X-ray single-crystal diffraction data. All trisulfidomolybdates A2[MoOS3] (A = NH4/K/Rb/Cs) are isotypic with the tetrasulfido salts, exhibiting the β -K2[SO4] type (orthorhombic, space group Pnma, Z = 4; for A = Rb: a = 940.62(4), b = 713.32(4), c = 1164.56(5) pm, R1 = 0.0281). In contrast, the disulfidomolybdates exhibit a rich crystal chemistry, forming three different structure types depending on the preparation conditions and the size of the A cation: All four cations form salts crystallizing with the (NH4)2[WO2S2] structure type (monoclinic, space group C2/c, Z = 4, for A = Rb: a = 1144.32(11), b = 732.60(4), c = 978.99(10) pm, β = 120.324(7)°, R1 = 0.0274). For the three alkali metal cations a second polymorph with a new structure type (monoclinic, space group P21/c, Z = 4) is observed in addition (for A = Rb: a = 674.83(2), b = 852.98(3), c = 1383.10(9) pm, β = 115.19(1)°, R1 = 0.0216). The cesium salt also crystallizes with a third modification of another new structure type (orthorhombic, space group Pbcn, Z = 4, a = 915.30(6), b = 777.27(7), c = 1120.02(7) pm, R1 = 0.0350). Only for K, an anhydrous monosulfidomolybdate could be obtained (K2[MoO4] structure type, monoclinic, space group C2/m, Z = 4, a = 1288.7(3), b = 615.7(2), c = 762.2(1) pm, β = 109.59(1)°, R1 = 0.0736). The intramolecular chemical bonding in the molybdate anions is discussed and compared with the respective vanadates. Hereby aspects like bond lengths, bond strengths and force constants derived from Raman spectroscopy, are taken into account. Especially for the polymorphic disulfido salts, in-depth analyses of the local coordination numbers and the packing of the ions are presented. The gradual bathochromic shift of the crystal color with increasing S content and increasing size of the counter cations A and molar volumes (for the polymorphic forms), respectively, is in accordance with the increase of the experimental (UV/Vis spectroscopy) and calculated (FP-LAPW band structure theory) band gaps.

Change in Microstructure Parameters of Porous Media Over Representative Elementary Volume for Porosity

The concept of representative elementary volume (REV) is critical to understand and predict the behavior of effective properties of complex heterogeneous materials in a multiscale manner. Porosity is usually used to define the REV of a given material, if porosity is stationary over consecutive volumetric increments, the sample is considered as REV regardless of the behavior of other microstructure properties. To investigate this common assumption, the change of key microstructure parameters such as local void ratio, sphericity, angularity, and coordination number of particles was computed over different volumes of REV for porosity. X-ray computed tomography was used to obtain three-dimensional volumes of natural sand packed at four different particle fragments. Efficient and robust algorithms were used to compute the microstructure properties from three-dimensional images. Findings revealed that the REVmin for porosity may not be adequate to be considered as a REV for parameters such as particle size distribution, local void ratio and coordination number. The REVmin for these parameters was observed to be larger than the REVmin for porosity.

Water: Two Liquids Divided by a Common Hydrogen Bond

The structure of water is the subject of a long and ongoing controversy. Unlike simpler liquids, where atomic interactions are dominated by strong repulsive forces at short distances and weaker attractive (van der Waals) forces at longer distances, giving rise to local atomic coordination numbers of order 12, water has pronounced and directional hydrogen bonds which cause the dense liquid close-packed structure to open out into a disordered and dynamic network, with coordination number 4-5. Here I show that water structure can be accurately represented as a mixture of two identical, interpenetrating, molecular species separated by common hydrogen bonds. Molecules of one type can form hydrogen bonds with molecules of the other type but cannot form hydrogen bonds with molecules of the same type. These hydrogen bonds are strong along the bond but weak with respect to changes in the angle between neighboring bonds. The observed pressure and temperature dependence of water structure and thermodynamic properties follow naturally from this choice of water model, and it also gives a simple explanation of the enduring claims based on spectroscopic evidence that water is a mixture of two components.

A study of mechanical homogeneity in as-cast bulk metallic glass by nanoindentation

A surface softening effect induced during copper-mould suction casting of bulk metallic glass is investigated as a function of rod diameter and glass fragility index, m, by nanoindentation. A reduction in hardness and reduced modulus at the rod surface is found to be favoured in small diameter castings and in fragile systems, respectively resulting from limited in-situ annealing and from a greater diversity of metastable atomic environments in the potential energy landscape of fragile glasses. Enhanced propensity for shear transformation zone nucleation in the low moduli surface is explained in terms of reduced atomic connectivity arising from a reduction in local co-ordination number and a lowering of the shear modulus. Finally, the structure and mechanical diversity that is possible in as-cast bulk metallic glass rods is explored through a relative quantification of shear modulus and plastic zone size across the whole as-cast state and in a single rod. These findings illustrate the sensitivity of bulk metallic glass to preparation, especially in respect of thermal history, potentially making replication of mechanical data between researchers problematic.

3D clustering DEM simulation and non-invasive experimental verification of shear localisation in irregular particle assemblies

This study investigated the particle shape effect on shear localisation in granular materials using discrete element method (DEM) simulations and experimental verification. A clustering DEM model (based on irregular particles) was developed and compared with a conventional DEM model (based on spherical particles). The clustering DEM model incorporated the real particle shapes through 3D digital image analysis of the microstructures of aggregate assemblies. Simulation results based on the two DEM models were compared with experimental quantifications, and then the capabilities of the two models to predict the micro- and macro-behaviour of granular materials were evaluated. The comparisons include global deformations, material peak strength and residual strength, stress and strain distribution, local coordination number distribution, and local void ratio distribution, and particle kinematics at the microscopic level.

Structural effects on trends in the deposition and dissolution of metal-supported metal adstructures

A simple thermodynamic formalism is combined with Density Functional Theory calculations to determine periodic trends in the reversible deposition/dissolution potentials of admetals on a variety of transition metal substrates. For each admetal/substrate combination (81 in total), the deposition/dissolution potential shift (referenced to the corresponding potential of the admetal in its bulk, elemental form) is calculated for isolated adatoms, for dimers, and for more extended kink structures. Clear periodic trends are found for the potential shifts across the space of different admetals and substrates. In addition, for the significant majority of these admetal/substrate systems, the structural effects are found to be a strong function of the local coordination number of the metal atoms, thereby verifying an important assumption that has been widely used in semiempirical models of deposition and dissolution.

Representative elementary volume analysis of porous media using X-ray computed tomography

The concept of representative elementary volume (REV) is critical to understand and predict the behaviour of effective parameters of complex heterogeneous media (e.g., soils) in a multiscale manner. Porosity is commonly used to define the REV of a given sample. In this paper we investigated whether the use of a REV for porosity can be used as a REV for other parameters such as particle size distribution, local void ratio and coordination number. X-ray computed tomography was used to obtain 3D images (i.e., volumes) of natural sand systems with different particle size distributions. 3D volumes of four different systems were obtained and a REV analysis was performed for these parameters utilizing robust 3D algorithms. Findings revealed that the REV min for porosity may not be adequate to be considered as a REV for parameters such as particle size distribution, local void ratio and coordination number. The REV min for these parameters was observed to be larger than the REV min for porosity. Heterogeneity of the systems was found to be an important factor to determine the REV for the parameters analyzed in this paper. The REV analysis revealed that as the uniformity coefficient increased, a larger volume was required to obtain the REV min for the distribution of particle sizes and coordination number whereas a smaller volume was required to obtain the REV min for local void ratio. Therefore, determination of the REV for parameters described in this paper or any microscale parameter of concern should not be derived based on REV for porosity and should be determined based on their distributions over different volumes.

A model for mesoscale patterns in motile populations

Experimental observations of cell migration often describe the presence of mesoscale patterns within motile cell populations. These patterns can take the form of cells moving as aggregates or in chain-like formation. Here we present a discrete model capable of producing mesoscale patterns. These patterns are formed by biasing movements to favor a particular configuration of agent–agent attachments using a binding function f ( K ) , where K is the scaled local coordination number. This discrete model is related to a nonlinear diffusion equation, where we relate the nonlinear diffusivity D ( C ) to the binding function f . The nonlinear diffusion equation supports a range of solutions which can be either smooth or discontinuous. Aggregation patterns can be produced with the discrete model, and we show that there is a transition between the presence and absence of aggregation depending on the sign of D ( C ) . A combination of simulation and analysis shows that both the existence of mesoscale patterns and the validity of the continuum model depend on the form of f . Our results suggest that there may be no formal continuum description of a motile system with strong mesoscale patterns.

Evidence for electronic gap-driven metal-semiconductor transition in phase-change materials

Phase-change materials are functionally important materials that can be thermally interconverted between metallic (crystalline) and semiconducting (amorphous) phases on a very short time scale. Although the interconversion appears to involve a change in local atomic coordination numbers, the electronic basis for this process is still unclear. Here, we demonstrate that in a nearly vacancy-free binary GeSb system where we can drive the phase change both thermally and, as we discover, by pressure, the transformation into the amorphous phase is electronic in origin. Correlations between conductivity, total system energy, and local atomic coordination revealed by experiments and long time ab initio simulations show that the structural reorganization into the amorphous state is driven by opening of an energy gap in the electronic density of states. The electronic driving force behind the phase change has the potential to change the interconversion paradigm in this material class.

Investigation of the Sintering of Heterogeneous Powder Systems by Synchrotron Microtomography and Discrete Element Simulation

Collective particle behavior such as interparticle coordination and particle rearrangement plays a significant role in the sintering of heterogeneous powder systems. Those phenomena have been investigated by in situ X‐ray microtomography and discrete element simulation (DEM). In situ 3D images of sintering copper‐based systems have been obtained at the European Synchrotron Research Facilities. The sintered systems comprise a dense packing of atomized copper powder with a size range of 0–63 μm and the same powder including artificial pores. Quantitative analysis of these images provided valuable data on local strain, coordination number, and particle movement. The sintering of the same systems has been simulated with the discrete element code dp3D. From this set of information, the importance of collective behavior on densification and microstructural evolution is assessed and the relevance of DEM to describe it is discussed.