Intermediate Range Order Research Articles

Intermediate Range Order in Metal-Ammonia Solutions: Pure and Na-Doped Ca-NH3.

The local and intermediate range ordering in Ca–NH3 solutions in their metallic phase is determined through H/D isotopically differenced neutron diffraction in combination with empirical potential structure refinements. For both low and high relative Ca concentrations, the Ca ions are found to be octahedrally coordinated by the NH3 solvent, and these hexammine units are spatially correlated out to lengthscales of ∼7.4–10.3 Å depending on the concentration, leading to pronounced ordering in the bulk liquid. We further demonstrate that this liquid order can be progressively disrupted by the substitution of Ca for Na, whereby a distortion of the average ion primary solvation occurs and the intermediate range ion–ion correlations are disrupted.

Neural Network Quantum Molecular Dynamics, Intermediate Range Order in GeSe2, and Neutron Scattering Experiments.

A remarkable property of certain covalent glasses and their melts is intermediate range order, manifested as the first sharp diffraction peak (FSDP) in neutron-scattering experiments, as was exhaustively investigated by Price, Saboungi, and collaborators. Atomistic simulations thus far have relied on either quantum molecular dynamics (QMD), with systems too small to resolve FSDP, or classical molecular dynamics, without quantum-mechanical accuracy. We investigate prototypical FSDP in GeSe2 glass and melt using neural-network quantum molecular dynamics (NNQMD) based on machine learning, which allows large simulation sizes with validated quantum mechanical accuracy to make quantitative comparisons with neutron data. The system-size dependence of the FSDP height is determined by comparing QMD and NNQMD simulations with experimental data. Partial pair distribution functions, bond-angle distributions, partial and neutron structure factors, and ring-size distributions are presented. Calculated FSDP heights agree quantitatively with neutron scattering data for GeSe2 glass at 10 K and melt at 1100 K.

Structure of soda-lime-aluminosilicate glasses as revealed by in-situ synchrotron powder diffraction experiments

The atomic structure of a soda-lime-aluminosilicate network glass that underwent two different thermal treatments has been investigated by in-situ Synchrotron Powder Diffraction experiments in the 30–1000 ∘C temperature range. First Sharp Diffraction Peak analysis has been initially performed to investigate intermediate range order characteristics: it provided information on volume variations upon heating and cooling, and final relaxation level as a function of the different thermal treatment. Pair Distribution Function analysis has been performed by Empirical Potential Structure Refinement modeling: it pointed out the short range order peculiarities of the samples. In particular, the thermal expansion behaviour of different atomic pairs has been revealed, together with a wide range of structural features like coordination numbers and polymerization degree. The present work well describes the multiple over temperature phenomenology of the investigated composition and discloses how the local range behaves independently from the thermal history of the sample.

Relationship between the Relaxation of Ionic Liquid Structural Motifs and That of the Shear Viscosity.

In a set of recent articles, we have highlighted that friction is highly inhomogeneous in a typical ionic liquid (IL) with charge networks that are stiff and charge-depleted regions that are soft. This has consequences not only for the dynamics of ILs but also for the transport properties of solutes dissolved in them. In this article, we explore whether the family of alkylimidazolium ILs coupled with bis(trifluoromethylsulfonyl)imide (with similar Coulombic interactions but different alkyl tails), when dynamically “equalized” by having a similar shear viscosity, display q-dependent structural relaxation time scales that are the same across the family. Our results show that this is not the case, and in fact, the relaxation of in-network charge alternation appears to be significantly affected by the presence of separate polar and apolar domains. However, we also find that if one was to assign weight factors to the relaxation of the structural motifs, charge alternation always contributes about the same amount (between 62.1 and 66.3%) across systems to the running integral of the stress tensor correlation function from which the shear viscosity is derived. Adjacency correlations between positive and negative moieties also contribute an identical amount if a prepeak is not present (about 38%) and a slightly smaller amount (about 28%) when intermediate range order exists. The prepeak only contributes about 6% to viscoelastic relaxation, highlighting that the dynamics of the smaller scale motifs is the most important.

A Brief Guide to the Structure of High-Temperature Molten Salts and Key Aspects Making Them Different from Their Low-Temperature Relatives, the Ionic Liquids.

High-temperature molten salt research is undergoing somewhat of a renaissance these days due to the apparent advantage of these systems in areas related to clean and sustainable energy harvesting and transfer. In many ways, this is a mature field with decades if not already a century of outstanding work devoted to it. Yet, much of this work was done with pioneering experimental and computational setups that lack the current day capabilities of synchrotrons and high-performance-computing systems resulting in deeply entrenched results in the literature that when carefully inspected may require revision. Yet, in other cases, access to isotopically substituted ions make those pioneering studies very unique and prohibitively expensive to carry out nowadays. There are many review articles on molten salts, some of them cited in this perspective, that are simply outstanding and we dare not try to outdo those. Instead, having worked for almost a couple of decades already on their low-temperature relatives, the ionic liquids, this is the perspective article that some of the authors would have wanted to read when embarking on their research journey on high-temperature molten salts. We hope that this will serve as a simple guide to those expanding from research on ionic liquids to molten salts and vice versa, particularly, when looking into their bulk structural features. The article does not aim at being comprehensive but instead focuses on selected topics such as short- and intermediate-range order, the constraints on force field requirements, and other details that make the high- and low-temperature ionic melts in some ways similar but in others diametrically opposite.

Effect of Ligand Polarity on the Internal Dipoles and Ferroelectric Distortion in BaTiO3 Nanocubes.

Surface adsorbates and surrounding matrix species have been demonstrated to affect the properties of nanoscale ferroelectrics and nanoscale ferroelectric composites; potentially counteracting performance losses that can occur in small particle sizes. In this work, the effects of nonpolar oleic acid (OA) and polar tetrafluoroborate (BF4 - ) ligand capping on the surface of various sizes of BaTiO3 nanocubes have been investigated with combined neutron diffraction and neutron pair distribution function (PDF), density functional theory (DFT), and ab initio molecular dynamics (AIMD) methods. The low real space PDF region provides an unobstructed view of rhombohedral (split short and long) Ti-O distances in BaTiO3 nanocubes, mimicking the well-established order-disorder local structure found in bulk BaTiO3 . Interestingly, the intermediate-range order in nanocubes is found to be orthorhombic, rather than tetragonal. It is concluded that polar ligands adsorbed at BaTiO3 surfaces stabilize the correlation length scale of local rhombohedral distortions in ferroelectric nanoparticles relative to nonpolar ligands.

Laser-induced structural modification in calcium aluminosilicate glasses using molecular dynamic simulations

Glass structures of multicomponent oxide systems (CaO–Al2O3–SiO2) are studied using a simulated pulsed laser with molecular dynamics. The short- and intermediate-range order structures revealed a direct correlation between the transformation of Al(IV) to Al(V), regions of increased density following laser processing, inherent reduction in the average T–O–T (T = Al, Si) angle, and associated elongation of the T–O bonding distance. Variable laser pulse energies were simulated across calcium aluminosilicate glasses with high silica content (50–80%) to identify densification trends attributed to composition and laser energy. High-intensity pulsed laser effects on fictive temperature and shockwave promotion are discussed in detail for their role in glass densification. Laser-induced structural changes are found to be highly dependent on pulse energy and glass chemistry.

Cluster Morphology of Colloidal Systems With Competing Interactions

Reversible aggregation of purely short-ranged attractive colloidal particles leads to the formation of clusters with a fractal dimension that only depends on the second virial coefficient. The addition of a long-ranged repulsion to the potential modifies the way in which the particles aggregate into clusters and form intermediate range order structures, and have a strong influence on the dynamical and rheological properties of colloidal dispersions. The understanding of the effect of a long-ranged repulsive potential on the aggregation mechanisms is scientifically and technologically important for a large variety of physical, chemical and biological systems, including concentrated protein solutions. In this work, the equilibrium cluster morphology of particles interacting through a short-ranged attraction plus a long-ranged repulsion is extensively studied by means of Monte Carlo computer simulations. Our findings point out that the addition of the repulsion affects the resulting cluster morphology and allows one to have a full control on the compactness or fractal dimension of the aggregates at a given thermodynamic condition. This allows us to manipulate the reversible aggregation process and, therefore, to finely tune the resulting building blocks of materials at large length scales.

Structural and dynamic properties of lithium silicate liquid: A view from molecular dynamics simulation

In this work, we have investigated the liquid lithium silicate by the method of molecular dynamics simulation (MD) using the Born-Mayer pairwise potentials under compression. The simulations have been carried out by systems including various pressure values from 5 to 30 GPa. The microstructure of lithium silicate was clarified through the thorough analysis of short-range order (SRO) and intermediate range order (IRO). The short-range order was investigated through factors such as pair radial distribution functions (PRDFs), coordination distributions, angular distributions. The results show that the structure of the system has a transformation from low-coordination to high-coordination under the influence of compression. Besides, the analysis of intermediate range order shows that the structure network in model consists of SiOx and LiOx units connected to others via an Oxygen atom. In the case for the intermediate range order, the fraction of OTy bond calculated under compression shows that their bonding angle and bond length are very little dependent on the pressure changes. Furthermore, the dynamics of liquid lithium silicates calculated through the diffusion coefficients of the atoms shows that there is a dependence of atomic mobility on pressure. The obtained results of simulations are in good agreement with previous experimental and simulated data.

On the ultimate resolution of As2S3-based inorganic resists

The possibility of making lithographic masks based on As2S3 chalcogenide glass with ultimately small size of resist elements has been investigated. Interference lithography using synchrotron radiation with a wavelength of 13.4 nm, corresponding to extreme ultraviolet light, was used. High-quality masks with elements of 30 nm in size were obtained. In a dynamic light scattering study, resist particles of about 3 nm in size were found in the developer after the development process, which corresponds to the intermediate-range order parameter in the structure of the chalcogenide glass. It is assumed that this grain size determines the ultimate resolution, roughness and unevenness of the edge of the inorganic resist mask.

Complex Structure of Molten NaCl–CrCl3 Salt: Cr–Cl Octahedral Network and Intermediate-Range Order

The resurgence of the molten-salt nuclear reactors (MSRs) creates interesting problems in molten-salt chemistry. As MSRs operate, the composition and physical properties of salts change because of ...

Crystallinity, order, the thin-film silicon continuum, and the spectral dependence of the refractive index in thin silicon films grown through ultra-high-vacuum evaporation for a range of growth temperatures

Using ultra-high-vacuum evaporation, thin-films of silicon were grown. This paper starts with a placement of our thin-films into the overall thin-film silicon continuum. Then, our thin-film’s Raman spectra are examined. In particular, two short-range order Raman related measures, namely the breadth and position of the Raman spectrum’s transverse-optic-peak, and an intermediate-range order Raman related measure, namely the ratio of the integrated transverse-acoustic and transverse-optic-peak intensities, are plotted as functions of the growth temperature. We then correlate these changes found in the nature of the order found within our thin-films of silicon with the changes observed in the spectral dependence of the refractive index, focusing on the maximum of the refractive index over the spectral range under consideration in this analysis. We find that the maximum of the refractive index increases in response to increased ordering. Finally, the potential device implications of these results are explored.

Unique and generic structural features of cholinium amino acid-based biocompatible ionic liquids.

Cholinium amino acid-based (Ch-AA) biocompatible ionic liquids (bio-ILs) are synthesized from renewable components and are efficiently used for biomass processing. However, their microscopic structural features that lead to their application as biomass solvents remain undetermined. Herein, we use atomistic simulations to investigate the structures of six different Ch-AA bio-ILs up to the nanometer length scale and demonstrate that, depending on the anion side chain structure, the respective IL exhibits structural ordering at different length scales. All the six Ch-AA bio-ILs investigated here show a generic feature of having a strong hydrogen bonding network between the hydroxyl group of the cholinium cation and the carboxyl group of the amino acid anions. We illustrate that each of these bio-ILs also displays a unique feature. Distinctive intermediate range structural ordering leads to heterogeneity in methioninate- and phenylalaninate-based ILs caused by the anion side chain segregation. Intermediate range ordering is not observed in glutaminate- and glutamate-based ILs because significant anion side chain and backbone interactions hinder the formation of side chain clusters. Interestingly, for the cysteinate-based IL, the side chains do not interact with the backbones and the intermediate range ordering is not observed because of a shorter anionic side chain.

Elastic properties and structures of pyrope glass under high pressures

Abstract X-ray diffraction analysis and ultrasonic measurements of a glass with the pyrope composition were conducted to determine its structural and elastic properties at pressures from 1 atm to 12.9 GPa. Our results indicate that its structural evolution is closely related to changes in the compression wave velocity (VP), shear wave velocity (VS), and Poisson ratio. We observed three modes of pyrope glass compression. Moderate shrinkage in the intermediate-range ordered structure occurred at pressures below 6 GPa. Significant shrinkage in the intermediate-range ordering was observed at pressures between 6 and 9 GPa. We observed changes in the short-range ordered structure at pressures above 9 GPa, which were associated with an increase in the coordination number of tetrahedral cations. The absolute values of VP and VS in pyrope glass are similar to those in magnesium-bearing silicate glasses with enstatite and diopside compositions. However, the velocities are higher than those observed in sodium aluminum silicate glasses with jadeite and albite compositions. This indicates that the velocities are governed by the initial density of a glass, which is determined by its chemical composition. In terms of pressure, the velocity minimum in pyrope glass occurs at ~5 GPa, which is similar to the velocity minima in fully polymerized glasses, such as jadeite and albite. The degree of polymerization in pyrope glass is intermediate, and it has a relatively polymerized network. A drastic increase in velocity was observed when the pyrope glass was subjected to pressures above 7–8 GPa, and the velocity exceeded that observed in silicate glasses. Densification phenomenon, such as an increase in the Al coordination number, was efficiently promoted. This was because the cationic field strength of Mg2+ exceeds those of typical non-network forming cations. Magnesium cations may have an important role in controlling the behavior of silicate glass, and partially melted mantle becomes enriched with Mg under pressure. Studying Mg-bearing aluminosilicate glasses can thus help us to better understand the behavior of magma deep in the interior of the Earth.

The microstructure of liquid Lead Silicate under PbO content

The structural characteristics of liquid lead silicate xPbO(1-x)SiO2 are performed by means of molecular dynamics simulation at ambient pressure using the Born-Mayer pairwise potentials. The simulations have been performed on the systems which include different samples with a large range of PbO content ( 0.05 ⩽ x ⩽0.75). The short and intermediate range order in liquid xPbO(1-x)SiO2 have been analysed via pair radial distribution function (PRDFs), coordination distribution, angular distribution and bond length distribution. Calculations show that most of the basic structural units in liquid lead silicate network are SiO4, PbO2 and PbO3 at low ratio of PbO and SiO4, PbO3 and PbO4 at high ratio of PbO. The distribution of O-T-O bond angle and T-O bond length (T is Si or Pb) in SiO4, PbO4 and PbO5 basic structural units are slightly changed when PbO content increase. In intermediate range order, the topology structure of OTy linkages (y=2,3,4) is also investigated in detail.

Static and dynamic ionic structure of molten CaCl2 via first-principles molecular dynamics simulations

Molten CaCl2 could further broaden and open up new solar power applications due to its high melting point and chemical stability. However, few studies have focused on the ionic structure of molten CaCl2 with the experimental limitations such as extremely high temperature, high corrosiveness, and water absorbency, so that static and dynamic structure of molten CaCl2 has not been well understood. In this work, the static and dynamic ionic structure of molten salts was investigated by conducting first-principles molecular dynamics simulations. Structural features like short-range order and intermediate-range order are observed in molten CaCl2. The 6-coordinated structure stays dominant in the CaCl2 melt among the temperature range of 1100–1500 K. The network structure is filled with distorted octahedron which tends to be rutile structure, and these octahedrons are linked via corner-sharing and edge-sharing models. Among the temperature range of 1100–1500 K, the increase of temperature can accelerate the movement of ions, but there is no obvious effect on the local ionic structure of molten CaCl2. Analysis in this paper may be supportive to fill the gap in understanding the ion behavior of molten CaCl2 and other divalent chlorides.

Complex Structure of Molten NaCl-CrClx Salts: Octahedra Network and Intermediate-Range Order

In the last decade, there has been a sustained resurgence of the Molten-Salt Nuclear Reactor (MSR) concept. As the MSRs operate, chemical compositions and physical properties of salts change because of the creation of fission products, and the effects of corrosion and radiolysis. Therefore, computer simulations of the microscopic structure of multi-component molten salts are necessary to predict changing thermophysical properties. However, first-principles simulations inevitably involve simplifying approximations and therefore require careful experimental validation. Here we present detailed experimental and simulation studies of the structure of molten NaCl-CrCl3 and NaCl-CrCl2 near-eutectic mixtures. These compositions have been chosen to elucidate the behavior of Cr ions in the melts since Cr is the most important corrosion product. Although the corrosion will result in relatively small quantities of Cr in the melt, we used much larger Cr concentration to measure the structure around Cr ions with high resolution. The results of X-ray and neutron diffraction with isotopic substitution, X-ray absorption spectroscopy, and ab initio molecular-dynamic simulations are in agreement. In NaCl-CrCl3 we found CrCl6 3+ octahedra and their networks, as well as intermediate-range order, which manifests itself in the prepeak at ~ 1 Å-1 with non-monotonic temperature behavior.Figure 1 shows one of the main results of this work, namely the comparison of measured and calculated total and partial PDFs of the molten NaCl-CrCl3 near-eutectic mixture. The consistency between the measured and calculated PDFs is remarkable. Neutron diffraction with isotopic substitution allows for a high-fidelity determination of the distances between Cr ions and other species in the melt. By utilizing a combination of neutron diffraction with isotopic substitution of 53Cr, X-ray diffraction, and ab-initio molecular dynamic simulations, we discovered the formation of chains of CrCl6 3- corner-, edge-, and face-sharing octahedra, with Cr-Cl distances of 2.4 Å, and Cl-Cl distance of 3.3 Å. We also found evidence for intermediate-range order that is likely related to inter-chain correlations. In this regard, these ternary salt shows a structure that is reminiscent of that of KCl-MgCl2, LiF-BeF2, and NaCl-UCln.Neutron and X-ray diffraction augment each other to determine partial PDFs Neutrons are particularly useful for studies involving, for example, Cr and Ni, where isotopes with a large difference in neutron cross-section are available. But when isotopic substitution is not achievable, the combination of X-rays and neutrons can extract the information about partial PDFs not otherwise available. The combination of neutrons and X-rays is also useful to overcome experimental constraints, such as compatibility between the salts and crucibles, potential radiation damage to crucibles, and sometimes large background from the sample environment. For example, quartz crucibles are suitable for both neutrons and X-rays and can be used with chloride salts but not with fluoride salts. The latter can be contained in vanadium crucibles for neutrons experiments. Our measurements demonstrate the relative strengths and weaknesses of both techniques for understanding the structure of multi-component salts. In conclusion, the understanding of the structure around Cr ions will be used to calculate properties of molten salt components (e.g., diffusivity) in the presence of Cr impurities. These properties will affect macroscopic thermo-physical properties such as thermal conductivity and viscosity of melts containing that, in turn, are necessary for designing safe and efficient MSRs. The extension of these methods to study melts containing up to 10 components will be discussed.We acknowledge useful discussions with Richard Mayes, Stephen Raiman, Jake Mcmurray (ORNL), and Raluca Scarlat (UC Berkeley). This material is based upon work supported by the Department of Energy under Award Number DE-NE0008751. This research used resources at the Spallation Neutron Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated by the Oak Ridge National Laboratory, and the beamline 28-ID-1 (PDF) of the National Synchrotron Light Source II, a DOE Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. Figure 1

Structural and dynamical investigation of Mg2SiO4 liquid

We have numerically studied structure and dynamics in Mg2SiO4 liquid. Eight models at pressure up to 40 GPa and at a temperature of 3500 K have been constructed by molecular dynamics simulation. The structural characteristics such as short-, intermediate-range order (denoted as SRO, IRO, respectively) structure and network structure are analyzed via the radial distribution functions, coordination units, bond angle, bond length as well as the number of corner-, edge- and face-sharing bonds and characteristics of all type OTy linkages. To clarify the dynamical properties, the models at different pressures are relaxed in NVE ensemble for a long time. The diffusivity of the individual atomic species is determined through the dependence of the mean-squared displacement on number of MD steps. Under compression, the local environment around Si and Mg atoms as well as the IRO structure in Mg2SiO4 changes significantly. The degree of polymerization of the Si–O network increases with pressure. The Si–O bonds are broken; the Mg atoms tend to incorporate into the Si–O network via both bridging oxygen (BO) and non-bridging oxygen (NBO) to form the –Si–O–Mg– network in Mg2SiO4 liquid. In the considered pressure range, the Mg atom always diffuses faster than the O atom and the O atom diffuses faster than the Si atom. The diffusivity of Si, Mg and O atom decreases with increase in pressure.

Ab Initio Study of Hydrolysis Effects in Single and Ion-Exchanged Alkali Aluminosilicate Glasses

Hydrolysis in alkali-doped aluminosilicate glasses is one of the most complicated mechanisms in glass science. There remain many fundamental and unresolved issues with implications on their potential applications. Herein, we address this challenge by carrying out detailed calculations on the structure and properties of both anhydrate (dry) and hydrated alkali aluminosilicate glasses on carefully constructed models. Specifically, the Na-, (Na + K)- and K-doped aluminosilicate glasses with compositions (SiO2)0.6 (Al2O3)0.2 (Na2O)0.2-x (K2O)x (x = 0, 0.10 and 0.20) are simulated using ab initio molecular dynamics (AIMD). The local short- and intermediate-range order in these glasses are analyzed in terms of atomic pair distribution, coordination number, bond length and bond angle distributions to delineate the subtle variations due to different alkali size and hydrolysis. The electronic structure, interatomic bonding, mechanical and optical properties for these models are calculated and validated with available experimental data. We use the novel concept of total bond order density (TBOD), the quantum mechanically derived metric, to characterize the internal cohesion and strength in the simulated glasses. Detailed analysis of the hydrolysis mechanism enables us to provide information on the complex interplay of various participating elements and their interactions at the atomic level. Such detailed information provides new platform of knowledge which is crucial for understanding the issues related to glass corrosion and durability and ways and means for their special applications in commercial glass products. Both un-dissociated molecular water and dissociated water in the form of hydroxyl group exist in the hydrated models in the presence of alkali ions. For the first time, we observed the opposite mixed alkali effect in the Poisson's ratio for anhydrate and hydrated glasses.

SEM-Drude Model for the Accurate and Efficient Simulation of MgCl2-KCl Mixtures in the Condensed Phase.

There is a long history of models that to different extents reproduce structural and dynamical properties of high-temperature molten salts. Whereas rigid ion models can work fairly well for some of the monovalent salts, polarizability is fundamentally important when small divalent or multivalent cations are combined with significantly polarizable anions such as Cl- to form networked liquids that display a first sharp diffraction peak. There are excellent polarizable ion models (PIMs) for these systems, but there has been little success with the less expensive Core-Shell type models, which are often described as unwieldy or difficult to fit. In this article, we present the Sharma-Emerson-Margulis (SEM)-Drude model for MgCl2/KCl mixtures that with the same ingredients used in the latest and most accurate PIM models overcome the aforementioned obstacles at significantly less computational cost; structural and dynamical properties are for all practical purposes very similar to what we obtain from the PIM but typical simulations can be more than 30 times faster. This has allowed us not only to expand our recent studies on the temperature and composition dependence of intermediate range order in MgCl2/KCl mixtures but also to access transport properties that were simply too costly to properly sample in our recently published studies.