Structure Of Amorphous Materials Research Articles

Structural mechanism of glass transition uncovered by unsupervised machine learning

Uncovering the structural origins of the ubiquitous dynamic arrest phenomenon at the glass transition has long been a challenge due to the difficulty in identifying a rational structural representation from a disordered medium. To address this challenge, we propose a novel approach based on unsupervised learning to define a set of structural fingerprints. In this approach, complex local atomic environments, ranging from short to medium range, are captured by the discretized radial distribution function and projected onto a simple two-dimensional space using a neural network-based autoencoder. This two-dimensional space is characterized by two static structural indicators, P1 and P2, providing a comprehensive and user-friendly representation of the mysterious “glassy structure”. By employing Gaussian mixture modeling, the structural space is autonomously divided into three sections, each representing a unique cluster with similar environments. These indicators not only elucidate the glass transition but also allow for the quantitative prediction of activation barriers for local structural excitations. Furthermore, the unsupervised clustering technique can distinguish between the structural features of “hard zones” and “soft zones”, as well as recently proposed superfast “liquid-like” atoms in glass. This unsupervised machine learning approach demonstrates the utility of seemingly agnostic local structure in amorphous materials, offering insights into the long-sought structural origins of the glass transition.

Structure of amorphous materials in the NASICON system NaTi2Si x PO12

The structure of glasses in the sodium (Na) super-ionic conductor (NASICON) system NaTi2Si x PO12 with x = 0.8 and x = 1.0 was explored by combining neutron and high-energy x-ray diffraction with 29Si, 31P and 23Na solid-state nuclear magnetic resonance (NMR) spectroscopy. The 29Si magic angle spinning (MAS) NMR spectra reveal that the silica component remains fully polymerized in the form of Si4 units, i.e. the silicon atoms are bound to four bridging oxygen atoms. The 31P{23Na} rotational echo adiabatic passage double resonance (REAPDOR) NMR data suggest that the 31P MAS NMR line shape originates from four-coordinated P n units, where n = 1, 2 or 3 is the number of bridging oxygen atoms per phosphorus atom. These sites differ in their 31P-23Na dipolar coupling strengths. The results support an intermediate range order scenario of a phosphosilicate mixed network-former glass in which the phosphate groups selectively attract the Na+ modifier ions. Titanium takes a sub-octahedral coordination environment with a mean Ti–O coordination number of 5.17(4) for x = 0.8 and 4.86(4) for x = 1.0. A mismatch between the P–O and Si–O bond lengths of 8% is likely to inhibit the incorporation of silicon into the phosphorus sites of the NASICON crystal structure.

The Impact of Energy Filtering on Fluctuation Electron Microscopy

Abstract Fluctuation electron microscopy (FEM) analyzes intensity fluctuations within diffraction patterns in order to draw conclusions regarding the structure of amorphous materials by calculating the normalized variance V(k,R). Ideally, such experiments only evaluate elastically diffracted electrons. However, an undesired inelastic background intensity is always present and degrades the FEM data. Energy filtered FEM experiments were performed on amorphous germanium created by self-ion implantation. FEM data were acquired in a transmission electron microscope at 60 and 300 kV with different electron doses as well as varying energy filter slit widths at two sample thicknesses. Generally, the measurements reveal that energy filtering greatly improves FEM data at both beam energies and sample thicknesses by removing a certain amount of the inelastic background intensity in the diffraction patterns. The narrower the energy filter, the larger the normalized variance. This brings energy filtered FEM data closer to the normalized variance determined by simulations under idealized conditions. Furthermore, preliminary results indicate that the medium range order length scale extracted from the pair-persistence analysis used in FEM is strongly affected by energy filtering.

Structure of Amorphous Two-Dimensional Materials: Elemental Monolayer Amorphous Carbon versus Binary Monolayer Amorphous Boron Nitride.

The structure of amorphous materials has been debated since the 1930s as a binary question: amorphous materials are either Zachariasen continuous random networks (Z-CRNs) or Z-CRNs containing crystallites. It was recently demonstrated, however, that amorphous diamond can be synthesized in either form. Here we address the question of the structure of single-atom-thick amorphous monolayers. We reanalyze the results of prior simulations for amorphous graphene and report kinetic Monte Carlo simulations based on alternative algorithms. We find that crystallite-containing Z-CRN is the favored structure of elemental amorphous graphene, as recently fabricated, whereas the most likely structure of binary monolayer amorphous BN is altogether different than either of the two long-debated options: it is a compositionally disordered "pseudo-CRN" comprising a mix of B-N and noncanonical B-B and N-N bonds and containing "pseudocrystallites", namely, honeycomb regions made of noncanonical hexagons. Implications for other nonelemental 2D and bulk amorphous materials are discussed.

A medium-range structure motif linking amorphous and crystalline states.

Amorphous materials have no long-range order, but there are ordered structures at short range (2-5 Å), medium range (5-20 Å) and even longer length scales1-5. While regular6,7 and semiregular polyhedra8-10 are often found as short-range ordering in amorphous materials, the nature of medium-range order has remained elusive11-14. Consequently, it is difficult to determine whether there exists any structural link at medium range or longer length scales between the amorphous material and its crystalline counterparts. Moreover, an amorphous material often crystallizes into a phase of different composition15, with very different underlying structural building blocks, further compounding the issue. Here, we capture an intermediate crystalline cubic phase in a Pd-Ni-P amorphous alloy and reveal the structure of the medium-range order, a six-membered tricapped trigonal prism cluster (6M-TTP) with a length scale of 12.5 Å. We find that the 6M-TTP can pack periodically to several tens of nanometres to form the cube phase. Our experimental observations provide evidence of a structural link between the amorphous and crystalline phases in a Pd-Ni-P alloy at the medium-range length scale and suggest that it is the connectivity of the 6M-TTP clusters that distinguishes the crystalline and amorphous phases. These findings will shed light on the structure of amorphous materials at extended length scales beyond that of short-range order.

On the Method of Constructing the Radial Distribution Function in the Structure of Amorphous Materials on the Basis of Diffraction Data

The method of constructing the radial distribution function, proposed by the authors previously, has been developed. The method includes calculation of the normalization factor and subtraction of the background caused by incoherent scattering in the amorphous material. Software is developed based on the proposed algorithms. The reported technique for processing the angular dependence of the scattered radiation intensity is approved on well-studied amorphous germanium and tungsten trioxide WO3. This technique makes it possible to perform a reliable (accurate to few percent) normalization of intensity and determine the coordination number n1 for an arbitrarily specified density of material, independent of the shape of the first coordination peak. In sum, a universal software tool has been elaborated, which makes it possible to investigate the structure of amorphous and polycrystalline inorganic materials by constructing the radial distribution functions, using diffraction of radiation of any type in these materials. All calculations are performed in the automatic mode.

Assessing the utility of structure in amorphous materials.

This paper presents a set of general strategies for the analysis of structure in amorphous materials and a general approach to assessing the utility of any selected structural description. Two measures of structure are defined, "diversity" and "utility," and applied to two model glass forming binary atomic alloys, Cu50Zr50 and a Lennard-Jones A80B20 mixture. We show that the change in diversity associated with selecting Voronoi structures with high localization or low energy, while real, is too weak to support claims that specific structures are the prime cause of these local physical properties. In addition, a new structure-free measure of incipient crystal-like organization in mixtures is introduced, suitable for cases where the stable crystal is a compound structure.

2pA_SS3-7Direct observation and modeling of local atomic structures of amorphous materials

2pA_SS3-7Direct observation and modeling of local atomic structures of amorphous materials

Reversible Aggregation and Colloidal Cluster Morphology: The Importance of the Extended Law of Corresponding States.

Cluster morphology of spherical particles interacting with a short-range attraction has been extensively studied due to its relevance to many applications, such as the large-scale structure in amorphous materials, phase separation, protein aggregation, and organelle formation in cells. Although it was widely accepted that the range of the attraction solely controls the fractal dimension of clusters, recent experimental results challenged this concept by also showing the importance of the strength of attraction. Using MonteCarlo simulations, we conclusively demonstrate that it is possible to reduce the dependence of the cluster morphology to a single variable, namely, the reduced second virial coefficient, B_{2}^{*}, linking the local properties of colloidal systems to the extended law of corresponding states. Furthermore, the cluster size distribution exhibits two well-defined regimes: one identified for small clusters, whose fractal dimension, d_{f}, does not depend on the details of the attraction, i.e., small clusters have the same d_{f}, and another related to large clusters, whose morphology depends exclusively on B_{2}^{*}, i.e., d_{f} of large aggregates follows a master curve, which is only a function of B_{2}^{*}. This physical scenario is confirmed with the reanalysis of experimental results on colloidal-polymer mixtures.

Local atomic order of a metallic glass made visible by scanning tunneling microscopy

Exploring the atomic level structure in amorphous materials by STM becomes extremely difficult due to the localized electronic states. Here we carried out STM studies on a quasi-low-dimensional film of metallic glass Zr65Cu27.5Al7.5 which is ‘ultrathin’ compared with the localization length and/or the length scale of short range order. The local electronic structure must appear more inherent, having states at Ef available for tip-sample tunneling current. To enhance imaging contrasts between long-range and short-range orders, the highly oriented pyrolytic graphite was chosen as substrate, so that the structural heterogeneity arising from competition between the glass former ability and the epitaxy can be ascertained. A chemical order predicted for this system was observed in atomic ordered regimes (1–2 monolayers), accompanied with a superstructure with the period Zr–Cu(Al)–Zr along three hexagonal axes. The result implies a chemical short range order in disordered regimes, where polyhedral clusters are dominant with the solute atom Cu(Al) in the center. An attempt for the structural modelling was made based on high resolution STM images, giving icosahedral order on the surface and different Voronoi clusters in 3D space.

Alkane-Metal Interfacial Structure and Elastic Properties by Molecular Dynamics Simulation.

The structure of amorphous materials near the interface with an ordered substrate can be affected by various characteristics of the adjoining phases, such as the lattice spacing of the adherent surface, polymer chain length, and adhesive strength. To discern the influence of each of these factors, four FCC metal lattices are examined for three chain lengths of n-alkane and van der Waals interfacial interactions are controlled by adjusting the Lennard-Jones 12-6 potential parameters. The role of interaction strength is investigated for a single chain length and substrate combination. Four nanoconfined systems are also analyzed in terms of their mechanical strength. A strong layering effect is observed near the interface for all systems. The distinctiveness of polymer layering, i.e., the maximum density and spatial extent, exhibits a logarithmic dependence on the interaction strength between polymer and substrate. Congruency with the substrate lattice parameter further enhances this effect. Moreover, the elastic modulus of the alkane phase as a function of layer thickness indicates that the effects of ordering within the structure extend beyond the immediately obvious interfacial region.

Divergence of Voronoi Cell Anisotropy Vector: A Threshold-Free Characterization of Local Structure in Amorphous Materials.

Characterizing structural inhomogeneity is an essential step in understanding the mechanical response of amorphous materials. We introduce a threshold-free measure based on the field of vectors pointing from the center of each particle to the centroid of the Voronoi cell in which the particle resides. These vectors tend to point in toward regions of high free volume and away from regions of low free volume, reminiscent of sinks and sources in a vector field. We compute the local divergence of these vectors, where positive values correspond to overpacked regions and negative values identify underpacked regions within the material. Distributions of this divergence are nearly Gaussian with zero mean, allowing for structural characterization using only the moments of the distribution. We explore how the standard deviation and skewness vary with the packing fraction for simulations of bidisperse systems and find a kink in these moments that coincides with the jamming transition.

Structural Evaluation of Amorphous MoS3 Positive Electrode Active Materials in All-Solid-State Lithium Secondary Batteries

Sulfur is an attractive active material for positive electrode because sulfur has a high theoretical capacity. High capacity of the batteries can be achieved by using active materials with a large content of sulfur. Low electronic conductivity is a drawback of sulfur. We thus focus on a transition metal trisulfides such as TiS3 with high electronic conductivity. Moreover, amorphization of active materials is potentially capable of achieving higher capacity and cyclability because of open and random structure in amorphous materials. Recently, we have found that amorphous TiS3 prepared by ball milling had better cyclability than crystalline TiS3 [1, 2]. Very recently, we have reported that amorphous molybdenum trisulfide (a-MoS3) electrode active materials prepared by mechanical milling showed high reversible capacity (about 670 mAh g-1) for 60 cycles in the all-solid-state lithium batteries with Li2S-P2S5 electrolytes at room temperature [3]. However, the electrochemical reaction mechanism of a-MoS3 has not been identified yet. In order to improve the battery performance, it is important to understand the reaction mechanisms of a-MoS3 in the all-solid-sate lithium batteries. In this study, we investigated microstructures and morphologies of a-MoS3 electrodes before and after charge-discharge processes. a-MoS3 was prepared by mechanical milling of the mixture of molybdenum metal and sulfur. The obtained active materials were applied as a positive electrode to all-solid-state cells with sulfide solid electrolytes. Structures and morphologies of a-MoS3 electrodes before and after charge-discharge tests were analyzed by XRD, XPS, SEM and HR-TEM. The HR-TEM observations of a-MoS3 electrodes during the 1st discharge test revealed that the lattice fringes due to MoS2 layer structure disappeared gradually. After the 1st full discharge, the lattice fringes were not observed and the electron diffraction showed halo pattern. However, the lattice fringes like MoS2 were observed again after the 1st charge. These results of HR-TEM observations suggested that the reversible structural changes of a-MoS3 occurred. On the other hand, Li2S nano crystals were observed in HR-TEM image of a-MoS3 after the 10th discharge. The HR-TEM image after the 10th charge showed no lattice fringes of Li2S and MoS2. It is concluded that MoS3 after the 10th charge was completely amorphous. Reference [1] A. Hayashi, et al., Chem. Lett., 41 (2012) 886. [2] T. Matsuyama, et al., J. Solid State Electrochem., 17 (2013) 2697. [3] T. Matsuyama, et al., J. Mater. Chem. A, submitted for publication Acknowledgment This research was financially supported by ALCA-SPRING project.

Molecular Design of Amorphous Porous Organic Cages for Enhanced Gas Storage

Porous molecular solids are garnering increasing attention with examples of high surface areas and applications in molecular separations. Recently, amorphous networks of molecular cages have shown increased porosity with respect to their crystalline counterparts. However, the structures of amorphous materials cannot be precisely elucidated by X-ray diffraction techniques, thus molecular simulations are vital to understanding their pore structures and the origin of porosity. Here, we use GPU-accelerated molecular dynamics simulations as an efficient methodology to construct representative amorphous network structures. We employ Voronoi network analysis of amorphous networks of seven previously reported cage molecules to provide insight into structure–property relationships. Accordingly, we apply this understanding to delineate synthetic design features that give rise to highly porous analogues of chemically robust cages constructed from carbon–carbon bonds.

Revealing ordering and structural changes at glass transition

ABSTRACTOrdering types in the disordered structure of amorphous materials and structural changes which occur at glass-liquid transition are discussed revealing medium range order and reduction of topological signature of bonding system.

Atomistic simulations of diamond-like carbon growth

Diamond-like carbon (DLC) films are composed of carbon bonds with different hybridizations, including sp2, sp3 and even sp1. Understanding the atomic bonding structure is essential to understanding the properties and optimizing the process parameters of the films. Because of the limited analytical tools for characterizing the atomic bonding structure in amorphous materials, computational research at the atomistic scale could provide significant insight into the structure–property relationships in diamond-like carbon films and has been applied to understanding the various phenomena occurring during DLC film growth. The contributions of the atomistic simulations and electronic structure calculations pertain mainly to three important issues: (i) the sp3 bond formation and stress generation mechanisms, (ii) the stress reduction mechanism by metal incorporation, and (iii) the impact angle-dependent surface smoothening/roughening mechanisms.

Structural signature of slow dynamics and dynamic heterogeneity in two-dimensionalcolloidal liquids: glassy structural order

Glassy states are formed if crystallization is avoided upon cooling or increasing density.However, the physical factors controlling the ease of vitrification and the nature of glasstransition remain elusive. Among various glass-forming systems, colloidal liquids are one ofthe most ideal glass-forming systems because of the simplicity and controllability of theinteractions. We use numerical simulations of two-dimensional polydisperse andbinary hard discs to tackle both of these longstanding questions. For polydispersesystems, we systematically control the polydispersity, which can be regarded asthe strength of frustration effects on crystallization. We reveal that crystal-likehexatic order grows in size and lifetime with an increase in the colloid volumefraction or with a decrease in polydispersity (or frustration). We stress that hexaticordering in hard disc systems is a direct consequence of dense packing and amanifestation of low configurational entropy. Our study suggests an intriguingscenario that the strength of frustration controls both the ease of vitrificationand the nature of the glass transition. Vitrification may be a process of hiddencrystal-like ordering under frustration for this system. This may provide not only aphysical basis for glass formation, but also an answer to another longstandingquestion on the structure of amorphous materials: ‘order in disorder’ may be anintrinsic feature of a glassy state of many materials. For binary mixtures, onthe other hand, the relevant structural feature linked to slow dynamics is nothexatic order, but an amorphous structure of low structural entropy. These resultssuggest that slow dynamics is associated with bond orientational order linked tothe crystal for a weakly frustrated system, whereas to amorphous structures oflow configurational entropy for a strongly frustrated system. This suggests anintrinsic link between structure and dynamics in glass-forming materials: slowdynamics is linked to structuring (‘glassy ordering’) towards low configurationalentropy. We discuss the nature of ‘glassy order’ responsible for slow dynamics.

Application of electron optical techniques to the study of amorphous materials

Despite the sub-angstrom resolution capability of modern transmission electron microscopes, elucidation of the structure of amorphous materials by direct-imaging methods is still challenged by problems of projection and data presentation identified over 20 years ago. Classical diffraction analysis, leading to the radial distribution function not only retains its essential importance but has been extended to deal with small scattering volumes in inhomogeneous samples by the use of focused electron probe illumination but retaining orientation averaging. Fluctuation microscopy provides a significant advance in the detection and statistical representation of medium-scale spatial variations in amorphous structure. Issues of scattering coherence are crucial in both these approaches. Further opportunities for use of focused probes include the study of angular correlations in nanodiffraction patterns as well as fine structure in energy loss spectra. Finally, there may be a return to the direct imaging approach thanks to recent advances in electron tomography and in aberration-corrected depth sectioning.

Adjusting and understanding the properties and crystallisation behaviour of amorphous trehalose as a function of spray drying feed concentration

Trehalose is known to stabilise proteins and peptides in the amorphous state. It is believed that crystallisation to a dihydrate is an important aspect of this protection, with trehalose acting as a desiccant to keep water away from, and prevent damage to, the biomolecules. The structure of amorphous materials has an impact on their ability to crystallise (possibly due to the presence of different amounts of short range order). In this study, repeat batches of trehalose were spray dried from different solution concentrations in water (0.5, 5.0 and 10.0% w/v). Each feed concentration resulted in amorphous spheres. Those from the highest feed concentration were found to be the largest particle size (due to more rapid onset of drying) whereas the lowest feed concentration resulted in the smallest size. The high feed concentration resulted in material that reproducibly and readily crystallised when exposed to water vapour, however the excess water (that over and above the amount needed to form the crystalline dihydrate) was not readily desorbed, presumably as it was entrapped between the recently formed crystals. The most dilute feed concentration resulted in samples that exhibited great variability between batches, often requiring a larger mass of water to be sorbed before the crystallisation would begin than was needed for samples from higher feed concentrations. The exception to this was one batch that was observed by NIR to have the aspects of crystalline structure in its amorphous state (presumably due to longer drying times allowing some short range order). Whilst these samples allowed desorbed water to clear the crystallised material more readily than was observed from higher feed concentration samples, the variability was a perceived disadvantage. A reasonable compromise was the material formed from intermediate feed concentration which showed less variability than seen for low feed materials and greater dissipation of desorbed water than for high feed material. It is concluded that the properties of amorphous trehalose are altered as a consequence of processing and care must be taken to optimise this when attempting to stabilise macromolecular drugs.

Structure of amorphous alumina tubes studied by positron annihilation lifetime spectroscopy

Positron annihilation lifetime spectroscopy (PALS) is used to investigate the detailed structure of amorphous alumina tubes prepared via thermohydrolysis of anhydrous AlCl3 . The results demonstrate that PALS is a sensitive probe of the structure of amorphous materials, capable of distinguishing between the structures of amorphous alumina tubes and amorphous alumina prepared by a standard procedure, which are shown to differ slightly in the concentration and size of free-volume elements. The observed distinctions can be understood in terms of the anisotropic aggregation of primary alumina particles during hydrolysis in aqueous media or thermohydrolysis in water vapor.