Non-crystalline Materials Research Articles

Висмут қоспасы бар радиацияға төзімді наноөлшемді GE2SB2TE5 қабықшалардағы ауысу және жады эффектісінің ерекшеліктері

The physics and technology of noncrystalline materials is one of the rapidly developing areas of condensed matter physics, materials science, and nanotechnologies. Among the variety of materials with a non-crystalline structure, a special place is occupied by the class of chalcogenide glassy semiconductors (CGS) with unique properties and phenomena, such as the switching effect, photostructural transformations. The switching effect is observed in thin CGS films and is associated with a fast-reversible phase transition of the structure from the glassy state to the crystalline state under the action of voltage pulses or laser radiation. Based on this effect, a separate class of information carriers has been formed for non-volatile storage devices such as PCM (Phase Change Memory) with high performance and reliability. Materials for use in PCM devices are CGSs of complex compositions lying on the GeTe-Sb2Te3 quasi-binary boundary, and thin films of Ge2Sb2Te5 composition are the most promising for use in PCM devices. The ability to control the electronic properties of such functional materials based on CGS significantly expands the scope of their application and is an important scientific and practical task, since traditional methods for purposefully changing the electronic properties of crystalline semiconductors, such as doping during synthesis or the thermal diffusion method, turned out to be ineffective for CGS. Impurities introduced into CGS at low concentrations by the methods indicated above usually do not show electrical activity, since they are compensated by their charged structural defects, and an increase in the impurity concentration leads to significant crystallization. In this work, the atomic composition of Ge2Sb2Te5 nanosized films with a bismuth content of more than 12 at.%, obtained by ion-plasma deposition in the direct current mode, was studied by the method of energy dispersive analysis using a scanning electron microscope. In addition, the current–voltage characteristics of the samples were measured. It has been found that the transition voltage and the time, which characterize the switching and memory effects, depend significantly on the film thickness and composition. The addition of bismuth reduces the film switching time. A decrease in the film thickness leads to a decrease in the threshold voltage Uth and the switching time. We believe that the results of this work are useful for the development of optical memory based on phase change materials.

Mechanochemistry in Portugal-A Step towards Sustainable Chemical Synthesis.

In Portugal, publications with mechanochemical methods date back to 2009, with the report on mechanochemical strategies for the synthesis of metallopharmaceuticals. Since then, mechanochemical applications have grown in Portugal, spanning several fields, mainly crystal engineering and supramolecular chemistry, catalysis, and organic and inorganic chemistry. The area with the most increased development is the synthesis of multicomponent crystal forms, with several groups synthesizing solvates, salts, and cocrystals in which the main objective was to improve physical properties of the active pharmaceutical ingredients. Recently, non-crystalline materials, such as ionic liquids and amorphous solid dispersions, have also been studied using mechanochemical methods. An area that is in expansion is the use of mechanochemical synthesis of bioinspired metal-organic frameworks with an emphasis in antibiotic coordination frameworks. The use of mechanochemistry for catalysis and organic and inorganic synthesis has also grown due to the synthetic advantages, ease of synthesis, scalability, sustainability, and, in the majority of cases, the superior properties of the synthesized materials. It can be easily concluded that mechanochemistry is expanding in Portugal in diverse research areas.

The chemistry of melting oxynitride phosphate glasses

Abstract Oxide glasses are the most commonly studied non-crystalline materials in Science and Technology, though compositions where part of the oxygen is replaced by other anions, e.g. fluoride, sulfide or nitride, have given rise to a good number of works and several key applications, from optics to ionic conductors. Oxynitride silicate or phosphate glasses stand out among all others because of their higher chemical and mechanical stability and their research continues particularly focused onto the development of solid electrolytes. In phosphate glasses, the easiest way of introducing nitrogen is by the remelting of the parent glass under a flow of ammonia, a method that allows the homogeneous nitridation of the bulk glass and which is governed by diffusion through the liquid-gas reaction between NH3 and the PO4 chemical groupings. After nitridation, two new structural units appear, the PO3N and PO2N2 ones, where nitrogen atoms can be bonded to either two or three neighboring phosphorus, thus increasing the bonding density of the glass network and resulting in a quantitative improvement of their properties. This short review will gather all important aspects of the synthesis of oxynitride phosphate glasses with emphasis on the influence of chemical composition and structure.

Phase Transformations from Nanocrystalline to Amorphous (Zr70Ni25Al5)100-xWx (x; 0, 2, 10, 20, 35 at. %) and Subsequent Consolidation.

Glasses, which date back to about 2500 BC, originated in Mesopotamia and were later brought to Egypt in approximately 1450 BC. In contrast to the long-range order materials (crystalline materials), the atoms and molecules of glasses, which are noncrystalline materials (short-range order) are not organized in a definite lattice pattern. Metallic glassy materials with amorphous structure, which are rather new members of the advanced materials family, were discovered in 1960. Due to their amorphous structure, metallic glassy alloys, particularly in the supercooled liquid region, behave differently when compared with crystalline alloys. They reveal unique and unusual mechanical, physical, and chemical characteristics that make them desirable materials for many advanced applications. Although metallic glasses can be produced using different techniques, many of these methods cannot be utilized to produce amorphous alloys when the system has high-melting temperature alloys (above 1500 °C) and/or is immiscible. As a result, such constraints may limit the ability to fabricate high-thermal stable metallic glassy families. The purpose of this research is to fabricate metallic glassy (Zr70Ni25Al5)100-xWx (x; 0, 2, 10, 20, and 35 at. %) by cold rolling the constituent powders and then mechanically alloying them in a high-energy ball mill. The as-prepared metallic glassy powders demonstrated high-thermal stability and glass forming ability, as evidenced by a broad supercooled liquid region and a high crystallization temperature. The glassy powders were then consolidated into full-dense bulk metallic glasses using a spark plasma sintering technique. This consolidation method did not result in the crystallization of the materials, as the consolidated buttons retained their short-range order fashion. Additionally, the current work demonstrated the capability of fabricating very large bulk metallic glassy buttons with diameters ranging from 20 to 50 mm. The results indicated that the microhardness of the synthesized metallic glassy alloys increased as the W concentration increased. As far as the authors are aware, this is the first time this metallic glassy system has been reported.

Thermal Conductivity of Sodium Silicate Glasses and Melts: Contribution of Diffusive and Propagative Vibration Modes

The thermal conductivity of silicate melts and glasses is an important physical property for understanding the temperature distribution in high-temperature metallurgical processes; however, the mechanism of heat conduction in these non-crystalline materials remains unclear. Two types of vibration modes must be considered to understand the mechanism of heat conduction, namely, propagative and diffusive vibration modes. In the present study, we carefully derived the thermal conductivity of pure silica and sodium disilicate glasses and melts, and estimated the contribution of the diffusive vibration mode using a recently developed model. The results indicated that the diffusive vibration mode was not dominant in the silicate non-crystalline materials, whereas the propagative vibration mode (i.e., phonons) was dominant in the heat conduction of silicate glasses and melts, which is in contrast with borate glasses.

Three-dimensional atomic packing in amorphous solids with liquid-like structure.

Liquids and solids are two fundamental states of matter. However, our understanding of their three-dimensional atomic structure is mostly based on physical models. Here we use atomic electron tomography to experimentally determine the three-dimensional atomic positions of monatomic amorphous solids, namely a Ta thin film and two Pd nanoparticles. We observe that pentagonal bipyramids are the most abundant atomic motifs in these amorphous materials. Instead of forming icosahedra, the majority of pentagonal bipyramids arrange into pentagonal bipyramid networks with medium-range order. Molecular dynamics simulations further reveal that pentagonal bipyramid networks are prevalent in monatomic metallic liquids, which rapidly grow in size and form more icosahedra during the quench from the liquid to the glass state. These results expand our understanding of the atomic structures of amorphous solids and will encourage future studies on amorphous-crystalline phase and glass transitions in non-crystalline materials with three-dimensional atomic resolution.

ゆらぎで探る溶液の構造－混合状態の直接表現－

Radial distribution functions are commonly used to study the structures of many non-crystalline materials. The present author proposed a completely different method to describe the solution structure by expressing the inhomogeneity in distribution of molecules and in concentration as “density fluctuation” and “concentration fluctuation”, respectively; namely the structure of a solution is described in terms of the “mixing state” or “mixing scheme.” This paper introduces the fluctuations, as well as Kirkwood-Buff Integrals. Fluctuations of solutions become more pronounced in the mesoscale region. The relationship with solution thermodynamics, which represents the macroscopic limit, is also discussed. The features and cautions of experiments to measure the fluctuations are described. Finally, as analytical examples, temperature and concentration dependences of mixing schemes for two solution systems with upper critical and lower critical solution temperatures are presented.

Accuracy of the independent atom approximation in digital tomosynthesis Monte Carlo simulations

Monte Carlo (MC) codes serve as the gold standard simulation tool during design and optimisation of x-ray imaging systems. Such simulations often model Rayleigh scattering based on the Independent Atom Approximation Model (IAM). This model neglects the low range molecular interference (MI) effects of non-crystalline materials such as human tissues. Previous work has found discrepancies in the simulated images of planar x-ray images between IAM and MI models. However, insignificant differences were found for computed tomography (CT) reconstructions. In this work we present Geant4 MC simulations of a flat panel source digital tomosynthesis (DT) system for human extremities. Results show that with a 1:9 scatter to primary ratio (SPR) in the x-ray projections, the DT reconstructions are insensitive to the differences of the IAM and MI models. Therefore, MC codes that use the IAM model are sufficient for the study of DT systems. That is because DT algorithms have a larger effect on image quality than the few percent change in the noise due to a physical model and noise suppression methods make this change even less important. Dependency of this conclusion on SPR must be considered in other DT modalities where SPR might be larger.

The Solution Chemistry of Mixing States Probed via Fluctuations: a Direct Description of Inhomogeneity in Mixing

Abstract Radial distribution functions are commonly used to represent the structures of solutions, which represent the probability of finding another atom in the shell at a distance r from the atom of interest. This method has been used to study the structures of many non-crystalline materials. However, the information gained in this method is microscopic, and is limited to the first or second nearest neighbors from the featured atom. The present author proposed a completely different method to describe the solution structure by expressing the inhomogeneity in distribution of atoms and molecules and in concentration as “density fluctuation” and “concentration fluctuation”, respectively; namely the structure of a solution is described in terms of the “mixing state” or “mixing scheme.” This paper introduces density fluctuation and concentration fluctuation, as well as Kirkwood-Buff Integrals. Fluctuations of solutions become more pronounced in the mesoscale region. The relationship with solution thermodynamics, which represents the macroscopic limit, is also discussed. The features and cautions of experiments to measure the fluctuations are described. Finally, as analytical examples, temperature and concentration dependences of mixing schemes for two solution systems with upper critical and lower critical solution temperatures are presented.

The energy landscape governs ductility in disordered materials.

Based on their structure, non-crystalline phases can fail in a brittle or ductile fashion. However, the nature of the link between structure and propensity for ductility in disordered materials has remained elusive. Here, based on molecular dynamics simulations of colloidal gels and silica glasses, we investigate how the degree of structural disorder affects the fracture of disordered materials. As expected, we observe that structural disorder results in an increase in ductility. By applying the activation-relaxation technique (an open-ended saddle point search algorithm), we demonstrate that the propensity for ductility is controlled by the topography of the energy landscape. Interestingly, we observe a power-law relationship between the particle non-affine displacement upon fracture and the average local energy barrier. This reveals that the dynamics of the particles upon fracture is encoded in the static energy landscape, i.e., before any load is applied. This relationship is shown to apply to several classes of non-crystalline materials (oxide and metallic glasses, amorphous solid, and colloidal gels), which suggests that it may be a generic feature of disordered materials.

XAFS studies of glass structure

X-ray absorption fine structure (XAFS) spectroscopy has become one of the most important techniques to characterise the local coordination of specific atomic species present in condensed matter. The possibility of investigating the structure of noncrystalline materials has made XAFS very attractive for the study of the short range structure of the glassy state. The XAFS acronym denotes the structure present above the x-ray absorption edges; such a structure is distinguished as x-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS). XANES carries information on the atomic arrangement around the absorbing species: distances and bonding angles. XANES also provides information on the electronic states in the proximity of the conduction band: different chemical environments may be identified from known features in the spectrum. EXAFS provides information on the average distance and radial distribution function of the nearest neighbours of the absorbing species. In the last decade, the enormous progress of experimental techniques and the excellent quality of experimental spectra, consequent to the development of the synchrotron radiation facilities, have stimulated the evolution of the XAFS theory and analysis procedures. This paper is a survey on the theoretical development of XAFS and on the methods for determining the short range information in disordered materials. Applications of XAFS to some vitreous systems are reported.

The structure of oxide glasses studied by high-energy x-ray diffraction

The use of high‑energy x‑ray diffraction techniques with the latest generation synchrotron sources has created new approaches to study quantitatively the structure of noncrystalline materials. Recently, this technique has been combined with neutron diffraction at pulsed source to provide more detailed and reliable structural information not previously available. This article reviews and summarises recent results obtained from the high energy x‑ray diffraction on several oxide glasses, SiO2, B2O3 and PbSiO3, using bending magnet beamlines at SPring‑8. In particular, it addresses the structural models of the oxide glasses obtained by the reverse Monte Carlo (RMC) modelling technique using both the high energy x‑ray and neutron diffraction data.

Predicting the refractive index of amorphous materials using the Bruggeman effective medium approximation.

Previous studies have shown that the Lorentz-Lorenz relationship, or molar refractivity/specific refractivity effective medium approximation, enables a reasonable prediction of the refractive index of amorphous water ice, given the refractive index of crystalline water ice. In the current study, we show that the Bruggeman effective medium approximation provides an even closer match to measurements of the refractive index of several amorphous materials, given the refractive index of their crystalline phase. We show that the Bruggeman effective medium approximation provides a good match to measurements of the refractive index of amorphous ice as well. Thus, assuming that the volume fraction of the scattering centers is a constant for a given amorphous material (with respect to a given range of wavelengths) seems to be a more robust assumption than assuming that the molar mass and molar refractivity or specific refractivity are preserved in going from the crystalline state to the amorphous state of the same material. Our results have implications for astrophysics applications, as well as for the optics of non-crystalline materials in general.

Phonon-mediated lipid raft formation in biological membranes

Short-wavelength collective molecular motions, also known as phonons, have recently attracted much interest in revealing dynamic properties of biological membranes through the use of neutron and X-ray scattering, infrared and Raman spectroscopies, and molecular dynamics simulations. Experimentally detecting unique vibrational patterns such as, shear phonon excitations, viscoelastic crossovers, transverse acoustic phonon gaps, and continuous and truncated optical phonon modes in cellular membranes, to name a few, has proven non-trivial. Here, we review recent advances in liquid thermodynamics that have resulted in the development of the phonon theory of liquids. The theory has important predictions regarding the shear vibrational spectra of fluids, namely the emergence of viscoelastic crossovers and transverse acoustic phonon gaps. Furthermore, we show that these vibrational patterns are common in soft (non-crystalline) materials, including, but not limited to liquids, colloids, liquid crystals (mesogens), block copolymers, and biological membranes. The existence of viscoelastic crossovers and acoustic phonon gaps define the self-diffusion properties of cellular membranes and provide a molecular picture of the transient nature of lipid rafts (Bolmatov et al., 2020). Importantly, the timescales (picoseconds) for the formation and dissolution of transient lipid rafts match the lifetime of the formation and breakdown of interfacial water hydrogen bonds. Apart from acoustic propagating phonon modes, biological membranes can also support more energetic non-propagating optical phonon excitations, also known as standing waves or breathing modes. Importantly, optical phonons can be truncated due to the existence of finite size nanodomains made up of strongly correlated lipid–cholesterol molecular pairs. These strongly coupled molecular pairs can serve as nucleation centers for the formation of stable rafts at larger length scales, due to correlations of spontaneous fluctuations (Onsager's regression hypothesis). Finally and importantly, molecular level viscoelastic crossovers, acoustic phonon gaps, and continuous and truncated optical phonon modes may offer insights as to how lipid–lipid and lipid–protein interactions enable biological function.

Non-porous organic crystals and their interaction with guest molecules from the gas phase

Some organic molecules encapsulate solvents upon crystallization. One class of compounds that shows a high propensity to form such crystalline solvates are tetraaryladamantanes (TAAs). Recently, tetrakis(dialkoxyphenyl)-adamantanes have been shown to encapsulate a wide range of guest molecules in their crystals, and to stabilize the guest molecules against undesired reactions. The term ‘encapsulating organic crystals’ (EnOCs) has been coined for these species. In this work, we studied the behavior of three TAAs upon exposition to different guest molecules by means of sorption technique. We firstly measured the vapor adsorption/desorption isotherms with water, tetrahydrofuran and toluene, and secondly, we studied the uptake of methane on dry and wet TAAs. Uptake of methane beyond one molar equivalent was detected for wet crystals, even though the materials showed a lack of porosity. Thus far, such behavior, which we ascribe to methane hydrate formation, had been described for porous non-crystalline materials or crystals with detectable porosity, not for non-porous organic crystals. Our results show that TAA crystals have interesting properties beyond the formation of conventional solvates. Gas-containing organic crystals may find application as reservoirs for gases that are difficult to encapsulate or are slow to form crystalline hydrates in the absence of a host compound.Wet tetraaryladamantane crystals take up methane in form of methane hydrate structure I, even though they appear non-porous to argon.

Emerging Role of Non-crystalline Electrolytes in Solid-State Battery Research

As the need for new modalities of energy storage becomes increasingly important, all-solid-state secondary ion batteries seem poised to address a portion of tomorrow’s energy needs. The success of such batteries is contingent on the solid-state electrolyte meeting a set of material demands, including high bulk and interfacial ionic conductivity, processability with electrodes, electrode interfacial stability, thermal stability, etc. The demanding criteria for an ideal solid-state electrolyte has translated into decades of research devoted to discovering new electrolytes and modifying their structure/processing to improve their properties. While much research has focused on the electrolyte properties of polycrystalline ceramics, non-crystalline materials (glasses, amorphous solids, and partially crystallized materials) have demonstrated unique advantages in processability, stability, tunability, etc. These non-crystalline electrolytes are also fundamentally interesting for their potential contributions toward understanding ionic conduction in the solid state. In this review, we first review a decade of advances in two distinct families of non-crystalline lithium-ion electrolytes: lithium thiophosphate and lithium phosphate oxynitride. In doing so, we demonstrate two pathways for non-crystalline electrolytes to address the barriers towards development of all-solid-state batteries, viz., interfacial stability and conduction. Finally, we conclude with some discussion of the development of fundamental models of ionic conduction in the non-crystalline state, including the ongoing debate between strong and weak electrolyte theories. Collectively, these discussions make a promising case for the role of non-crystalline electrolytes in the next generation of energy storage technology.

Photonic amorphous topological insulator

The current understanding of topological insulators and their classical wave analogs, such as photonic topological insulators, is mainly based on topological band theory. However, standard band theory does not apply to amorphous phases of matter, which are formed by non-crystalline lattices with no long-range positional order but only short-range order, exhibiting unique phenomena such as the glass-to-liquid transition. Here, we experimentally investigate amorphous variants of a Chern number-based photonic topological insulator. By tuning the disorder strength in the lattice, we demonstrate that photonic topological edge states can persist into the amorphous regime prior to the glass-to-liquid transition. After the transition to a liquid-like lattice configuration, the signatures of topological edge states disappear. This interplay between topology and short-range order in amorphous lattices paves the way for new classes of non-crystalline topological photonic bandgap materials.

Numerical modeling of nonohmic percolation conduction and Poole–Frenkel laws

We present a numerical model that simulates the current–voltage (I–V) characteristics of materials exhibiting percolation conduction. The model consists of a two dimensional grid of exponentially different resistors in the presence of an external electric field. We obtained exponentially nonohmic I–V characteristics validating earlier analytical predictions and consistent with multiple experimental observations of the Poole–Frenkel laws in noncrystalline materials. The exponents are linear in voltage for samples smaller than the correlation length of percolation cluster L and square root in voltage for samples larger than L.

Local thickness and composition measurements from scanning convergent-beam electron diffraction of a binary non-crystalline material obtained by a pixelated detector

The nanometer-scale spatial distributions of local thickness or composition of noncrystalline materials are generally measured by spectroscopy with scanning transmission electron microscopy (STEM). Since spectroscopy requires a high electron dose and causes irradiation damage, alternative non-spectroscopic methods are required to measure the local thickness or composition of electron-sensitive noncrystalline materials. Here, we focus on the radial distribution function (RDF) of the electron diffraction of non-crystalline materials. We confirm that the RDF of the electron diffraction obtained by simulation contains information on the thickness and composition. Next, we demonstrate the determination of both thickness and composition from experimentally obtained RDFs. Although some constraints are required, we determine the local thickness and composition of a BaO-SiO2 glass sample by comparing the RDFs of diffraction measured by a high-speed pixelated detector with those of the simulated diffractions. Collaterally, this determination method can improve the quality of STEM images.

Accurate contrast determination for X-ray speckle visibility spectroscopy.

X-ray speckle visibility spectroscopy using X-ray free-electron lasers has long been proposed as a probe of fast dynamics in noncrystalline materials. In this paper, numerical modeling is presented to show how the data interpretation of visibility spectroscopy can be impacted by the nonidealities of real-life X-ray detectors. Using simulated detector data, this work provides a detailed analysis of the systematic errors of several contrast extraction algorithms in the context of low-count-rate X-ray speckle visibility spectroscopy and their origins are discussed. Here, it was found that the finite detector charge cloud and pixel size lead to an unavoidable `degeneracy' in photon position determination, and that the contrasts extracted using different algorithms can all be corrected by a simple linear model. The results suggest that experimental calibration of the correction coefficient at the count rate of interest is possible and essential. Thisallows computationally lightweight algorithms to be implemented for on-the-fly analysis.