Medium-range Order Research Articles

Revealing the relationship between liquid fragility and medium-range order in silicate glasses

Despite decades of studies, the nature of the glass transition remains elusive. In particular, the sharpness of the dynamical arrest of a melt at the glass transition is captured by its fragility. Here, we reveal that fragility is governed by the medium-range order structure. Based on neutron-diffraction data for a series of aluminosilicate glasses, we propose a measurable structural parameter that features a strong inverse correlation with fragility, namely, the average medium-range distance (MRD). We use in-situ high-temperature neutron-scattering data to discuss the physical origin of this correlation. We argue that glasses exhibiting low MRD values present an excess of small network rings. Such rings are unstable and deform more readily with changes in temperature, which tends to increase fragility. These results reveal that the sharpness of the dynamical arrest experienced by a silicate glass at the glass transition is surprisingly encoded into the stability of rings in its network.

In situ observation of medium range ordering and crystallization of amorphous TiO2 ultrathin films grown by atomic layer deposition

The evolution of medium range ordering (MRO) and crystallization behavior of amorphous TiO2 films grown by atomic layer deposition (ALD) were studied using in situ four-dimensional scanning transmission electron microscopy. The films remain fully amorphous when grown at 120 °C or below, but they start showing crystallization of anatase phases when grown at 140 °C or above. The degree of MRO increases as a function of temperature and maximizes at 140 °C when crystallization starts to occur, which suggests that crystallization prerequires the development of nanoscale MRO that serves as the site of nucleation. In situ annealing of amorphous TiO2 films grown at 80 °C shows enhancement of MRO but limited number of nucleation, which suggests that post-annealing develops only a small portion of MRO into crystal nuclei. The MRO regions that do not develop into crystals undergo structural relaxation instead, which provides insights into the critical size and degree of ordering and the stability of certain MRO types at different temperatures. In addition, crystallographic defects were observed within crystal phases, which likely negate corrosion resistance of the film. Our result highlights the importance of understanding and controlling MRO for optimizing ALD-grown amorphous films for next-generation functional devices and renewable energy applications.

Medium Range Ordering in liquid Al-based alloys: towards a machine learning approach of solidification

Ab initio molecular dynamic simulations of liquid Al93Cr7 and Al83Zn10Cr7 alloy have evidenced the presence of an icosahedral short range order (iSRO) which develops into icosahedral medium range order (iMRO) as the melt is undercooled. This atomic arrangement accounts for the presence of Dynamic Heterogeneities characterized by Al fast-dynamics regions and Cr-rich slow-dynamics regions. Characterisation of the medium range order was carried out by a direct connectivity approach. However, given the small size of the simulation (256 atoms), such characterisation remains partial. In order to better describe both iMRO formation and more dilute alloys closer to industrial compositions, a new modelling strategy has been initiated to allow in the long term for large-scale atomic-level simulations. Molecular Dynamics (MD) of million to billion atoms may indeed lead to meaningful results. Exploitation of such large amounts of MD-generated big data can be carried out by means of Machine Learning (ML) tools which provide relevant and powerful analysis methods. An unsupervised ML approach based on topological descriptors using persistent homology concepts is proposed to reveal the structural features of atomic arrangements without a priori knowledge on the studied system. This approach has been applied so far to pure Al melts. Both translational and orientational orderings are thus evidenced together with nucleation pathways, whose revealed features are beyond the hypotheses of the Classical Nucleation Theory.

A fractal structural feature related to dynamic crossover in metallic glass-forming liquids.

The dynamic crossover in supercooled liquids initially predicted by model coupling theory has been widely accepted, but its underlying structural origin is still an open issue for glass-forming liquids. By molecular dynamics simulations of binary CuZr liquids, the present work verifies that high pressure could enhance this crossover, facilitating the studies on the structural features at the crossover temperature Tc. We discover that the topological connectivity of icosahedral clusters is responsible for this dynamic crossover, rather than all clusters. Tc is the temperature at which the connectivity degree between these clusters reaches a maximum and the dynamic heterogeneity begins to keep stable. Below Tc, the fractal topological structures appear in the medium-range order scale. The icosahedral clusters with a certain connectivity pattern can be regarded as a fractal structural unit. By employing the established fractal analysis method, the fractal dimension D of the icosahedral network is calculated. Our results indicate that the D value increases monotonically with increasing pressure and the fractal behavior of the icosahedral network is an inherent feature of metallic glasses. We also find similar fractal behavior in clusters with high local five-fold symmetry. Our findings shed light on the origin of a dynamic crossover in the deep supercooled region of metallic glasses and also demonstrate the important role of icosahedral clusters in uncovering the fractal behavior of metallic glass.

Quantifying chemical fluctuations around medium-range orders and its impact on dislocation interactions in equiatomic CrCoNi medium entropy alloy

Equiatomic multi-principal element alloys manifest unique mechanical properties derived from the single-phase solid solutions with compositional disorders. Recently discovered medium-range orders (MROs) with a size of 1–5 nm in CrCoNi medium entropy alloy acclaimed measurable impact on mechanical properties, which, however has been questioned by density functional theory calculations. Here we report that the formation of MROs is accompanied by the redistribution of constituent elements, and the subtle chemical composition fluctuations could by directly probed and quantified by using a state-of-the-art aberration-corrected transmission electron microscope. The presence of such MROs contributes to an appreciable increase in yield strength (∼40 MPa) with a higher work hardening rate, originating from the strong dislocation interactions with MROs at the incipient plastic deformation. These findings demonstrate that MROs have a significant impact on reducing the mean free path of full/partial dislocations along a specific slipping plane, offering a new avenue for strengthening of equiatomic element alloys by tuning local composition and atomic configurations.

Investigation of Medium Range Order Defects in CuxZr100-x (x = 50, 56, 60, 64) Metallic Glasses Using Reverse Monte Carlo Modeling

The identification of glassy defects in amorphous materials is a long-standing but imperative problem which hinders our deep understanding of the structural origin of mechanical behavior in metallic glasses (MGs). Here, a combination of experiments and numerical simulations were used to reconstruct the atomic packing of MGs. Using the integration of synchrotron X-ray diffraction (XRD) datasets, ab initio molecular dynamics simulations, as well as reverse Monte Carlo simulation, we determined the three-dimensional atomic positions of a series of binary MGs CuxZr100-x (x = 50, 56, 60, 64). Then we uncovered the connection of short-range clusters as well as the nature of the medium range order (MRO). It turns out that full icosahedral tend to connect to each other forming the back bones, with dimensions positively correlated with the Cu content. By quantifying the discontinuity of full icosahedral networks, we identified the MRO defects which were found to be highly influenced by the macroscopic chemical contents. Here, the density of MRO defects is growing with the decrease of Cu contents. These results suggest the reason for the stable kinetic properties and good glass forming ability of the Cu64Zr36 system, which is rich in full icosahedral clusters <0,0,12,0> but a lack of MRO defects.

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.

Phase stability and optoelectronic properties of lead-free CsSn1−xGexI3 mixed halide perovskites: A first-principles study

In this study, the physical properties of inorganic mixed CsSn1−xGexI3 halide perovskites are investigated by performing first-principles calculations. The results reveal that the mixed phase forms solid solutions without a long-range order in the atomic arrangement, which helps increase the carrier lifetime by facilitating the separation of electron–hole pairs. CsSn1−xGexI3 favors to adopt an orthorhombic CsSnI3 structure with highly disordered Sn–Ge arrangements in Sn-rich compositions, whereas it is likely to crystallize into a rhombohedral CsGeI3 structure maintaining some medium-range order in Ge-rich compositions. Regardless of the crystal shape, the bandgap of the mixed phase is less than 2 eV, which indicates that it can efficiently absorb visible light. In addition, electrons and holes exhibit relatively small effective carrier masses (<0.6 m0, where m0 is the electron rest mass), which indicates high mobility. By demonstrating the high phase stability and advantageous photophysical properties of CsSn1−xGexI3, this work paves the way for the development of high-performance photovoltaic devices.

Persistent homology: A tool to understand medium-range order glass structure

Glass structure remains puzzling to scientists, especially due to the challenges in characterizing their structural order beyond the first coordination shell, i.e., the so-called medium-range order. Structural method development is therefore needed to advance our understanding of, e.g., structure-property relations in these disordered materials. To this end, we here review the fundamentals, applications and perspectives of an interesting new approach, namely persistent homology , which is a type of topological data analysis. This method allows for the analysis of both ring- and void-type structures in materials without making any assumptions of the network structure. As discussed herein, it has recently been used to analyze atomic position data (as obtained from atomistic simulations or reverse Monte Carlo) of glasses, especially regarding their medium-range order structure. We also discuss the opportunities in coupling persistent homology analyses with machine learning calculations as well as the open questions and challenges that require further investigations. • We provide an introduction to the use of persistent homology. • Recent uses of persistent homology in glass science are reviewed. • Perspectives on applying persistent homology in glass science are identified.

Fabricating polymer/HEA-hybrid topological lattice structure for enhanced mechanical properties

Metamaterials such as architected lattice structures have aroused broad interest in being applied as mechanical supports for their lightweight and custom-shaped capabilities. Although various prior efforts have been devoted, a multiscale fabrication of micro-nano lattice structures without penalizing the mechanical properties is still a challenging but highly desirable task. Here we put forward a strategy to produce a mechanically enhanced micro-nano lattice structure by conformally depositing a CoCrFeNiTi high-entropy alloy (HEA) coating layer onto a three-dimensional (3D) printed polymer skeleton. The template for the 3D printing employs a six-membered tricapped trigonal prism (6M-TTP) structure derived from a medium-range order structure motif in amorphous alloys. The topological complexity of the 6M-TTP can substantially avoid the stress concentration by offering stress-release channels, while the HEA film incorporating with amorphous and nanocrystalline constituents can further reinforce the lattice architecture through its size hardening effect. Benefitting from the above, the fabricated polymer/HEA-hybrid lattice exhibits a high specific compressive strength (∼0.055 ​MPa ​kg-1 m3 at a density below 500 ​kg ​m-3), a superior elastic recoverability (∼70% recovery rate under >30% compression), an enhanced plasticity (40% strain) and a high specific modulus (0.135 ​MPa ​kg-1 m3). Our strategy initiates a perspective way to fabricate multiscale micro-nano lattice structures with improved mechanical properties, which could be extended to widespread metamaterial research.

The kinetics of reentrant glass transition in metallic liquids

Reentrant glass transition is a new glass-transition phenomenon in which a devitrified liquid reenters a second glassy state and is well-known in some colloidal systems. Reentrant glass transition has been recently discovered in metallic-liquid systems but its ongoing mechanisms remain mostly unknown. Here, the reentrant glass transition of a series of Pd-Ni-P metallic liquids are investigated using fast-calorimetric analyses and ab-initio molecular dynamics simulation. While the amount of heat release of reentrant glass transition in different Pd-Ni-P liquids are similar, the change of liquid kinetics itself results in different reentrant-glass-transition behaviors: the transition kinetics is governed by the fast hopping of P atoms rather than the cooperative liquid motions related with vitrification. The reentrant glass transition is found to be related to medium-range structural ordering process through vertex-sharing of the P-centered polyhedral clusters. The kinetics map of liquid freezing for Pd-Ni-P glass-forming liquids over broad ranges of heating-rate and composition is presented.

Role of S/Se replacement on the structure of Ge20Se80-xSx glasses

The effect of S-content on the medium-and short-range order structure of Ge20Se80−xSxleft(x=0,15,30 mathrm {at}%right), has been investigated by using high-energy X-ray diffraction. Medium-range order changes, as illustrated by the parameters of the first sharp diffraction peak, were discussed in the light of the microcrystalline model. The short-range order parameters have been obtained by the analysis of the first two peaks in the curve of the total radial distribution function. The current results indicate that S-atoms enter the host network of the stiffness composition (Ge20Se80) as compensation of Se-atoms.

Liquid-liquid transition and inherited signatures in Zr-Cu-Ni-Al metallic glasses

Up to now, the research of liquid-liquid transition (LLT) has significantly improved the understanding of its liquid structure and evolution, which is crucial to addressing the longstanding problem with glass formation. However, it remains unclear how LLT affects the vitrification and structures of glasses, due to the challenge in experimentally exploring LLT in high-temperature melt well above the liquidus temperature. In the present study, an experiment is conducted to observe LLT in three Zr-Cu-Ni-Al melts at around 1450 K from both dynamic and thermodynamic perspectives. It is discovered that superheated fragility is closely associated with glass-forming ability. By controlling the melt temperature, two types of metallic glasses are obtained, showing difference in crystallization behaviors, which is attributed to the competition between crystal-like clusters and icosahedral-like clusters. Prior to the LLT, the formation of crystal-like clusters is dominant. Afterwards, the LLT promoted formation of icosahedral-like clusters becomes dominant. Due to the inherited signatures in glass, LLT is manifested by the changes in both medium-range order structure and short-range order structure. This study further reveals the pattern of microstructural evolution accompanied by LLT in high-temperature multi-component melts, which is conducive to the control on structure and properties of glasses from liquids.

Structural length-scale of β relaxation in metallic glass.

Establishing the structure-property relationship is an important goal of glassy materials, but it is usually impeded by their disordered structure and non-equilibrium nature. Recent studies have illustrated that secondary (β) relaxation is closely correlated with several properties in a range of glassy materials. However, it has been challenging to identify the pertinent structural features that govern it. In this work, we show that the so-called polyamorphous transition in metallic glasses offers an opportunity to distinguish the structural length scale of β relaxation. We find that, while the glass transition temperature and medium-range orders (MROs) change rapidly across the polyamorphous transition, the intensity of β relaxation and the short-range orders (SROs) evolve in a way similar to those in an ordinary reference glass without polyamorphous transition. Our findings suggest that the MRO accounts mainly for the global stiffening of the materials and the glass transition, while the SRO contributes more to β relaxation per se.

Structure of amorphous TeO2 revisited: A hybrid functional ab initio molecular dynamics study

The atomic-scale structure of glassy ${\mathrm{TeO}}_{2}$ has been the subject of a longstanding debate. We resort to first-principles molecular dynamics with a careful choice of the exchange and correlation functional to achieve a good agreement with experimental findings for the topology of glassy and molten ${\mathrm{TeO}}_{2}$. We show that only molecular dynamics at the hybrid functional level of theory is able to reproduce a correct description of the medium-range order in the glass. Based on a Wannier-function decomposition of the electronic structure, we show that the coordination number of tellurium is around 4 and that there is a non-negligible fraction of nonbridging oxygen atoms. An analysis of the net atomic charge distribution shows that an increase of the Te-O-Te bridge asymmetry strongly correlates with the charge on the oxygen atom. Additionally, we find that the oxygen bridge asymmetry increases with temperature, which strengthens the short-range disorder in molten ${\mathrm{TeO}}_{2}$, and consequently lowers the coordination number of Te. These results provide a revisited picture of the ${\mathrm{TeO}}_{2}$ network connectivity and its evolution as a function of temperature.

Structure dependence of fracture toughness and ionic conductivity in lithium borophosphate glassy electrolytes for all-solid-state batteries

Glass materials are potential candidates as solid electrolytes for batteries, but the atomistic origins of the variations in their properties and functionalities with composition are not well understood. Here, based on combined experimental and simulation techniques, we investigate the structural origin of the variation in fracture toughness and ionic conductivity of lithium borophosphate glass electrolytes with varying compositions. We focus on these properties since they are critically important for mechanical stability and electrochemical performances of glassy electrolytes. To this end, we have performed molecular dynamics simulations combined with X-ray total scattering experiments to provide the atomic picture of the disordered structure of borophosphate glass. The mechanical properties have been characterized through single-edge precracked beam measurements and axial tensile simulations. We find that the deformation and fracture behaviors of the electrolytes are governed by bond switching events of boron, which dissipate the strain energy during fracture. The migration of lithium ions in the electrolyte network is facilitated by hopping between superstructural rings, which reflects the important role of medium-range order structure in determining the lithium-ion diffusion. These findings have important implications for the design of future glassy electrolytes.

Interstitial-driven local chemical order enables ultrastrong face-centered cubic multicomponent alloys

Multicomponent alloys of the Fe-Mn-Co-Cr-Ni family with face-centered cubic (fcc) structure exhibit many excellent properties. However, they usually show limited yield strength, which cannot meet the demand for practical applications. Here, we report a universal strategy for designing ultrastrong and ductile fcc multicomponent alloys, by introducing interstitial-driven local chemical order (LCO) through simple thermomechanical processing. In a prototype FeMnCoCrN multicomponent alloy processed by partial-recrystallization annealing, a high-density of fine laths containing interstitial-driven LCO domains (with both short- and medium-range orders) are predominant. Those laths evolve from planar dislocation slip bands promoted by the intrinsic short-range order of the alloy under prior cold deformation. Owing to the hardening effect of the LCO-laths, an ultra-high yield strength of 1.34 GPa is achieved, while deformation twinning contributes to a uniform elongation of 13.9%. This design strategy, which is also successfully verified in a multicomponent austenitic steel, provides a new paradigm for developing high-performance fcc materials at low cost.

Short- and medium range order in GeTe4-Ag glasses

Short- and medium range order glasses of GeTe4-Ag approximate composition have been studied by neutron diffraction, X-ray diffraction and extended X-ray absorption fine structure spectroscopy. Chemical correlations have been determined by fitting multiple datasets simultaneously with the reverse Monte Carlo simulation technique. It has been found that Ge atoms remain basically fourfold coordinated by Te while Te atoms have mostly two Ge/Te neighbours. The topology of the GeTe4 host network does not change upon adding Ag. Similarly to binary Ge-Te glasses, neighbouring GeTe4 tetrahedra are predominantly in corner sharing configuration in all investigated GeTe4-Ag compositions. Ag atoms bind mostly to Te and the average total coordination number of Ag does not change significantly with increasing Ag content.

Identification of medium range order defects and their critical effect on spallation of Cu64Zr36 metallic glass

The absence of a well-defined structural defect is a longstanding problem, acting as the obstacle for understanding of mechanical behavior in metallic glasses. Here, through a compelling integration of experimental and computational technologies containing Synchrotron X-ray diffraction, extended X-ray absorption fine structure spectroscopy (EXAFS) as well as atomistic simulations, we present more direct evidences for the existence of medium range order (MRO) defects based on the discontinuity of stiff icosahedral network. As revealed by the numerical simulation of spallation process for two Cu64Zr36 metallic glasses with different cooling history, the MRO defects can be used as the efficient predictor of void nucleation, as evidenced by the strong correlation between high MRO defects location and subsequent void sites. This observation implies that the initial distribution as well as further generation of MRO defects play a critical role for the mechanical failure process of metallic glasses. It can be also extended to experimental conditions since the atomic packing of metallic glasses in molecular dynamic (MD) simulations is in great agreement with that obtained from reality as characterized by results of X-ray and EXAFS.

Imaging Atomic Structure, Strain, and Disorder By Atomic Electron Tomography

Knowledge of the atomic structure of materials is critical to understanding their functionality in many fields such as biology, microelectronics, condensed matter, and nanotechnology. Traditionally, x-ray crystallography has been the main method used to solve structures of all types. New techniques such as micro electron diffraction and single particle cryo electron microscopy are now utilizing electrons to study crystals that cannot be crystallized at the scales necessary for x-ray experiments. However, these techniques require ensembles of identical unit cells (like molecules or proteins) to solve the structure from a series of images or diffraction patterns. The need to average over many unit cells means that important material properties such as defects, dopants, and disorder are not captured. Investigations of unique arrangements of atoms requires a direct imaging method that does not rely on structural averaging or the assumption of crystallinity.Transmission electron microscopy (TEM) provides the ability to directly image atomic structure, and the development of powerful, stable aberration correctors in the last decade now routinely provide sub-Angstrom imaging resolution. Scanning TEM (STEM) has also become an effective method for measuring the atomic structure of inorganic materials based on so-called Z-contrast where the image intensities are proportional to the atomic number of the material being imaged. However, by their nature TEMs are limited to producing only two-dimensional projections of a structure. Electron tomography is a technique that can reconstruct the three-dimensional structure of unique nanoscale objects from a series of two-dimensional images acquired at different viewing angles. Tomography was traditionally limited to nanoscale resolution due to experimental difficulties and a lack of high-quality reconstruction algorithms.We have developed atomic electron tomography (AET) by utilizing the sub-Angstrom real-space resolution of STEM with advanced tomographic reconstruction algorithms to solve the structure of materials at the single atom level without averaging or the assumption of crystallinity. This talk will present recent progress and capabilities in the field of AET for reconstructing order and disorder in materials with 20 picometer precision. Our success in resolving chemical order/disorder in metallic FePt nanoparticles was extended to include the time domain by capturing nucleation and growth or ordered phases in the same nanoparticle. Also, the ability to reconstruct atomic structure without the need for averaging or crystallinity led to the achievement of directly imaging three-dimensional atomic arrangements in amorphous solids. We determined the chemical and structural disorder of an eight-element metallic glass to quantitatively characterize short- and medium-range order, and we determined the disordered atomic packing of monatomic amorphous materials.The field of electron microscopy is rapidly changing with the advent of improved detectors, aberration correctors, and monochromators. We are exploring the use of these new hardware to implement new imaging algorithms such as ptychography to allow AET to explore all new classes of materials especially lightly scattering materials like oxygen. We have also established the Materials Databank to share experimentally determined 3D atomic structures with other scientists. As more structures and classes of materials are solved, we expect AET to become an important technique in the field of atomic structural characterization.