Medium-range Order Research Articles

Transmittance enhancement of amorphous SiO2 via ultraviolet irradiation to eliminate atomic defects

Optical absorption defects seriously degrade the performance of amorphous-SiO2. Laser/UV conditioning (LC/UC) emerge as promising methods for defects elimination, yet its mechanism is still debated due to insufficient defect characterization methods. In this study, the experiments and machine learning potential were used to characterize the defects in Raman, phonon/vibrational density of states, and XRD spectra, the UC mechanism was implied. The UC transforms the atomic defects into medium-range order structure (n-fold rings) to reduce the absorption, the mobile oxygen O0 reduces the energy barrier in the reaction and shorter wavelength of UC is suggested. Experimental results validated that the absorption coefficient below 300 nm after 254 nm UC decreases 62% higher than that after 365 nm UC.

Medium-Range Structural Order as the Driver of Activated Dynamics and Complexity Reduction in Glass-Forming Liquids.

We analyze in depth the Elastically Collective Nonlinear Langevin Equation theory of activated dynamics in metastable liquids to establish that the predicted inter-relationships between the alpha relaxation time, local cage and collective elastic barriers, dynamic localization length, and shear modulus are causally related within the theory to the medium range order (MRO) static correlation length. The latter grows exponentially with density for metastable hard sphere fluids and as a nonuniversal inverse power law with temperature for supercooled liquids under isobaric conditions. The physical origin of predicted connections between the alpha time and other metrics of cage order and the thermodynamic inverse dimensionless compressibility is fully established. It is discovered that although kinetic constraints from the real space first coordination shell are important for the alpha time, they are of secondary importance compared to the consequences of the more universal MRO correlations in both the modestly and deeply metastable regimes. This understanding sheds new light on the theoretical basis for, and prior successes of, the predictive mapping of chemically complex thermal liquids to effective hard sphere fluids based on matching their dimensionless compressibilities, a scheme we call "complexity reduction". In essence, the latter is equivalent to the physical requirement that the thermal liquid MRO correlation equals that of its effective hard sphere analog. The mapping alone is shown to provide a remarkable level of quantitative predictive power for the glass transition temperature Tg of 21 molecular and polymer liquids. Predictions for the chemically specific absolute magnitude and growth with cooling of the MRO correlation length are obtained and lie in the window of 2-6 nm at Tg. Dynamic heterogeneity, elastic facilitation, and beyond pair structure issues are briefly discussed. Future opportunities to theoretically analyze the equilibrated deep glass regime are outlined.

Exploring Structural Anisotropy in Amorphous Tb-Co via Changes in Medium-Range Ordering.

Amorphous thin films grown by magnetron co-sputtering exhibit changes in atomic structure with varying growth and annealing temperatures. Structural variations influence the bulk properties of the films. Scanning nanodiffraction performed in a transmission electron microscope (TEM) is applied to amorphous Tb17Co83 (a-Tb-Co) films deposited over a range of temperatures to measure relative changes in medium-range ordering (MRO). These measurements reveal an increase in MRO with higher growth temperatures and a decrease in MRO with higher annealing temperatures. The trend in MRO indicates a relationship between the growth conditions and local atomic ordering. By tilting select films, the TEM measures variations in the local atomic structure as a function of orientation within the films. The findings support claims that preferential ordering along the growth direction results from temperature-mediated adatom configurations during deposition, and that oriented MRO correlates with increased structural anisotropy, explaining the strong growth-induced perpendicular magnetic anisotropy found in rare earth-transition metal films. Beyond magnetic films, we propose the tilted FEM workflow as a method of extracting anisotropic structural information in a variety of amorphous materials with directionally dependent bulk properties, such as films with inherent bonding asymmetry grown by physical vapor deposition.

Short and medium range order in the rapidly solidified metallic liquid Ta: Atomic packing, connection modes, and pressure effect

In this study, molecular dynamics (MD) simulations were utilized to explore the Short and Medium-Range Order (MRO) in the rapidly solidified metallic liquid tantalum (Ta). Radial distribution function (RDF) and Voronoi tessellation analysis (VTA) techniques were employed to thoroughly explore the effect of pressure on the connectivity and structural properties at the Short-Range Order (SRO) and MRO levels. Our findings indicate that, at a quenching rate of 1013 K s-1, glassy states are achieved at or below 20 GPa, while crystalline phases emerge at 25 GPa. VTA analysis indicates a significant alteration in the local structure of glassy Ta with increasing pressure. Specifically, the fraction of icosahedral-like clusters decreases while the fraction of crystal-like clusters rises notably.Furthermore, we highlight that icosahedral-like clusters strongly tend to form 3-atom connection mode, while crystal-like clusters prefer 2-atom and 4-atom connection modes. Notably, icosahedral-like clusters are identified as the primary contributors to the emergence of the left sub-peak in the second peak of the RDF. In contrast, all cluster types contribute to the appearance of the right sub-peak.

Elucidating temperature-dependent local structure change and optical properties in GeTe phase-change material

Phase-change memory emerges as a top contender for non-volatile data storage applications. We report here a systematic change in local structure and crystallization kinetics of binary GeTe thin films using temperature-dependent resistivity measurements, which offers single-stage crystallization at around 187 °C, corroborated with x-ray diffraction. Furthermore, the change in chemical bonding upon crystallization is determined through x-ray photoelectron spectroscopy core level spectra, which reveals the existence of Ge and Te components that align with the GeTe crystal structure. Also, an investigation was carried out employing a UV–Vis–NIR spectrophotometer to explore the evolution of optical bandgaps (Eg), Tauc parameter (B) representing the local disorder, and Urbach energy (Eu) of the GeTe material, as it undergoes the transition from a disordered amorphous state to a crystalline state. As crystallization progresses, a consistent shift of Eg from 0.92 to 0.70 eV corresponds to as-deposited amorphous at room temperature and crystalline at 250 °C, respectively. In addition, the reduction in Eu (from 199.87 to 141.27 meV) and a sudden increase of B around crystallization temperature is observed upon increasing temperature, indicating direct observation of enhanced medium-range order and distortion in short-range order, respectively, in GeTe thin films, revealing improved structural and optical properties. These enhancements make the GeTe material ideal for data storage applications of phase-change memory for next-generation computing technology.

Revealing hidden medium-range order in silicate glass formers using many-body correlation functions

Revealing hidden medium-range order in silicate glass formers using many-body correlation functions

Li2O concentration influenced local structure and properties of molten LiCl salt by machine learning driven molecular dynamics simulation

Pyroprocessing is one of the promising technologies for the reprocessing of spent nuclear fuel. In the electrolytic reduction process of pyroprocessing, LiCl serves as the electrolyte with a minor addition of Li2O as the initial oxygen ion input. Investigating the impact of Li2O concentration on the molten salt system is essential for enhancing the efficiency of electrolytic reduction. In the present study, the LiCl-Li2O system is simulated using deep potential molecular dynamics simulation. The properties including self-diffusion coefficient, shear viscosity, ionic conductivity, heat capacity, radial distribution functions, coordination numbers, short-range order, medium-range order, Voronoi structure, and angle distribution functions are explored at different Li2O concentrations. This study reveals that the variations of physical properties at different Li2O concentrations in LiCl molten salt can be attributed to the changes in local structure. For example, unnormal high proportions of two Voronoi structures are found in the molten salt when the Li2O concentration increases, which may explain the changes of properties. These findings offer valuable insights for refining the parameters of electrolytic reduction and elucidating the correlation between microscopic and macroscopic phenomena.

Unveiling the thermodynamic landscape of liquid Ti–Al–Ni alloys through first-principles simulations

We conducted ab initio molecular dynamics simulations to systematically examine the composition-dependent thermodynamic properties and atomic-scale interactions in liquid Ti–Al–Ni alloys throughout the entire ternary phase space at 2033 K. The calculated enthalpies of mixing demonstrated exothermic tendencies, with a distinct minimum in the composition of Ti0.0Al0.50Ni0.50, indicating significant Al–Ni attractive interactions. Incorporating ternary interaction parameters into the Redlich-Kister-Muggianu equation enabled accurate modeling of the complex variations in mixing enthalpy. Analysis of partial pair correlation functions and structure factors revealed chemical and topological short-range ordering (SRO), as well as medium-range ordering, within the liquid alloy. Quantifying deviations from ideal configurational entropy clarifies the coupling between chemical SRO and topological SRO, significantly impacting the overall Gibbs energy of mixing. This comprehensive atomistic study provides insights into the thermodynamics of Ti–Al–Ni alloys, paving the way for tailoring their properties for high-performance applications.

Pressure dependence of intermediate-range order and elastic properties of glassy Baltic amber.

Amber is a unique example of a fragile glass that has been extensively aged below its glass transition temperature, thus reaching a state that is not accessible under normal experimental conditions. We studied the medium-range order of Baltic amber by x-ray diffraction (XRD) at high pressures. The pressure dependences of the low-angle XRD intensity between 0 and 5 Å^{-1} were measured from 0 to 7.3 GPa by the energy-dispersive XRD. The first diffraction peak at 1.1 Å^{-1} and ambient pressure has a doublet structure consisting of the first sharp diffraction peak (FSDP) at 1.05 Å^{-1} and the second feature at 1.40 Å^{-1}. The peak position and the width of the FSDP increase as the pressure increases, while the intensity of the FSDP decreases. Below P_{0}=2.4 GPa, the rapid increase of the FSDP peak position was observed, while above P_{0}, the gradual increase was observed. Below P_{0}, voids and holes in a relatively low-density state are suppressed, whereas above P_{0}, the suppression becomes mild. Such a change suggests the crossover from the low- to high-density state at P_{0}. There is a close correlation between the pressure dependence of XRD and previously reported sound velocity results. The correlation between the mean-square fluctuation of the shear modulus on the nanometer scale and fragility in amber and other glass formers is also discussed.

Effect of local chemical order on monovacancy diffusion in CoNiCrFe high-entropy alloy

High-entropy alloys (HEAs) are promising materials for nuclear applications. Understanding the influence of local chemical order (LCO) on defect diffusion and evolution in these alloys is crucial for enhancing their resistance to radiation damage. In this study, we used molecular dynamics simulation to investigate the effect of LCO on monovacancy diffusion in CoNiCrFe HEA. Alongside the Warren-Cowley parameter, which quantifies the degree of local elemental ordering, we propose a new parameter to characterize the spatial scale of LCOs. Based on their degree, LCO structures have been classified as either chemical medium-range order (CMRO) or chemical short-range order (CSRO). Our study reveals a non-monotonic variation in vacancy diffusion coefficients, transitioning from random solid solution to CSRO and then to CMRO structures. Moreover, we observe a preference for vacancy diffusion in low-energy Fe, Co-rich regions, with their spatial distribution and spatial connectivity significantly influencing the vacancy diffusion. Due to the spatial scale of the inhomogeneity introduced by LCO, both global average diffusion parameters and single migration barriers cannot fully reflect the actual diffusion dynamics. Therefore, our study emphasizes the importance of understanding the preferred diffusion pathways and the associated energy landscapes to fully assess the defect diffusion dynamics in HEAs. This deeper investigation into localized diffusion behaviors influenced by LCO is crucial for evaluating and enhancing the radiation damage tolerance of HEAs.

Different impact of recovery annealing and thermal pressure treatments on the microstructure and mechanical properties of rejuvenated Cu64Zr36 metallic glasses: A molecular dynamics study

The interpenetrating connection of icosahedra network (ICOIN) was termed as structural skeleton, which could quantify the strength and ductility of MGs. The impact of different rejuvenation methods on medium-range order (MRO) alterations was rarely studied. This work employed molecular dynamics simulations to examine the difference in MRO evolution of metallic glasses treated by recovery annealing and thermal-pressure treatment during uniaxial tension. All of the specimens obtain enhanced ductility due to improved potential energy. During tension, ICOI of all rejuvenated specimens exhibits an increased destruction rate and a decreased transformation rate. However, there are some converse characteristics in the specimens under different rejuvenation methods. The rejuvenated specimens treated by recovery annealing exhibit increased free volume and decreased ICOIN density. After thermal-pressure treatment, the rejuvenated specimens demonstrated decreased free volume and enhanced ICOIN density. During tension, the enhancement of icosahedral free volume in specimens treated by thermal pressure largely decreased the ICOIN density, thereby increasing the ductility. After tension, the rejuvenated specimens under high pressure have a significant reduction in ICOIN density compared to those under recovery annealing. The significance of this work is to accurately regulate the ICOIN density of rejuvenated specimens.

Unveiling the Formation Mechanism of Medium Range Ordering in Zr-based Bulk Metallic Glasses Using Angular Correlation Analysis of 4D-STEM

Unveiling the Formation Mechanism of Medium Range Ordering in Zr-based Bulk Metallic Glasses Using Angular Correlation Analysis of 4D-STEM

Connecting Medium Range Ordering to Topological Properties of Amorphous Bi2Se3

Connecting Medium Range Ordering to Topological Properties of Amorphous Bi2Se3

Evolution of short-range order and dynamics of medium-range order influence on the glass-forming ability of CuZrAg alloys

The glass-forming ability (GFA) of metallic glasses (MGs) is influenced by both short-range order (SRO) and medium-range order (MRO). However, experimentally examining the dynamic evolution of these microstructures across various time scales remains challenging. Therefore, this study employs molecular dynamics (MD) simulations to investigate the evolution of SRO and relaxation dynamics of MRO impacting on the GFA of Cu60Zr40-xAgx (x = 5, 10, 15, 20, 25) ternary alloys during their rapid solidification process. The GFA is characterized by the width of the supercooled liquid region, with Cu60Zr35Ag5 exhibiting the most prominent GFA among the five alloys. The reverse tracking method was utilized to analyze the inheritance and transformation of defective icosahedral clusters within the SRO. The defective icosahedral heritability and transformation rate are indicative of a more ordered and stable structure, significantly influencing the GFA. Furthermore, the decrease of Ag content leads to the increase of the average size of MRO clusters. The systems with larger MRO nanoclusters exhibit pronounced dynamic heterogeneity and a robust “cage effect”, thus hindering crystallization and promoting GFA. This study approaches GFA analysis from a microstructural evolution and dynamics perspective, aiming to provide theoretical backing for the discovery of alloys with superior GFA.

Comparative assessment and experimental analysis of atomic coordination and mechanical properties of sputter deposited Ti-rich TiN, TiC, and TiCN thin films

In the present work, Titanium nitride (TiN)150 nm, Titanium on Titanium nitride (TiN75 nm/Ti75 nm), Titanium on Titanium carbonitride (TiCN75 nm/Ti75 nm), and Titanium on Titanium carbide (TiC75 nm/Ti75 nm) films were deposited on Si (100), quartz, and glass substrates through dc-magnetron sputtering. Vacuum annelation was carried at 600ºC for 1 hr to form TiN (m-TiN), Ti-rich TiN (b-TiN), Ti-rich TiC (b-TiC) and Ti-rich TiCN (b-TiCN) films. b-TiCN showed less optical absorbance and band gap compared to b-TiN and b-TiC films and more detrimental types of defects, which might lead to the degradation of mechanical properties in b-TiCN film. The shifting of the Ti-N core orbital towards higher binding energy showed the higher bond strength of the b-TiN film. The ITA/ITO ratio increased from 0.8 to 2.37 for b-TiC to b-TiN films, signifying the deterioration in medium-range order. The defect parameter, ID/IG, increased from 0.63 to 0.71, indicating that b-TiCN has higher defects than b-TiC films. The valence band spectra revealed that b-TiN and b-TiC valence bands are present above the Fermi level but below in the case of b-TiCN films. X-ray absorption spectroscopy confirmed that the Ti L, N K, and C K-edge were observed at 462.38, 404.53, and 287.33 eV, respectively. The highest Young’s modulus and hardness were 97.43 and 18.47 GPa, respectively, measured by a Berkovich nanoindenter for b-TiN thin films.

First-Principles Investigation of the Effects of UF4 and ThF4 Fuels on the Structural, Dynamic, and Thermodynamic Properties of LiF-NaF.

An in-depth understanding and characterization of molten salt properties are necessary for the optimized design, efficient operation, and safety assurance of molten salt reactors (MSRs). Investigating molten salt properties in experimental settings can be challenging and time-consuming due to the high temperatures of interest, the salt's corrosiveness, purity and composition control, and health and safety concerns. Therefore, it is beneficial to perform computational screening to assist in the ultimate experimental measurements. Herein, we used first-principles molecular dynamics simulations to calculate several thermophysical, structural, and dynamic properties of eutectic LiF-NaF with fuel additives UF4 and ThF4. We found that with the incorporation of uranium or thorium, a prepeak appears in the structure factor, indicative of a medium-range structural ordering. Furthermore, we explore the mechanism through which these structural changes enhance shear stress correlations, thereby increasing the salt's viscosity. This work highlights the importance of studying the atomic-scale structure of molten salts and how the addition of fuel elements can substantially affect it.

Correlations between shadow glass transition, enthalpy recovery and medium range order in a Pd40Ni40P20 bulk metallic glass

Two distinct enthalpic states were induced in a Pd40Ni40P20 bulk metallic glass through cyclic annealing at 473 K (0.81Tg) and 558 K (0.96Tg), targeting individual β or α relaxation modes, here referred to as the so-called shadow glass transition (SGT) and enthalpy recovery, respectively. Fluctuation electron microscopy (FEM) was employed to systematically investigate the medium-range order (MRO) within the amorphous structure in the post-annealed state and subsequent quenching after the SGT. Comparing the results with literature data from the same glass quenched from SCL samples, a significant increase in MRO was observed after annealing at 0.81Tg. Conversely, samples quenched directly after the SGT exhibit a reduced MRO, similar to those quenched from the supercooled liquid (SCL). These findings suggest a correlation between the SGT and a decrease in MRO within the glass, a phenomenon discussed in the context of underlying structural rearrangements associated with β relaxation, encompassing the SGT. Additionally, prolonged annealing at 0.96Tg led to glass separation in the material, with remixing observed as an endothermic transformation in the SCL. The origin of this transformation is discussed as either a potential liquid–liquid or glass–liquid transition and their connection to modifications in medium-range order.

Characterization of Z cluster connectivity in CuZr metallic glasses.

Previous studies have proposed that the backbone of metallic glasses consists mainly of high-centrosymmetric structures, particularly Z clusters, which are responsible for the strength of the glass matrix. However, exploring these networks involves medium-range order analysis, a topic still not fully understood in the literature. This study investigates the atomic connectivity of CuZr metallic glasses by analyzing Z clusters using complex networks to establish their relationship with the mechanical behavior. Our results reveal higher connectivity and larger network sizes in the sample exhibiting the most pronounced stress overshoot, while the opposite trend is observed in samples with less pronounced stress overshoot. Metrics, such as density and clustering coefficient, further validate the correlation between Z cluster connectivity and mechanical behavior. These findings underscore the critical role of Z cluster connectivity in understanding the mechanical response of metallic glasses. Molecular dynamics simulations were conducted using the LAMMPS software. Atomic interactions in Cu Zr metallic glasses were modeled using the embedded atom method, and compression tests were performed to assess the mechanical response. Atomic connectivity was examined through complex network analysis based on Z clusters, utilizing the NetworkX library for the Python programming language. Within this framework, parameters such as the average coordination number, network size, and network density were calculated, revealing the relationship between the interpenetrating Z cluster structure and the mechanical response of the samples.

Normal-to-Supercooled Liquid Transition in Molecular Glass-Formers: A Hidden Structural Transformation Fuelled by Conformational Interconversion.

Molecular dynamics and transport coefficients change significantly around the so-called Arrhenius crossover in glass-forming systems. In this article, we revisit the dynamic processes occurring in a glass-forming macrocyclic crown thiaether MeBzS2O above its glass transition, revealing two crossover temperatures: TB at 309 and TA at 333 K. We identify the second one as the Arrhenius crossover that is closely related to the normal-to-supercooled liquid transition in this compound. We show that the transformation occurring at this point goes far beyond molecular dynamics (where the temperature dependence of structural relaxation times changes its character from activation-like to super-Arrhenius), being reflected also in the internal structure and diffraction pattern. In this respect, we found a twofold local organization of the nearest-neighbor molecules via weak van der Waals forces, without the formation of any medium-range order or mesophases. The nearest surrounding of each molecule evolves structurally in time due to the ongoing fast conformational changes. We identify several conformers of MeBzS2O, demonstrating that its lowest-energy conformation is preferred mainly at lower temperatures, i.e., in the supercooled liquid state. Its increased prevalence modifies locally the short-range intermolecular order and promotes vitrification. Consequently, we indicate that the Arrhenius transition is fuelled rather by conformational changes in this glass-forming macrocyclic crown thiaether, which is a different scenario from the so-far existing concepts. Our studies combine broadband dielectric spectroscopy (BDS), X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, molecular dynamics (MD) simulations, and density functional theory (DFT) calculations.

Influence of Zr element on the atomic structure of Al-Cu alloy liquid

The relationship among chemical composition, structure and glass-forming ability in multicomponent alloys is a long-standing puzzle due to the complexity of compositions. In this work, Al77.8Cu22.2 and Al70Cu20Zr10 amorphous alloy liquids were investigated by combining high-energy X-ray diffraction at a synchrotron facility, ab-initio molecular dynamics and reverse Monte Carlo simulations. The analysis of short- and medium-range order structures of two liquid alloys show that the addition of Zr element increases the number of cluster types and makes the distribution of cluster content more uniform, and enhances the structural heterogeneity and correlation of clusters with high five-fold symmetry. These findings reveal the influence of Zr element on the atomic structure of the Al-Cu liquid alloy and deepen the understanding of Al-based metallic glasses formation.