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This contribution focuses on the structural origin of flexible to rigid transitions and the possible underlying intermediate phase which have been reported to occur in a variety of network glasses such as chalcogenides or modified oxides. Here, using molecular dynamics simulations of densified glass-forming liquids, 2SiO$_2$-Na$_2$O, which are known to display a numerical reversibility window as a signature of an intermediate phase, we focus on structural functions emphasizing topological ordering using the Bhatia–Thornton formalism. Results not only reveal that densified silicates display topological ordering on lengthscales of about 25 Å, but also display obvious threshold behaviors close to the isostatic condition when the network undergoes a flexible to rigid transition. The mechanical constraint count of the atomic network structure reveals that a typical lengthscale characterizing the decay of topological correlations emerges for stressed rigid systems at $\simeq $3.5 Å, whereas small wavevector oscillations are found to be minimal when the isostatic condition is merely satisfied. An additional analysis building on diffusivity and liquid entropy suggests that the locus of flexible to rigid transitions has also connections with water-like anomalies of densified tetrahedral liquids.

The structural properties of glassy diboron trioxide, g-B2O3, are investigated from ambient to high pressure conditions using two types of atomic force-field models that account for many-body effects. These models are parameterized by a dipole- and force-fitting procedure of reference datasets created via first-principles calculations on a series of configurations. The predictions of the models are tested against experimental data, where particular attention is paid to the structural transitions in g-B2O3 that involve changes to both the short- and medium-range order. The models outperform those previously devised, where improvement originates from the incorporation of two key physical ingredients, namely, (i) the polarizability of the oxide ion and (ii) the ability of an oxide ion to change both size and shape in response to its coordination environment. The results highlight the importance of many-body effects for accurately modeling this challenging system. [ABSTRACT FROM AUTHOR]

Features intrinsic to disorder and network aspects are ubiquitous in structural glasses. Among this important class of materials, chalcogenide glasses are special—they are built of short‐range covalent forces, making them simpler than silicate glasses that possess mixed ionic and covalent forces. Selenium‐based glasses also display complex elastic phase transitions that have been described from various models, including mean‐field approaches to molecular simulations. These point to the presence of two sharply defined elastic phase transitions, a rigidity and stress transition that are non‐mean‐field in character, and separate the three distinct topological phases of flexible, isostatically rigid, and stressed‐rigid. This article reviews the physics of these glassy networks. The elastic phases and glass transition temperature are explained on a molecular level in terms of topological constraint theory (TCT), connectivity, and the open degrees of freedom. The broader aspects of TCT in relation to phase change materials, high‐k dielectrics, and cements are also commented upon. [ABSTRACT FROM AUTHOR]

There is much to learn from simulation studies of polyamorphism achieved for systems with different bonding environments. Chalcogenide glasses such as Ge–Se glasses undergo an elastic phase transition involving important changes in network connectivity. Stimulated by recent developments of topological constraint theory, we show that the concept of rigidity can be extended to a broader range of thermodynamic conditions including densified glasses. After having validated our structural first principles molecular dynamics models with experimental data over a broad pressure range for GeSe4, we show that the onset of polyamorphism is strongly related to the constraint density measuring the degree of rigidity of the network backbone, while voids and cavities in the structure collapse at very small pressures. This leads to the identification that the progressive onset of higher coordinated species typical of high pressure phases is responsible for the onset of stressed rigidity, although the constraint analysis also indicates progressive stiffening of bonding angles. Results are compared to stoichiometric and stressed rigid GeSe2 and to isostatic As2Se3 and then generalized to other compositions in the Ge–Se binary under pressure. [ABSTRACT FROM AUTHOR]

Silicate glasses can be seen as connectivity networks where specific tools are used for the understanding of the behavior with composition of their chemical and physical properties. These tools make use of the effective interatomic interactions and mechanical constraints such as those used for analyzing the stability of macroscopic trusses. We first describe how complex interactions and coordination numbers can be efficiently handled within a general framework. The ingredients of the general framework, also known as mean-field bond constraint theory, are then presented. The theory predicts flexible to stressed-rigid elastic phase transitions at threshold compositions (or mean-coordination numbers). Applications to alkali- and alkaline-earth-silicates are considered and experimental signatures of the phase transition identified. Finally, we focus on the latest development in the field, namely the appearance of a new self-organized intermediate phase between flexible and stressed-rigid phases. Experimental signatures of these phases are reviewed, and the tools of adaptative silicate networks used to understand their features. Keywords: Network glasses, silicates, rigidity, constraints

We investigate the dynamical properties of liquid GexSe100-x as a function of Ge content by first-principles molecular dynamic simulations for a certain number of temperatures in the liquid state. The focus is set on ten compositions (where x ≤ 33%) encompassing the reported flexible to rigid and rigid to stressed-rigid transitions. We examine diffusion coefficients, diffusion activation energies, glassy relaxation behavior, and viscosity of these liquids from Van Hove correlation and intermediate scattering functions. At fixed temperature, all properties/functions exhibit an anomalous behavior with Ge content in the region 18%-22%, and provide a direct and quantitative link to the network rigidity. [ABSTRACT FROM AUTHOR]

Seven magnesium‐containing aluminoborosilicate glasses, with three to five oxides, have been studied through comprehensive multinuclear solid‐state NMR (11B, 27Al, 29Si, 23Na, 17O, and 25Mg) and Raman spectroscopy. The progressive addition of cations and the substitution of sodium and calcium by magnesium illuminate the impact of magnesium on the glass structure. The proportion of tri‐coordinated boron drastically increased with magnesium addition, demonstrating the poor charge‐compensating capabilities of magnesium in tetrahedral boron units. Oxygen‐17 NMR showed the formation of mixing sites containing both Na and Mg near nonbridging oxygen sites. Furthermore, a high magnesium content appears to result in the formation of two subnetworks (boron and silicon rich) with different polymerization degrees as well as to promote the formation of high‐coordination aluminum sites (Al[V] and Al[VI]). Finally, magnesium coordination ranging from 4 to 6, with a mean value shifting from 5 to 6 along the series, suggests that magnesium might endorse an intermediate role in these glasses. [ABSTRACT FROM AUTHOR]

A typical binary amorphous telluride GeTe2 is investigated from first‐principles molecular dynamics simulations. After a comparison with chemical analogs from neutron or X‐ray diffraction experiments, such as GeO2 or GeSe2, the structure of this material is studied by examining real and reciprocal space properties. It is found that the base geometrical motifs of the germanium atom can be either in tetrahedral or in defected coordinations involving pyramidal units. A review of previous results for other compositions reveals that such binary Ge tellurides contain soft tetrahedra, at variance with lighter chalcogenides, such as GeS and GeSe, and are characterized by an increased angular bending motion (typically 20°) as compared with, e.g., GeS (5°). In addition, for amorphous Ge‐rich materials, GeTe2 and GeTe, a secondary tetrahedral geometry appears, related to the presence of GeGe bonds, having a larger mean angle of about 125°. These typical features not only relate to characteristics observed from scattering experiments but may also be a crucial feature for the understanding of the phase‐change phenomena. [ABSTRACT FROM AUTHOR]

The observation of reversibility windows (RWs) in binary GexS100−x, AsxS100−x and ternary GexAsxS100−2x glass systems permits constructing a global topological phase diagram in the GeAsS composition triangle. The RW denotes glass compositions for which the enthalpy of relaxation at Tg vanishes, leading to thermally reversing glass transitions. The phase diagram permits delineating glass compositions that are in the flexible phase (FP), intermediate phase (IP), and the stressed rigid phase (SRP) for all ternary GexAsxS100−2x glass compositions. For the IP compositions examined, a general lowering of the molar volumes, Vm, is observed in relation to those for non‐IP compositions, giving rise to a volumetric window. Melt dynamics of IP compositions reveal a fragility index, m < 20, lower than that for non‐IP melts for which m > 20 displays a fragility window, underscoring in part the reason for the delayed homogenization of melts in these sulfide glasses. The observations of the three types of window characteristics of IP compositions are feasible only when the homogeneous bulk glasses are synthesized, in which the variance ⟨Δx⟩ in As and Ge concentrations, x, across batch compositions is less than 0.01%. This is established by Fourier transform–Raman profiling of each batch of composition that is synthesized. [ABSTRACT FROM AUTHOR]

The structural properties of liquid GeSe2 have been calculated by first-principles molecular dynamics by using a periodic simulation box containing N = 480 atoms. This has allowed a comparison with previous results obtained on a smaller system size (N = 120) [M. Micoulaut, R. Vuilleumier, and C. Massobrio, Phys. Rev. B 79, 214205 (2009)]. In the domain of first-principles molecular dynamics, we obtain an assessment of system size effects of unprecedented quality. Overall, no drastic differences are found between the two sets of results, confirming that N = 120 is a suitable size to achieve a realistic description of this prototypical disordered network. However, for N = 480, short range properties are characterized by an increase of chemical order, the number of Ge tetrahedra coordinated to four Se atoms being larger. At the intermediate range order level, size effect mostly modify the low wavevector region (k ∼1 Å-1) in the concentration-concentration partial structure factor. [ABSTRACT FROM AUTHOR]

Select Raman scattering and modulated differential scanning calorimetry (DSC) experiments on specially homogenized binary GexSe100–x and GexS100–x chalcogenide glasses, supported by ab initio MD simulations are undertaken. In both chalcogenides, the glasses exhibit a super‐strong behavior (with a melt fragility index, m(x) < 20) in the 20(1)% < x < 26(1)% composition range, defining the fragility window, and with m(x) > 20 as one goes away from the fragility window. Remarkably, molecular dynamics (MD) simulations confirm the existence of the fragility window. Furthermore, in both chalcogenide glasses, the enthalpy of relaxation at Tg, ΔHnr(x), becomes minuscule (≈0) in the 20(1)% < x < 26(1)% range, defining the reversibility window, and with ΔHnr(x) increasing by an order magnitude abruptly with x outside that window. Thus, super‐strong melts formed in the fragility window give rise to isostatically rigid glasses in the reversibility window and fragile melts formed at x < 20(1)% and at x > 26(1)% give rise to flexible‐ and stressed‐rigid glasses, respectively. Melt dynamics for the examined glass systems unequivocally encode the nature of the glass topological phases. [ABSTRACT FROM AUTHOR]

The structural and dynamical properties of densified sodium silicates are investigated using molecular dynamics (MD) simulations. From the analysis of the first sharp diffraction peak (FSDP) in the amorphous phase, it is shown that some of its characteristic parameters (position, width) in partial structure factors display minima in a certain pressure interval defining a window. The pressure window can be correlated with anomalies in transport properties (diffusion, viscosity) and their activation barriers. The count of topological constraints, sensitive to pressure and temperature, is also computed. In the pressure window, we find evidence that an adaptative behavior takes place as angular constraints soften and reduce the increasing network connectivity related strain induced by pressure. These findings display striking similarities with the Boolchand intermediate phase (IP) found in rigidity driven by composition. The present numerical results also suggest that structural signatures for the IP should be found from a detailed analysis of neutron structure factors involving the partials. Finally, on a more general ground, the present study links for the first time to the best of our knowledge characteristic features of the FSDP with transport properties in the liquid. [ABSTRACT FROM AUTHOR]

The structural, vibrational, and electronic properties of low-temperature supercooled GeTe4 are studied using density functional theory (DFT). Two models have been considered: an ordinary melt-quenched system containing a majority of defect-octahedral germanium atoms, and a relaxed one obtained from a quenched SiTe4 rescaled structure which contains mostly tetrahedral germanium, while leaving the Te environment preserved. The tetrahedral system exhibits an increased agreement with the experimental structure factor S( k), the pair distribution function g( r), and the infrared absorption spectrum. It is suggested that the fraction of tetrahedral Ge must be higher as usually believed. In addition, we provide the calculation of the 125Te wideline NMR spectra. While the latter do not agree with recent experimental findings, both systems exhibit important significant differences in the widths of the computed spectra. These exploratory calculations clearly show that NMR will be an interesting probe that deserves further investigations for the experimental determination of the local geometry. [ABSTRACT FROM AUTHOR]

An FT-Raman profiling method is developed to monitor growth of structural homogeneity of binary Ge xSe100- x melts in real time ( tR) non-invasively as starting materials are reacted over days. Raman spectra of quenched melts were acquired along a one inch long column of a sample in a quartz tube. In the first step of reaction, tR < 2 days, Ge-rich crystalline- and glassy- phases form and coexist with Se-rich glasses. In the second step, tR extending up to 7 days, local structures characteristic of melts/glasses form, and steadily homogenize as Ge and Se atoms diffuse. The process terminates when all Raman lineshapes taken along the length of a sample coalesce into unique profile. Several factors contribute to the long reaction time tR for melts to homogenize, including liquid density difference of Ge and Se, diffusion controlled nanoscale mixing of melts, batch dryness, batch sizes, and laser spot size. Physical properties of such homogeneous Ge xSe100- x glasses are found to be quite different from their inhomogeneous counterparts realized after the first step of reaction. Slow homogenization of chalcogenides melts may occur generally. Variations in physical properties of chalcogenide glasses possessing nominally the same composition may have their origin in structural heterogeneity and purity. [ABSTRACT FROM AUTHOR]

When investigated as a function of deformation rate and time, model nano‐crystals exhibit a clear threshold in mechanical and relaxation behavior, this observation being valid in both the Hooke (elastic) and plastic regime. At low deformation rate (ϵ˙<ϵ˙c≃5×105  s−1), materials are able to equilibrate with ongoing deformation, whereas ultra‐fast deformations bring the system out of equilibrium on the same nanosecond timescale. The time evolution of stress is investigated and suggests features that are typical of glassy relaxation with a stretched exponential behavior for stress relaxation that is only encountered for large deformation rates. Related relaxation times furthermore reveal a rapid increase with ϵ˙. These results actually link general aspects of plastic deformation, irreversibility and thermal equilibrium. [ABSTRACT FROM AUTHOR]

The process of ultrasonic shot peening has been studied from two aspects: (a) a 3D numerical model of shot motion in the peening chamber based on the theory of granular gases, (b) an experimental setup developed for measuring shot velocity distribution in the chamber and impact locations on the peened surface. The aim of such study is to propose an experimental method providing shot trajectories in order to validate the 3D model for the process parameter optimization, used for example in the aeronautic industry. The presented results illustrate the value of the model in the understanding and mastering of the process, as well as its usefulness, on one hand for the design of peening chambers, and on the other hand for a better definition of numerical models that help to predict residual stresses generated in the peened material. [ABSTRACT FROM AUTHOR]