Pockets, jumps and filaments: classifying ionic motion and determining the role of structure in electrochemical properties of 2LiS-GeSsuperionic glasses.

The structural properties of a typical solid electrolyte system (2LiS-GeS) is investigated from First principles molecular dynamics simulations. Results reveal that depolymerization of the base GeSnetwork by alkali additives takes place but appears reduced with respect to the corresponding sodium analog glass. Experimental structure functions are reproduced and reveal that the network is dominated by GeS4/2tetrahedra that are connected by edges (four-membered rings) and corners and disrupted by the addition of lithium, albeit a non-negligible fraction of connecting tetrahedra (units) are still present in the glass structure.

Evidence for Complex Dynamics in Glassy Fast Ion Conductors: The Case of Sodium Thiosilicates.

Classical molecular dynamics is used to study the dynamics of alkali ions in a promising fast ion conductor glass system, i.e., NaS-SiS.

Crucial role of S-rings in structural, relaxation, vibrational, and electronic properties of liquid sulfur close to the λ transition.

Liquid sulfur has been studied by density-functional based molecular dynamics simulations at different temperatures ranging from 400 up to 700 K across the well-documented λ transition. Structure models containing either a majority of S chains or S rings are considered and compared to experimental data from x-ray scattering. The comparison suggests a liquid structure of a majority of twofold sulfur at low temperature, dominated by S rings that open progressively upon temperature increase.

Influence of Magnesium on the Structure of Complex Multicomponent Silicates: Insights from Molecular Simulations and Neutron Scattering Experiments.

A series of multicomponent glasses containing up to five oxides are studied using classical molecular dynamics simulations and neutron scattering experiments. The focus is on the role of magnesium in determining the structural properties of these glasses and the possible mixed effect during a sodium/magnesium substitution. Calculated structure functions (pair correlation function and structure factor) rather accurately reproduce their experimental counterpart, and we show that more fine structural features are qualitatively reproduced well, despite some discrepancies in the preferential spatial distribution between sodium and magnesium to aluminum and boron, as well as the nonbridging oxygen, distribution.

Noble gas in densified liquid and amorphous silica and thermodynamic conditions for the emergence of bubbles.

Different noble gases (He, Ne, and Ar) containing densified silica liquids and glasses are investigated from molecular dynamics simulations at different system densities using a dedicated force field. The results for pure silica are first compared to reference potentials prior to an investigation of the thermodynamic diagram, the diffusivity, and the structure under different (T, P) conditions. It is found that the equation of state and the diffusivity are weakly sensitive to the nature of the incorporated noble gas, leading to a similar trend with density for all systems.

Dynamic and stress signatures of the rigid intermediate phase in glass-forming liquids.

We study the evolution of enthalpic changes across the glass transition of model sodium silicate glasses (NaO)(SiO), focusing on the detection of a flexible-rigid transition and a possible reversibility window in relationship with dynamic properties. We show that the hysteresis resulting from enthalpic relaxation during a numerical cooling-heating cycle is minimized for 12% ≤ x ≤ 20% NaO, which echoes with the experimental observation. The key result is the identification of the physical features driving this anomalous behavior.

Many-body effects at the origin of structural transitions in BO.

The structural properties of glassy diboron trioxide, g-BO, 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-BO that involve changes to both the short- and medium-range order.

Diffraction patterns of amorphous materials as a series expansion of neighbor distribution functions.

An exact analytical expression for the static structure factor [Formula: see text] in disordered materials is derived from Fourier transformed neighbor distribution decompositions in real space, and permits to reconstruct the function [Formula: see text] in an iterative fashion. The result is successfully compared to experimental data of archetypal glasses or amorphous materials (GeS, AsSe, GeTe), and links quantitatively knowledge of structural information on short and intermediate -range order with the motifs found on the diffraction patterns in reciprocal space. The approach furthermore reveals that only a limited number of neighbor shells is sufficient to reasonably describe the structure factor for k  >  2 [Formula: see text].

Decoding entangled transitions: Polyamorphism and stressed rigidity.

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.

Kinetics of Decelerated Melting.

Melting presents one of the most prominent phenomena in condensed matter science. Its microscopic understanding, however, is still fragmented, ranging from simplistic theory to the observation of melting point depressions. Here, a multimethod experimental approach is combined with computational simulation to study the microscopic mechanism of melting between these two extremes.

From elemental tellurium to GeSbTe melts: High temperature dynamic and relaxation properties in relationship with the possible fragile to strong transition.

We investigate the dynamic properties of Ge-Sb-Te phase change melts using first principles molecular dynamics with a special emphasis on the effect of tellurium composition on melt dynamics. From structural models and trajectories established previously [H. Flores-Ruiz et al.

Evidence for Anomalous Dynamic Heterogeneities in Isostatic Supercooled Liquids.

Upon cooling, the dynamics of supercooled liquids exhibits a growing transient spatial distribution of relaxation times that is known as dynamic heterogeneities. The relationship between this now well-established crucial feature of the glass transition and some underlying liquid properties remains challenging and elusive in many respects. Here we report on computer simulations of liquids with a changing network structure (densified silicates), and show that there is a deep and important link between the mechanical nature characterized by topological constraints and the spatial extent of such fluctuations.

Anomalous diffusion and non-monotonic relaxation processes in Ge-Se liquids.

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.

Universal amorphous-amorphous transition in GexSe100-x glasses under pressure.

Pressure induced structural modifications in vitreous GexSe100-x (where 10 ≤ x ≤ 25) are investigated using X-ray absorption spectroscopy (XAS) along with supplementary X-ray diffraction (XRD) experiments and ab initio molecular dynamics (AIMD) simulations. Universal changes in distances and angle distributions are observed when scaled to reduced densities. All compositions are observed to remain amorphous under pressure values up to 42 GPa.

Relaxation and physical aging in network glasses: a review.

Recent progress in the description of glassy relaxation and aging are reviewed for the wide class of network-forming materials such as GeO2, Ge x Se1-x , silicates (SiO2-Na2O) or borates (B2O3-Li2O), all of which have an important usefulness in domestic, geological or optoelectronic applications. A brief introduction of the glass transition phenomenology is given, together with the salient features that are revealed both from theory and experiments. Standard experimental methods used for the characterization of the slowing down of the dynamics are reviewed.

Revealing the role of molecular rigidity on the fragility evolution of glass-forming liquids.

If quenched fast enough, a liquid is able to avoid crystallization and will remain in a metastable supercooled state down to the glass transition, with an important increase in viscosity upon further cooling. There are important differences in the way liquids relax as they approach the glass transition, rapid or slow variation in dynamic quantities under moderate temperature changes, and a simple means to quantify such variations is provided by the concept of fragility. Here, we report molecular dynamics simulations of a typical network-forming glass, Ge-Se, and find that the relaxation behaviour of the supercooled liquid is strongly correlated to the variation of rigidity with temperature and the spatial distribution of the corresponding topological constraints, which ultimately connect to the fragility minima.

Liquid B2O3 up to 1700 K: x-ray diffraction and boroxol ring dissolution.

Using high energy x-ray diffraction, the structure factors of glassy and molten B2O3 were measured with high signal-to-noise, up to a temperature of T  =  1710(20) K. The observed systematic changes with T are shown to be consistent with the dissolution of hexagonal [B3O6] boroxol rings, which are abundant in the glass, whilst the high-T (>~1500 K) liquid can be more closely described as a random network structure based on [BO3] triangular building blocks. We therefore argue that diffraction data are in fact qualitatively sensitive to the presence of small rings, and support the existence of a continuous structural transition in molten B2O3, for which the temperature evolution of the 808 cm−1 Raman scattering band (boroxol breathing mode) has long stood as the most emphatic evidence.

Structural singularities in Ge(x)Te(100-x) films.

Structural and calorimetric investigation of Ge(x)Te(100-x) films over wide range of concentration 10 < x < 50 led to evidence two structural singularities at x ∼ 22 at. % and x ∼ 33-35 at. %.

Thermodynamic precursors, liquid-liquid transitions, dynamic and topological anomalies in densified liquid germania.

The thermodynamic, dynamic, structural, and rigidity properties of densified liquid germania (GeO2) have been investigated using classical molecular dynamics simulation. We construct from a thermodynamic framework an analytical equation of state for the liquid allowing the possible detection of thermodynamic precursors (extrema of the derivatives of the free energy), which usually indicate the possibility of a liquid-liquid transition. It is found that for the present germania system, such precursors and the possible underlying liquid-liquid transition are hidden by the slowing down of the dynamics with decreasing temperature.

Densified network glasses and liquids with thermodynamically reversible and structurally adaptive behaviour.

If crystallization can be avoided during cooling, a liquid will display a substantial increase of its viscosity, and will form a glass that behaves as a solid with a relaxation time that grows exponentially with decreasing temperature. Given this 'off-equilibrium' nature, a hysteresis loop appears when a cooling/heating cycle is performed across the glass transition. Here we report on molecular dynamics simulations of densified glass-forming liquids that follow this kind of cycle.

Structural, dynamic, electronic, and vibrational properties of flexible, intermediate, and stressed rigid As-Se glasses and liquids from first principles molecular dynamics.

The structural, vibrational, electronic, and dynamic properties of amorphous and liquid AsxSe1-x (0.10 View Article and Find Full Text PDF

Structure and topology of soda-lime silicate glasses: implications for window glass.

The structural and topological properties of soda-lime silicate glasses of the form (1-2x)SiO2-xNa2O-xCaO are studied from classical molecular dynamics using a Buckingham type potential. Focus is made on three compositions (x = 6%, 12%, and 18%) which are either silica-rich or modifier-rich. We compare the results to available experimental measurements on structural properties and find that the simulated pair correlation function and total structure factor agree very well with available experimental measurements from neutron diffraction.

Crucial effect of melt homogenization on the fragility of non-stoichiometric chalcogenides.

The kinetics of homogenization of binary AsxSe100 - x melts in the As concentration range 0% < x < 50% are followed in Fourier Transform (FT)-Raman profiling experiments, and show that 2 g sized melts in the middle concentration range 20% < x < 30% take nearly two weeks to homogenize when starting materials are reacted at 700 °C. In glasses of proven homogeneity, we find molar volumes to vary non-monotonically with composition, and the fragility index M displays a broad global minimum in the 20% < x < 30% range of x wherein M < 20. We show that properly homogenized samples have a lower measured fragility when compared to larger under-reacted melts.

Designing heavy metal oxide glasses with threshold properties from network rigidity.

Here, we show that a new class of glasses composed of heavy metal oxides involving transition metals (V2O5-TeO2) can surprisingly be designed from very basic tools using topology and rigidity of their underlying molecular networks. When investigated as a function of composition, such glasses display abrupt changes in network packing and enthalpy of relaxation at Tg, underscoring presence of flexible to rigid elastic phase transitions. We find that these elastic phases are fully consistent with polaronic nature of electronic conductivity at high V2O5 content.

Superstrong nature of covalently bonded glass-forming liquids at select compositions.

Variation of fragility (m) of specially homogenized Ge(x)Se(100-x) melts is established from complex specific heat measurements and shows that m(x) has a global minimum at an extremely low value (m = 14.8(0.5)) in the 21.