A Strong to Fragile Transition in a Model of Liquid Silica

Negative density-dependence of the structural relaxation time of liquid silica: insights from a comparative molecular dynamics study

In many tetrahedral network-forming liquids, structural relaxation is anomalously accelerated by compression over relatively low pressure ranges. Here, for silica, we study this problem through comparative molecular dynamics simulations using two different models. Under compression, the network structures are compacted by slight tuning of the intertetrahedral bond angles while nearly preserving the unit tetrahedral structure. The consequent structural changes are remarkable for length scales larger than the nearest neighbor ion-pair distances. Accompanying with such structural changes, the interactions of the nearest Si-O pairs remain almost unchanged, whereas those of other ion pairs are, on average, strengthened by the degree of compression. In particular, the enhancement of the net Si-O interactions at the next nearest neighbor distance, which assist an ion in escaping from the potential well, reduces the activation energy, leading to a significant acceleration of structural relaxation. The results of our comparative molecular dynamics simulations are compatible with the scenario proposed by Angell, and further indicate that the structural relaxation dynamics cannot be uniquely determined by the configurations but strongly depends on the details of the coupling between the structure and the interaction.

Oxygen Quadclusters in SiO_{2} Glass above Megabar Pressures up to 160 GPa Revealed by X-Ray Raman Scattering

As oxygen may occupy a major volume of oxides, a densification of amorphous oxides under extreme compression is dominated by reorganization of oxygen during compression. X-ray Raman scattering (XRS) spectra for SiO_{2} glass up to 1.6 Mbar reveal the evolution of heavily contracted oxygen environments characterized by a decrease in average O-O distance and the potential emergence of quadruply coordinated oxygen (oxygen quadcluster). Our results also reveal that the edge energies at the centers of gravity of the XRS features increase linearly with bulk density, yielding the first predictive relationship between the density and partial density of state of oxides above megabar pressures. The extreme densification paths with densified oxygen in amorphous oxides shed light upon the possible existence of stable melts in the planetary interiors.

Universal scaling in the aging of the strong glass former SiO2

We show that the aging dynamics of a strong glass former displays a strikingly simple scaling behavior, connecting the average dynamics with its fluctuations, namely, the dynamical heterogeneities. We perform molecular dynamics simulations of SiO2 with van Beest-Kramer-van Santen interactions, quenching the system from high to low temperature, and study the evolution of the system as a function of the waiting time tw measured from the instant of the quench. We find that both the aging behavior of the dynamic susceptibility χ4 and the aging behavior of the probability distribution P(fs,r) of the local incoherent intermediate scattering function fs,r can be described by simple scaling forms in terms of the global incoherent intermediate scattering function C. The scaling forms are the same that have been found to describe the aging of several fragile glass formers and that, in the case of P(fs,r), have been also predicted theoretically. A thorough study of the length scales involved highlights the importance of intermediate length scales. We also analyze directly the scaling dependence on particle type and on wavevector q and find that both the average and the fluctuations of the slow aging dynamics are controlled by a unique aging clock, which is not only independent of the wavevector q, but is also the same for O and Si atoms.

Computer simulations of glasses: the potential energy landscape

We review the current state of research on glasses, discussing the theoretical background and computational models employed to describe them. This article focuses on the use of the potential energy landscape (PEL) paradigm to account for the phenomenology of glassy systems, and the way in which it can be applied in simulations and the interpretation of their results. This article provides a broad overview of the rich phenomenology of glasses, followed by a summary of the theoretical frameworks developed to describe this phenomonology. We discuss the background of the PEL in detail, the onerous task of how to generate computer models of glasses, various methods of analysing numerical simulations, and the literature on the most commonly used model systems. Finally, we tackle the problem of how to distinguish a good glass former from a good crystal former from an analysis of the PEL. In summarising the state of the potential energy landscape picture, we develop the foundations for new theoretical methods that allow the ab initio prediction of the glass-forming ability of new materials by analysis of the PEL.

Dynamics in a tetrahedral network glassformer: vibrations, network rearrangements, and diffusion

We perform molecular dynamics simulation on a tetrahedral network glassformer using a model for viscous SiO2 by Coslovich and Pastore [J. Phys.: Condens. Matter 21, 285107 (2009)]. In this system, Si and O particles form a random network at low temperature T. We attach an ellipsoid to each particle to represent its time-averaged vibration tensor. We then examine the anisotropic vibrations of Si and O, where the ellipsoid orientations are correlated with the network. The ellipsoids exhibit marked vibrational heterogeneity. The configuration changes occur as breakage and reorganization of the network, where only one or two particles undergo large jumps at each rearrangement leading to diffusion. To the time-correlation functions, however, the particles surrounding these largely displaced ones yield significantly T-dependent contributions, resulting in a weak violation of the Stokes-Einstein relation. This crossover is mild in silica due to the small Si-O bond numbers per particle, while it is strong in fragile glassformers with large coordination numbers. On long timescales, jump events tend to occur in the same regions forming marked dynamic heterogeneity. We also calculate the diffusion constants and the viscosity. The diffusion obeys activation dynamics and may be studied by short-time analysis of irreversible jumps.

Temperature-dependent defect dynamics in the network glass SiO2

We investigate the long time dynamics of a strong glass former, SiO(2), below the glass transition temperature by averaging single-particle trajectories over time windows which comprise roughly 100 particle oscillations. The structure on this coarse-grained time scale is very well defined in terms of coordination numbers, allowing us to identify ill-coordinated atoms, which are called defects in the following. The most numerous defects are O-O neighbors, whose lifetimes are comparable to the equilibration time at low temperature. On the other hand, SiO and OSi defects are very rare and short lived. The lifetime of defects is found to be strongly temperature dependent, consistent with activated processes. Single-particle jumps give rise to local structural rearrangements. We show that in SiO(2) these structural rearrangements are coupled to the creation or annihilation of defects, giving rise to very strong correlations of jumping atoms and defects.

Revisiting the slow dynamics of a silica melt using Monte Carlo simulations

We implement a standard Monte Carlo algorithm to study the slow, equilibrium dynamics of a silica melt in a wide temperature regime, from 6100K down to 2750K . We find that the average dynamical behavior of the system is in quantitative agreement with results obtained from molecular dynamics simulations, at least in the long-time regime corresponding to the alpha -relaxation. By contrast, the strong thermal vibrations related to the boson peak present at short times in molecular dynamics are efficiently suppressed by the Monte Carlo algorithm. This allows us to reconsider silica dynamics in the context of mode-coupling theory, because several shortcomings of the theory were previously attributed to thermal vibrations. A mode-coupling theory analysis of our data is qualitatively correct, but quantitative tests of the theory fail, raising doubts about the very existence of an avoided singularity in this system. We discuss the emergence of dynamic heterogeneity and report detailed measurements of a decoupling between translational diffusion and structural relaxation, and of a growing four-point dynamic susceptibility. Dynamic heterogeneity appears to be less pronounced than in more fragile glass-forming models, but not of a qualitatively different nature.

Temperature dependence of spatially heterogeneous dynamics in a model of viscous silica

Molecular dynamics simulations are performed to study spatially heterogeneous dynamics in a model of viscous silica above and below the critical temperature of the mode coupling theory, T(MCT) . Specifically, we follow the evolution of the dynamic heterogeneity as the temperature dependence of the transport coefficients shows a crossover from non-Arrhenius to Arrhenius behavior when the melt is cooled. It is demonstrated that, on intermediate time scales, a small fraction of oxygen and silicon atoms are more mobile than expected from a Gaussian approximation. These highly mobile particles form transient clusters larger than that resulting from random statistics, indicating that the dynamics are spatially heterogeneous. An analysis of the clusters reveals that the mean cluster size is maximum at times intermediate between ballistic and diffusive motion, and the maximum size increases with decreasing temperature. In particular, the growth of the clusters continues into the crossover to Arrhenius behavior for the transport coefficients. These findings imply that the structural relaxation in silica cannot be understood as a statistical bond breaking process. Although the mean cluster sizes for silica are at the lower end of the spectrum of values reported in the literature for other model liquids, we find that spatially heterogeneous dynamics in models of strong and fragile glass formers are similar on a qualitative level. However, unlike the results for fragile liquids, we show that correlated particle motion along quasi-one-dimensional, stringlike paths is of little importance for the structural relaxation in this model of silica, suggesting that stringlike motion is suppressed by the presence of a network structure. To study transient clusters of localized particles, we calculate a generalized susceptibility corresponding to the self-part of a four-point time dependent density correlation function. We find that this generalized susceptibility is maximum on the time scale of the structural relaxation, where a strong increase of the peak height indicates a growing length of spatial correlations between localized particles upon cooling. Characterizing the local structural environments of the most mobile and the most immobile particles, respectively, we show that high particle mobility is facilitated by, but not limited to, the vicinity of defects in the network structure.

Spatially heterogeneous dynamics and dynamic facilitation in a model of viscous silica

We perform molecular dynamics simulations to study the structural relaxation dynamics of a model of viscous silica, the prototype of a strong glass former. We find that the melt dynamics are spatially heterogeneous regardless of whether the bulk relaxation is non-Arrhenius or Arrhenius, and cannot be understood as a statistical bondbreaking process. Further, we show that stringlike motion is suppressed by the covalent bondings, yet high particle mobility propagates continuously, supporting the concept of dynamic facilitation emphasized in recent theoretical work.

Structure and diffusion in amorphous aluminum silicate: A molecular dynamics computer simulation

The amorphous aluminum silicate (Al2O3)2(SiO2) [AS2] is investigated by means of large scale molecular dynamics computer simulations. We consider fully equilibrated melts in the temperature range 6100 K> or =T> or =2300 K as well as glass configurations that were obtained from cooling runs from T=2300 to 300 K with a cooling rate of about 10(12) K/s. Already at temperatures as high as 4000 K, most of the Al and Si atoms are fourfold coordinated by oxygen atoms. Thus, the structure of AS2 is that of a disordered tetrahedral network. The packing of AlO4 tetrahedra is very different from that of SiO4 tetrahedra in that Al is involved with a relatively high probability in small-membered rings and in triclusters in which an O atom is surrounded by four cations. We find as typical configurations two-membered rings with two Al atoms in which the shared O atoms form a tricluster. On larger length scales, the system shows a microphase separation in which the Al-rich network structure percolates through the SiO2 network. The latter structure gives rise to a prepeak in the static structure factor at a wave number q=0.5 A(-1). A comparison of experimental x-ray data with the results from the simulation shows good agreement for the structure function. The diffusion dynamics in AS2 is found to be much faster than in SiO2. We show that the self-diffusion constants for O and Al are very similar and that they are by a factor of 2-3 larger than the one for Si.

Pressure-induced structural changes in liquid SiO2 from Ab initio simulations

First-principles molecular dynamics simulations at constant pressure have been used to investigate the mechanisms of compression of liquid SiO2. Liquid SiO2 is found to become denser than quartz at a pressure of about 6 GPa, in agreement with extrapolations of lower pressure experimental data. The high compressibility of the liquid is traced to medium-range changes in the topology of the atomic network. These changes consist in an increase of network connectivity caused by the pressure-induced appearance of coordination defects.

Fragile-to-strong transition and polyamorphism in the energy landscape of liquid silica

Liquid silica is the archetypal glass former, and compounds based on silica are ubiquitous as natural and man-made amorphous materials. Liquid silica is also the extreme case of a 'strong' liquid, in that the variation of viscosity with temperature closely follows the Arrhenius law as the liquid is cooled toward its glass transition temperature. In contrast, most liquids are to some degree 'fragile', showing significantly faster increases in their viscosity as the glass transition temperature is approached. Recent studies have demonstrated the controlling influence of the potential energy hypersurface (or 'energy landscape') of the liquid on the transport properties near the glass transition. But the origin of strong liquid behaviour in terms of the energy landscape has not yet been resolved. Here we study the static and dynamic properties of liquid silica over a wide range of temperature and density using computer simulations. The results reveal a change in the energy landscape with decreasing temperature, which underlies a transition from a fragile liquid at high temperature to a strong liquid at low temperature. We also show that a specific heat anomaly is associated with this fragile-to-strong transition, and suggest that this anomaly is related to the polyamorphic behaviour of amorphous solid silica.