Evidence for Anomalous Dynamic Heterogeneities in Isostatic Supercooled Liquids

Ionic Conductivity in Disordered Media: Molecular Flexibility as a New Paradigm to Enhance Ion Motion in Glassy Electrolytes

We investigate the role of molecular flexibility on the electrical transport properties of model electrolytes containing ions and an underlying disordered network structure with changing connectedness. Rather than focusing on the effect of ion content in a stoichiometric network former AY_{2} (e.g., SiS_{2}), we explore the possibility of increasing the Y∶A ratio (flexibility index m) in order to reduce connectivity and to promote the occurrence of flexible modes and topological degrees of freedom in the network structure. At fixed ion content and below a certain threshold modifier composition x_{c}, topological constraint counting indicates that a mean-field stress-to-flexible transition is expected for a flexibility index m_{c}, and an ion hopping model predicts a substantial increase of conductivity once m>m_{c}. Molecular dynamics simulations on a typical amorphous electrolyte, xNa_{2}S-(1-x)SiS_{m}, independently and quantitatively confirm the prediction as anomalous changes with m are obtained, and these manifest by waterlike diffusivity anomalies, and a substantial increase of ionic conductivity upon moderate change of m. The analysis disentangles contributions from mobility and the free carrier rate in the electrical transport, and finally suggests that molecular flexibility can serve as an efficient way for conductivity enhancement in all solid-state batteries using amorphous electrolytes.

Ionic self-diffusion and the glass transition anomaly in aluminosilicates

The glass transition temperature (Tg) is the temperature after which a supercooled liquid undergoes a dynamical arrest. Usually, glass network modifiers (e.g., Na2O) affect the behavior of Tg. However, in aluminosilicate glasses, the effect of different modifiers on Tg is still unclear and shows an anomalous behavior. Here, based on molecular dynamics simulations, we show that Tg decreases with increasing charge balancing cation field strength (FS) in the aluminosilicate glasses, which is an anomalous behavior as compared to other oxide glasses. The results show that the origins of this anomaly come from the dynamics of the supercooled liquid above Tg, which in turn is correlated to pair excess entropy. Our results deepen our understanding of the effect of different modifiers on the properties of the aluminosilicate glasses.

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 (Na₂O)_x(SiO₂)_100-x, 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% Na₂O, which echoes with the experimental observation. The key result is the identification of the physical features driving this anomalous behavior. The intermediate-flexible boundary is associated with a dynamic onset with increasing depolymerization that enhances the growing atomic motion with a reduced internal stress, whereas the intermediate-stressed rigid boundary exhibits a substantial increase in the temperature at which the relaxation is maximum. These results suggest an essentially dynamic origin for the intermediate phase observed in network glass-forming liquids.

Relaxation oscillation of borosilicate glasses in supercooled liquid region

Most supercooled non-polymeric glass-forming melts exhibit a shear thinning phenomenon, i.e., viscosity decreases with increasing the strain rate. On compressing borosilicate glasses at high temperature, however, we discovered an interesting oscillatory viscous flow and identified it as a typical relaxation oscillation caused by the peculiar structure of borosilicate glass. Specifically, the micro-structure of borosilicate glass can be divided into borate network and silicate network. Under loading, deformation is mainly localized in the borate network via a transformation from the three coordinated planar boron to trigonal boron that could serve as a precursor for the subsequent formation of a BO₄ tetrahedron, while the surrounding silicate network is acting as a stabilization/relaxation agent. The formation of stress oscillation was further described and explained by a new physics-based constitutive model.