Origin of dynamical heterogeneities in borosilicate glass-forming systems

Dynamical heterogeneities give rise to spatiotemporal fluctuations yielding a broad distribution of relaxation rates in glass-forming systems. In alkali-(alumino)borosilicate glass, a highly complex glass system with a wide variety of structural components involving mixed network former and mixed network modifier effects, the factors governing dynamical heterogeneities in short-range order remain unknown. This paper reports an investigation of the dominant factors governing dynamical heterogeneities in borosilicate glass. The dynamical heterogeneities of two alkali-(alumino)borosilicate glasses are investigated using the isoconfigurational ensemble method via molecular dynamics simulations. We find that alkali ions drive high dynamical propensities in local spatial regions. The average number of bridging oxygens for both boron and silicon does not monotonically correlate with the corresponding calibrated dynamical propensity. The local alkali agglomeration affects the dynamical propensity of the alkali ion. The agglomeration of lighter-weighted alkali ions surrounding an alkali ion favors its high dynamical propensity, while the agglomeration of equal-weighted or heavier-weighted alkali ions favors its low dynamical propensity. The separation distance between the alkali ion and the alkali-modified boron or silicon atom affects the dynamical propensity of the alkali ion. Mixed-alkali environments perturb the relationship between the calibrated dynamical propensity of an alkali ion and its separation distance from its modified boron or its modified silicon atom.