Translational and reorientational heterogeneity in the glass-forming liquid Ca0.4K0.6(NO3)1.4

Abstract

Molecular dynamics (MD) simulations were performed in order to reveal dynamical heterogeneity in the glass-forming liquid Ca_0.4K_0.6(NO₃)_1.4 (CKN). A polarizable model for the nitrate anion has been used in a 10.0 ns simulation of 996 ions at 450 K, which is close to the previously estimated glass transition temperature for this model (T_g ∼ 380 K). Mobile and immobile ions within 500.0 ps time windows were identified, and short-range spatial correlations between them were analysed. Mobile ions are strongly correlated, but they are much less correlated with immobile ones. In the case of the NO₃⁻ species, the sub-ensemble of translationally mobile ions also results in a rapidly decaying reorientational correlation function. Thus, if one probes spatial heterogeneity of rotational degrees of freedom in CKN, one will be also probing spatial heterogeneity of translational degrees of freedom, and vice-versa. By using a reorientation analogous of the van Hove correlation function, small angle rotational diffusion in the dynamics of mobile anions, and large angular jumps in the dynamics of immobile ones were observed.