STRUCTURAL RELAXATION AND DYNAMICAL CORRELATIONS IN A MOLTEN STATE NEAR THE LIQUID GLASS-TRANSITION - A MOLECULAR-DYNAMICS STUDY

Molecular dynamics calculations have been used to study structural relaxation and dynamical correlations near the glass transition in the system [Ca(NO3)2]0.4 [KNO3]0.6. As in a typical molten salt, the overall structure is determined by charge ordering. However, the radial distribution function for Ca2+ ions is unusual in that even at high temperatures it shows a split first peak due to specific spatial correlations of the cations with the nitrate anions. Structural relaxation that accompanies cooling of the system has been characterized with the aid of the van Hove real-space correlation functions Gs(r,t) for the constituent atoms (Ca, K, N, O). The relaxation of the incoherent structure factor Fs (k,t), with a wave vector k near the peak of the static structure factor, has been investigated as a function of temperature. The results clearly reveal both the α and β relaxation processes; the former can be well represented by a master curve with a stretched exponential shape. An analysis of the susceptibility, which agrees qualitatively with neutron spin-echo data, suggests that the glass transition for the model occurs around 400 K. The relatively small discrepancy with the experimental transition temperature derived from neutron scattering data (366 K) is likely related to inadequacies in the model employed for the interionic interactions. The functions C1(t) and C2(t), which describe the reorientational relaxation of the threefold symmetry axes of the nitrate ions, are shown to exhibit a scaling behavior analogous to that of the structure factor. In the region of the glass transition, where translational diffusion has essentially stopped, the nitrate ions continue to flip predominantly about their twofold axes. © 1990 American Institute of Physics.