Atomistic mechanisms of phase separation and formation of solid solutions: model studies of NaCl, NaCl-NaF, and Na(Cl1-xBrx) crystallization from the melt.

The crystallization of sodium chloride from its melt and mixtures with other sodium halides is investigated by means of transition path sampling molecular dynamics simulations. From this we explore the nucleation mechanisms of both the solidification and the melting process at the atomistic level of detail. By incorporation of impurities the nucleation picture of the eutectic mixtures changes considerably. Doping the NaCl crystal with fluoride ions, we observed the substitutional defects to act as favored nucleation centers for the melting transition. This phenomenon plays a critical role during the solidification process of NaCl-NaF melts of low NaF concentration and is demonstrated to account for the segregation of fluoride ions. While NaCl-NaF corresponds to a eutectic system, we also investigated NaCl-NaBr mixtures. The bromide ions were observed to behave very similarly to chloride ions. As a consequence, no phase separation occurs and Na(Cl1-xBrx) solid solutions are formed. At the example of these two prototypes we demonstrate the study of the atomistic mechanisms related to phase separation processes and solid solution formation during the nucleation and growth of crystals from multinary melts.