Molecular Dynamics Simulations Of Phase Transition And Nucleation Of KBr Clusters

Since the phase transition and nucleation are microscopic processes, the experimental study of these processes is difficult. Despite the obvious importance of phase transition and nucleation in science and technology, there is not much knowledge about the details of atomic or molecular processes in such transformations. On the other hand, computer simulation provides a direct route from the microscopic details of a system to macroscopic properties of experimental interest.Molecular dynamics simulations are performed on (KBr)₁₀₈, (KBr)₂₅₆, (KBr)₅₀₀, and (KBr)₈₆₄ clusters to analyze structural, energetic, and kinetic aspects of phase transition. Born-Mayer-Huggins potential function was employed to reproduce melting and freezing when clusters are heated and cooled. Various diagnostic methods, including the total energy curves, the Lindemann indices, and pair-correlation functions, are used to characterize phase changes during heating and cooling stages. Results demonstrate that the melted clusters are not spherical in shape. The melting temperatures and heats of fusion decrease approximately linearly with the reciprocals of cluster radii. The melting point and heat of fusion for bulk KBr obtained by extrapolation are close to the experimental values.On the other hand, the homogeneous nucleation in the freezing of molten clusters comprised of 256, 500 and 864 K⁺Br^- ion pairs have been investigated in computer simulation. The nucleation rates tend to decrease with increasing cluster size andtemperature. The solid-liquid interfacial free energy σ_sl of 42.4-52.3mJ/m² is close tothe values predicted by Turnbull's relation and comparable to the experimental observation by Buckle and Ubbelohde. Sizes of critical nuclei inferred from classical nucleation theory are of 6.5-20.7 K⁺Br^- ionic pairs in the temperature range of 400-600K. The critical nucleus size at bulk freezing temperature obtained by extrapolation is about 45 K⁺Br^- ionic pairs, which is comparable to the experimental value of NaCl.