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Molecular dynamics simulations of alkali halide clusters Croteau, Timothʹe

Abstract

An investigation of the melting, freezing and structure of pure LiCl, KC1 and mixed LiCl-KCl clusters is presented. The results of molecular dynamics simulations of unconstrained neutral clusters with 6, 8, 10, 32, 64, 216, 512, and 1000 ions have been carried out using the Born-Mayer-Huggins potential based on a rigid ion (non-polarizable) approximation. Initial molecular dynamics studies of phase transitions in clusters of alkali halides have shown some very interesting characteristics, especially in the case of large size asymmetry where the ionic radii ratio, r+/r_ < 0.5. A comparison of the structures of KC1 and LiCl clusters coupled with the calculation of the mean square displacement and distribution of the ions about the center of mass allows us to discuss the relevance of size asymmetry effects on the melting temperature, and on the stability of different isomeric structures. The main conclusion regarding pure LiCl clusters is that the behaviour of the LiCl pairs is greatly influenced by their strong dipolar character. This is shown by the clear competition between ring-like and cubic structures. For LiCl clusters, the strong dipolar character favours the formation of less ordered expanded ring structures explaining the absence of a sharp melting transition. At the solid-liquid transition, the energies associated with the liquid and solid structures are very similar possibly explaining the early melting of LiCl when compared with the other alkali halide salts. The study of binary mixtures shows that the structures are insensitive t.o LiCl concentration for a broad range of composition. The cubic character of KC1 clusters tends to dominate over the strong dipolar character of LiCl clusters which creates a separation or segregation of the species. Specifically, ion segregation effects give rise to a cubic portion in the structure of clusters with a LiCl mole fraction as high as 0.333, whereas ring geometries only start to appear when the KC1 mole fraction is reduced to 0.093. Moreover, it is only when the mole fraction of either KC1 or LiCl has reached 0.95 that the properties of the pure cluster are observed.

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