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Determination of melting point trends of model salts by molecular dynamics simulations Lindenberg, Erin


We examine the melting point trends across sets of coarse grain model salts using NPT molecular dynamics simulations. The melting point trends are established relative to a charge-centered, size-symmetric salt that is closely akin to the restricted primitive model. Two of the common features of ionic liquids, namely size asymmetry and a distributed cation charge, are systematically varied in a set of model salts. We find that redistributing the cation charge in salts with size-symmetric, monovalent, spherical ions can reduce the melting temperature by up to 50% compared to the charge-centered case. Displacing the charge from the ion center reduces the enthalpy of the liquid more than that of the solid resulting in a lower melting point. We consider two sets of size-asymmetric salts with size ratios up to 3:1 using different length scales; the melting point trends are different in each set, but within each set we find salts that achieve a melting point reduction of over 60% from the charge-centered, size-symmetric case. The lowest melting point range we find is between 450 K and 500 K. We find diversity in the solid phase structures. For all size ratios with small cation charge displacements, the salts crystallize with orientationally disordered cations. For equal-sized ions, once the cation charge is moved far enough off-center, the salts become trapped in glassy states upon cooling and we find an underlying crystal structure (space group 111) that features orientationally ordered ion pairs. The salts with large size ratios and large cation charge displacements achieve the lowest melting points and also show premelting transitions at lower temperatures (two as low as 300 K). We find two types of premelting behaviour; some salts exhibit a fast ion conductor phase, where the smaller anions move through a face-centered cubic (fcc) cation lattice, whereas other salts have a plastic crystal phase composed of ion pairs rotating on an fcc lattice.

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