How distributed charge reduces the melting points of model ionic salts

Abstract

Using coarse grain model ions and NPT molecular dynamics simulations, we determine melting point trends across a set of salts beginning with a system closely akin to the restricted primitive model. 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. Upon cooling from the liquid phase, the model salts considered either crystallize as orientationally-disordered CsCl solids, or become trapped in glassy states, depending on the amount and extent of the cation charge redistribution. For the latter case, we find an orientationally-ordered crystal structure with space group 111 (P(42m)) underlying the glassy states. The structural and dynamical properties of both the solid and liquid phases of the model salts are discussed in some detail.