ABSTRACT
Since the work of Edwards et al. a vast array of molten salts has been studied, including mixtures and eutectics [2-5]. Beyond the binary ionic melt there are two important factors affecting the local order of the system, namely, the covalency of the interaction between unlike ions and the stoichiometry
of the ions in the system. We will not discuss the effect of stoichiometry since all the ionic liquids considered later are simple 1:1 salts, but the nature of the interaction between ions is of particular importance. For example, consider the series of silver halides from AgCl to AgI, where both neutron diffraction [6,7] and EXAFS [8-10] techniques have been employed to investigate the changes in liquid structure when progressing from the smaller, less-polarizable chloride anion to the larger, more-polarizable iodide anion. A roughly tetrahedral arrangement of ions is found for all salts near their respective melting points, with CNs calculated to be 3.8, 3.9, and 4.6 for AgCl, AgBr, and AgI, respectively [6]. On further heating the local order is seen to decrease for Cl-and Br-salts resulting in rst shell CNs whereas for AgI little signi cant change in the local structure occurs. This has been attributed to the effect of the increased covalent character associated with the Ag+/I-ion pair, compared with essentially ionic interactions found for the Ag+/Cl-and Ag+/Br-. Kawakita et al. also noted that the molten structures of AgCl and AgBr are different from the mineral structure with respect to the cation-anion distance [7]. In contrast, for AgI the cation-anion
distances between the solid and the melt phases are almost identical, again indicating that the cause is increased covalency between the Ag+/I-ion pair.
