Elucidating Ionic Correlations Beyond Simple Charge Alternation in Molten MgCl₂-KCl Mixtures

The development of technologies for nuclear reactors based on molten salts has seen a big resurgence. The success of thermodynamic models for these hinges in part on our ability to predict at the atomistic level the behavior of pure salts and their mixtures under a range of conditions. In this letter, we present high-energy X-ray scattering experiments and molecular dynamics simulations that describe the molten structure of mixtures of MgCl₂ and KCl. As one would expect, KCl is a prototypical salt in which structure is governed by simple charge alternation. In contrast, MgCl₂ and its mixtures with KCl display more complex correlations including intermediate-range order and the formation of Cl^--decorated Mg²⁺ chains. A thorough computational analysis suggests that intermediate-range order beyond charge alternation may be traced to correlations between these chains. An analysis of the coordination structure for Mg²⁺ ions paints a more complex picture than previously understood, with multiple accessible states of distinct geometries.