It seems that Cs ions do not perturb the established structure between Li and F ions. In fact, the Cs partial structure factor is similar to that of K in pure FLiNaK. Usually, anion-anion peaks correspond to anti-peaks in anion-cation partial S(Q)'s due to their opposite electric charges, sometimes called charge alternation. 14 But Cs-F (and K-F) correlations are di↵erent in that partial structure factors of Cs-F, K-F and F-F have peaks at the same distance of 2.5 Å 1 . We believe that such behavior is a result of large relative sizes of Cs and K compared to other species, especially Li. Table shows the ionic radii of the components of the salt to illustrate this point. Also, we do not see that addition of Cs changes the pre-peak structure at around 1 Å 1 . The only significant change is the height of both the shoulder at 2 Å 1 and the main peak at about 2.5 Å 1 . The shoulder at 2 Å 1 is from the peaks in K-K and Cs-Cs partial structure functions. As for the Li-F sub-system, the presence of Cs ions does not appear to a↵ect it structurally at least at the level of 5 mol% of CsF (consistent with the presence of a large number of K ions with the ionic size similar to that of Cs). We infer that Li mobility in FLiNaK is also una↵ected by Cs. We also infer that the addition of Cs impurities will have a much stronger e↵ect on the structure and dynamics of FLiBe (LiF-BeF2), which contains only small cations (Li and Be).