Interestingly, both models predict a peak at ⇠ 1 Å 1 in the partial structure factor of Licontaining pairs. Such a peak signifies medium-range order correlations in the Li structure that could a↵ect Li mobility. This correlation could be interpreted as Li-ion clustering or incomplete mixing, or even chains of Li-F tetrahedra. In this respect, the e↵ects of Cs are especially interesting because Cs ions are larger, sometimes much larger, than other low-Z alkali ions in FLiNaK and other nuclear fluoride or chloride salts. As such, even small amounts of Cs could a↵ect the liquid structure. Experimentally, NMR measurements of CsF-LiF mixtures seem to confirm the expected enhancement of Li clustering due to the size mismatch of Li and Cs ions. Previous neutron di↵raction measurements in FLiNaK did not have the resolution to reach 1 Å 1 , 13 while our measurements did. Thus, we probed the medium-range order and the e↵ect of Cs on the structure of FLiNaK directly but did not find an enhancement of Li clustering.