after the mixture was pre-melted. Fig. shows representative XRD patterns at 30 C, 443 C, and 519 C with identified phases overlaid for reference. At 30 C the XRD pattern shows all anticipated phases; LiF, NaF, and KF with the addition of peaks corresponding to CsLiF 2 . X-ray di↵raction determined that CsLiF 2 is monoclinic with the space group show that there is indeed a phase separation at temperatures where the mixture is solid. All of Cs species form CsLiF 2 with Li cations in equilibrium up to melting temperature. CsLiF 2 forms as a separate phase, which melts at 371 C, completely leaving a mixture of solid and liquid. The remaining solid phases diminish with increasing temperature forming a complete liquid at 459 C, very close to reported melting temperature of FLiNaK (454 C). Note that due to insu cient experimental data, Neumann-Kopp rule was applied to estimate the Gibbs free energy of pure LiF•CsF compound previously in the CALPHAD model. That same model used in this work results in a discrepancy in the melting temperature of LiCsF 2 between CALPHAD and our measurements. Therefore, CALPHAD model needs to be optimized to be consistent with experimental data.