Self-Diffusivity Measurement of Eutectic F⁷LiNaK with and without Additives Using Quasi-Elastic Neutron Scattering

The atomic scale relaxation dynamics of eutectic F⁷LiNaK (46.5 LiF-11.5 NaF-42 KF mol %, Li-7 enriched) were measured using quasi-elastic neutron scattering (QENS) over a temperature range of 500-750 °C. The effect of adding 0.988 mol % cerium, 0.499 mol % cesium, and 1.21 mol % zirconium individually to the dynamics of F⁷LiNaK was also investigated. The relaxation process in both pure and doped F⁷LiNaK molten salts was fit with a stretched exponential function and the temperature dependence follows an Arrhenius behavior over a wavevector transfer range of 0.4 Å^-1 < Q < 0.9 Å^-1. The measured activation energy for self-diffusion is E_a = 0.77 ± 0.02 eV/atom for pure molten F⁷LiNaK. The QENS response with additives added to F⁷LiNaK was also fit with a stretched exponential and the associated Arrhenius behavior was characterized with activation energies of E_a = 0.88 ± 0.01 eV/atom for zirconium (1.21 mol %), E_a = 1.02 ± 0.02 eV/atom for cerium (0.988 mol %), and E_a = 0.71 ± 0.03 eV/atom for cesium (0.499 mol %). The measured diffusivities are compared to those simulated with a neural network force field model by Lee et al. [ Lee, S.-C. Comparative Studies of the Structural and Transport Properties of Molten Salt FLiNaK Using the Machine-Learned Neural Network and Reparametrized Classical Forcefields.