Probing the Structure and Dynamics of Na⁺ Ionic Doped LiF Crystals in Fluoride Eutectic Salt by Solid-State NMR

In this study, it was demonstrated that Na+ ionic doped crystals could be formed during the solidification of FLiNaK melts. The ionic adulterating behavior between Li+, Na+, and K+ ions was investigated in detail by using various 1D/2D solid-state NMR methods and theoretical calculations. 2D 19F–23Na and 19F–7Li HETCOR NMR showed that the Na+ ions could be doped into LiF and KF crystals to generate new adulterating phases. Furthermore, the doped ionic Na+ ions exhibited different dynamics in LiF and KF crystals, which is correlated to the bonding length of the lattice and the Na–F coordinated interactions. This study provides more insights into the ionic doped crystal phase formation in such simple alkali metal fluorides, which may be of greater significance in the application as optical devices.