Probing the ionic structure of FLiNaK-ZrF4 salt mixtures by solid-state NMR.

ABSTRACT

In this study, by applying 19F, 23Na and 7Li high-resolution NMR methods, the evolution of the [Zr x F y ]4x-y local ionic structures in FLiNaK-ZrF4 salt mixtures were elucidated. K3ZrF7, Na3ZrF7 and Na7Zr6F31 crystal phases were identified when the melt salts were being solidified. The distribution of these [Zr x F y ]4x-y species was dependent on the content of ZrF4 in FLiNaK eutectic salts. Moreover, K3ZrF7 phase transition from an orthorhombic lattice into a disordered cubic lattice was clarified, thereby causing dynamics of the coordinated F- ions to be reduced and the well-ordered crystal lattices to be destroyed. These mentioned results provide a further insight into the Zr-F based ionic structure and the formation of the disordered Zr-F structure in ZrF4-based eutectic salts.