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

In this study, by applying ¹⁹F, ²³Na and ⁷Li high-resolution NMR methods, the evolution of the [Zr F ]⁴ 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 F ]⁴ 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.