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_xF_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_xF_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.