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

Area of Science:

• Materials Science
• Solid-State Chemistry
• Nuclear Engineering

Background:

• Molten salts are crucial in various industrial applications, including nuclear reactors and energy storage.
• Understanding the local ionic structure and phase behavior of fluoride salts is essential for optimizing their performance and safety.

Purpose of the Study:

• To elucidate the evolution of local ionic structures, specifically [ZrF4]4-, in FLiNaK-ZrF4 salt mixtures.
• To identify crystalline phases formed during the solidification of these salt mixtures.
• To investigate the phase transition of K3ZrF7 and its impact on ionic dynamics and crystal structure.

Main Methods:

• High-resolution Nuclear Magnetic Resonance (NMR) spectroscopy using 19F, 23Na, and 7Li isotopes.
• Analysis of spectral data to determine local coordination environments and phase identification.
• Crystallographic analysis to understand lattice structures and phase transitions.

Main Results:

• Identified K3ZrF7, Na3ZrF7, and Na7Zr6F31 crystal phases upon solidification of FLiNaK-ZrF4 melts.
• Demonstrated that the distribution of [ZrF4]4- species depends on the ZrF4 content in FLiNaK eutectic salts.
• Clarified the phase transition of K3ZrF7 from an orthorhombic to a disordered cubic lattice, leading to reduced F- ion dynamics and destruction of ordered lattices.

Conclusions:

• The study provides detailed insights into the Zr-F based ionic structure within FLiNaK-ZrF4 mixtures.
• The formation of disordered Zr-F structures in ZrF4-based eutectic salts has been further clarified.
• NMR spectroscopy is a powerful tool for characterizing local ionic structures and phase transitions in molten salt systems.