High-Temperature Magic-Angle Spin Nuclear Magnetic Resonance Reveals Sodium Ion-Doped Crystal-Phase Formation in FLiNaK Eutectic Salt Solidification

The new salt heating magic-angle spin (MAS) system for high-temperature nuclear magnetic resonance (HT-NMR) is achieved by a laser heating beam coupled with optical fiber. The formation of a sodium ion-doped crystal KF phase is revealed in FLiNaK ternary eutectic salt (46.5%LiF–11.5%NaF–42.0%KF), which is considered unable to exist at room temperature according to its phase diagram. Density functional theory (DFT) simulation proved that Na⁺ ion-doped crystal is clearly ascertained to develop with the solidification of the melt. Temperature dependence, relaxation, and variable contact time cross-polarization NMR studies provide insights into the unique dynamics and the spatial scale of the doped Na⁺ ions in KF crystals, which exhibit faster ionic vibration due to the weakening of coordinated interactions from the ambient F^– ions.