Nuclear Magnetic Resonance Studies of Ionic Coordination and Interactions in NaF-LaF₃ and LiF-LaF₃ Mixed Salts

Heavy-metal fluoride (HMF) glasses exhibit a combination of broad infrared transparency, low phonon energy, and potential fluoride-ion conductivity, rendering them promising candidates for optical and electrochemical applications. However, the atomic-scale environment of La³⁺ ions, which governs these properties, remains inadequately characterized. In this work, we systematically examine NaF-LaF₃ and LiF-LaF₃ binary mixtures as model systems using a suite of solid-state nuclear magnetic resonance (NMR) techniques. Our findings reveal markedly distinct behaviors of the alkali cations. NaF readily reacts with LaF₃ to form a crystalline NaLaF₄ phase, as unambiguously confirmed by ¹⁹F and ²³Na NMR, along with 2D ¹⁹F-²³Na HETCOR and CP/MAS experiments. In contrast, LiF exhibits no evidence of forming Li-La-F coordination structures, instead persisting as a phase-separated LiF/ LaF₃ composite. This divergence is attributed to the stronger Li-F bonding and the limited coordination flexibility of Li⁺, which hinders disruption of the LaF₃ lattice. These mechanistic insights highlight the critical influence of alkali cation identity on the structural evolution in mixed fluoride systems and offer valuable design principles for ZBLAN and related HMF glasses.