Nuclear Magnetic Resonance Studies of Ionic Coordination and Interactions in NaF‐LaF 3 and LiF‐LaF 3 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 La 3+ ions, which governs these properties, remains inadequately characterized. In this work, we systematically examine NaF–LaF 3 and LiF–LaF 3 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 LaF 3 to form a crystalline NaLaF 4 phase, as unambiguously confirmed by 19 F and 23 Na NMR, along with 2D 19 F– 23 Na 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/ LaF 3 composite. This divergence is attributed to the stronger Li–F bonding and the limited coordination flexibility of Li + , which hinders disruption of the LaF 3 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.