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.
Sun et al. (Thu,) studied this question.