The local coordination environment of lanthanide ions in molten fluoride salts governs their physicochemical behavior in advanced nuclear technologies, but the influence of outer-sphere cations remains poorly understood. Here, the local structure of Nd3+ and Ho3+ in AF melts (A = Li, Na, K, Cs) and CsF–NaF/NaF–CaF2 mixtures is investigated by in situ UV–Vis absorption spectroscopy of hypersensitive f–f transitions combined with ab initio molecular dynamics (AIMD). The splitting energy and number of resolved components of hypersensitive bands depend strongly on the outer-sphere cation, whereas the barycenter energy remains constant. Along the LiF → NaF → KF → CsF series, spectral parameters reveal progressive reduction of coordination distortion and a shift toward more symmetric environments. AIMD simulations for Nd3+ show the average coordination number decreases from ∼7–8 in LiF to predominantly 6 in CsF. These changes arise from coupled variations in the first and outer coordination spheres, with relative contributions governed by the ionic potential of the surrounding cations. The similar spectral trends observed for Nd3+ and Ho3+ indicate a shared response of these two ions to changes in the outer-sphere cation.
