Intergrowth Fluorite Slabs Modulate Interstitial Fluorine to Enhance Fluoride‐Ion Conductivity

ABSTRACT Layered fluorite‐type compounds are largely restricted to double‐layer ( n = 2) slabs, and thicker fluorite blocks remain difficult to stabilize and control. Among the few examples, MSnF 4 (M = Pb, Ba) with a quadruple layer exhibits high fluoride‐ion conductivity, yet its structural complexity obscures the atomistic ion transport mechanism. Here we report two intergrowth fluoroiodides, Sn 2 Pb 5 F 12 I 2 ( n = 3 + 4) and Sn 5 Pb 5 F 17 I 3 ( n = 3 + 4 + 3), in which triple and quadruple slabs are periodically intergrown and separated by iodine layers. Their fluoride‐ion conductivities were evaluated alongside those of double‐layer (PbFI), triple‐layer (Pb 2 BaF 5 I), and quadruple‐layer (Pb 4 F 7 I) fluoroiodides. While conductivity increases monotonically with slab thickness in the simple layered series, the intergrowth phases outperform Pb 4 F 7 I with quadruple layers despite incorporating less conductive triple layers. Sn 2 Pb 5 F 12 I 2 and Sn 5 Pb 5 F 17 I 3 exhibit room‐temperature conductivities of ∼0.8 × 10 −3 and ∼0.2 × 10 −3 S cm– 1 , respectively. This enhancement is associated with modulation of octahedral‐site occupancy enabled by intergrowth stacking, which tunes interstitial‐fluorine concentration without aliovalent cation substitution, thereby avoiding detrimental dopant–carrier interactions. These results establish fluorite‐slab thickness and intergrowth stacking as a unified, doping‐free structural strategy to control anion‐carrier density and ion mobility in layered fluorite materials.