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

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, MSnF₄ (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, Sn₂Pb₅F₁₂I₂ (n = 3 + 4) and Sn₅Pb₅F₁₇I₃ (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 (Pb₂BaF₅I), and quadruple-layer (Pb₄F₇I) fluoroiodides. While conductivity increases monotonically with slab thickness in the simple layered series, the intergrowth phases outperform Pb₄F₇I with quadruple layers despite incorporating less conductive triple layers. Sn₂Pb₅F₁₂I₂ and Sn₅Pb₅F₁₇I₃ 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.