Unlocking High Fluoride-Ion Conductivity by Regulating the Order–Disorder Transition in Bi-substituted PbSnF₄

Layered fluorite-type tetragonal β-PbSnF4 exhibits the highest fluoride-ion conductivity among solid electrolytes; however, its room-temperature performance is limited by a phase transition to the low-conductivity monoclinic α-PbSnF4, and guidelines for enhancing ionic conductivity via elemental substitution remain unclear. In this study, we report the synthesis of single-phase tetragonal PbSn1−xBixF4+x (x = 0–0.12) solid solutions through a two-step nonheating process: cooled ball milling to obtain a disordered fluorite-type cubic γ phase, followed by a water-vapor-induced transition to the tetragonal β phase. Density functional theory calculations show that polarized Sn lone pairs on the tetragonal surface interact favorably with water molecules, providing a thermodynamic driving force for the γ → β phase transition under humid conditions. β-PbSn0.96Bi0.04F4.04 achieves a fluoride-ion conductivity of 6.3 mS cm−1 at 298 K. Bi3+ substitution introduces interstitial fluoride ions into the SnSn interlayer, enabling local 3D conduction pathways. Molecular dynamics simulations show that the β → α transition in PbSnF4 is driven by ordering of the fluoride-ion sublattice coupled with tilting of SnF5 polyhedra. Bi3+ substitution disrupts this tilting, retaining a disordered fluoride-ion sublattice with high conductivity at room-temperature. This study provides design guidelines for enhancing the room-temperature ionic conductivity of fluoride-ion conductors.