Unlocking High Fluoride‐Ion Conductivity by Regulating the Order–Disorder Transition in Bisubstituted PbSnF ₄

Layered fluorite‐type tetragonal β ‐PbSnF 4 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 ‐PbSnF 4 , and guidelines for enhancing ionic conductivity via elemental substitution remain unclear. In this study, we report the synthesis of single‐phase tetragonal PbSn 1− x Bi x F 4+ 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 ‐PbSn 0.96 Bi 0.04 F 4.04 achieves a fluoride‐ion conductivity of 6.3 mS cm −1 at 298 K. Bi 3+ 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 PbSnF 4 is driven by ordering of the fluoride‐ion sublattice coupled with tilting of SnF 5 polyhedra. Bi 3+ 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.