Effect of Pb 6s2 lone pair on the potential flattening of fluoride-ion conduction in perovskite-type fluoride

Abstract

Materials containing ns² lone pairs exhibit superior fluoride-ion conductivity, acting as promising candidates for solid electrolytes in all-solid-state fluoride-ion batteries. However, the effect of lone pairs on fluoride-ion conduction remains unclear, especially for 6s² in Pb²⁺. This study investigated the relationships between the ionic conductivity, crystal structure, and electronic structure of CsPb_0.9K_0.1F_2.9. Cubic CsPb_0.9K_0.1F_2.9 exhibited a low activation energy of 7.9 kJ mol⁻¹, resulting in high conductivity at 223 K (1.0 × 10⁻³ S cm⁻¹, bulk conductivity). ¹⁹F nuclear magnetic resonance spectroscopy confirmed facile local migration of the fluoride ions with a low activation barrier of 3.8 kJ mol⁻¹. Theoretical calculations revealed that the fluoride ions migrated with a low migration energy via an exchange reaction between the Pb 6s lone pairs and fluoride ions. The localised lone pair in the PbF₅ polyhedron stabilised the saddle-point structure and mitigated the migration barrier. These findings are beneficial for material design, providing superionic conductivity with a low migration barrier for fluoride ions as well as other anions, such as oxide and chloride.