The Role of Lone Pairs in Fast Oxide Ion Conduction in Rhombohedral Bi_0.8Pr_0.2O_1.5

The oxide ion conductor Bi0.8Pr0.2O1.5 exhibits exceptional ionic conductivity, of 0.8 S cm–1 at 800 °C, driven by the material’s layered rhombohedral framework and a unique structural interplay between stereochemically active bismuth lone pairs (LPs) and mobile oxide ions. In this work, we combine reverse Monte Carlo analysis of total neutron scattering data with ab initio molecular dynamics methodology to unravel the role of Bi3+ 6s2 lone pairs in driving ion transport across the β2 ↔ β1 phase transition. We show that this transition is characterized by a redistribution of oxide ions into the van der Waals gap, accompanied by a reorientation of BiO4 moieties at the fluorite-block edges, facilitating enhanced oxide ion mobility via additional conduction pathways. This work establishes a mechanistic link between lone pair stereochemistry and high-temperature ion transport, offering new design principles for advanced solid electrolytes based on heavy p-block elements.