Pressure-Induced Stereochemistry and Lone Pair ( E ) Localization in BiF3: Ellipsoidal EBi 6s2 / EF 2s2 Volumes, Perichoresis, and Phase Transitions

This work investigates the high-pressure behavior of BiF3 polymorphs, with emphasis on the stereochemical activity and spatial localization of the 6s² lone electron pair (E) of Bi³⁺and the 2s² Es of fluoride anions. Using density functional theory (DFT) and electron localization function (ELF) analyses, we characterize the evolution of α-, β-, and γ-BiF3E polymorphs and report two novel high-pressure phases, δ-BiF3E and κ-BiF3E, stabilized at approximately 70 GPa and above 200 GPa, respectively. The β-phase undergoes a clear phase transition near 70 GPa, where structural gliding of fluorine layers induces a symmetry shift from Pnma to Cmcm space group, corresponding to the δ-BiF3E phase. The δ-phase features a base-centered orthorhombic framework with perichoretic localization of the Bi 6s² lone pair in multiple spatial positions. At pressures exceeding 200 GPa, a previously unobserved polymorph, κ-BiF3E, emerges with a penta-capped triangular antiprismatic coordination environment and a distorted screw-axis symmetry. In this paper “perichoresis” is introduced as an original conceptual tool to describe the simultaneous localization of lone pairs in multiple spatial domains without invoking electronic delocalization. The results provide insight into the high-pressure stereochemistry of heavy p-block compounds and offer a predictive model for lone pair behavior under compression