Decompression-Induced In-Plane Lone Pair Electrons in BiSF Synthesized under High Pressure

Stereochemically active lone pair electrons (LPEs) associated with p-block cations, such as Pb2+ and Bi3+, possess the capability of causing a reduction in the symmetry of crystal structures. However, LPEs leading to steric hindrance may impede structural stabilization; compounds with Bi3+, akin to those containing La3+ with comparable ionic radii, are not always encountered. In this study, we synthesized BiSF using high-pressure (5 GPa) synthesis. BiSF crystallizes in a PbFCl-type structure, exhibiting orthorhombic distortion due to stereochemically active LPEs oriented along the b axis. This contrasts sharply with other PbFCl-type tetragonal compounds (e.g., BiOF), which have van der Waals gaps formed by interlayer LPEs. First-principles calculations and in situ XRD indicate that BiSF is of ideal tetragonal symmetry under high pressure, featuring inactive LPEs. Upon depressurization, a notable expansion of the soft S2- anions occurs, enhancing hybridization between the Bi-6s/6p and S-3s/3p orbitals. This activation of the stereochemistry of LPEs results in a symmetry reduction. BiSF exhibits anisotropic thermal expansion, which is attributed to the contraction of the "LPE-mediated" Bi-F bonds. Our findings illustrate that the direction and shape of LPEs can be tailored during the postsynthesis decompression, underlining high-pressure synthesis as an effective approach for obtaining compounds with stereochemically active LPEs at ambient pressure.