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

• Published in: Chemistry of materials Vol. 36; no. 9; pp. 4495 - 4501
• Main Authors: Zhong, Chengchao, Tassel, Cédric, Kato, Daichi, Ogawa, Kanta, Niwa, Ken, Tomita, Osamu, Ito, Yukino, Kawaguchi, Shogo, Hasegawa, Masashi, Abe, Ryu, Kageyama, Hiroshi
• Format: Journal Article
• Language: English
• Published: American Chemical Society 14.05.2024
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/acs.chemmater.4c00114

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.