Pressure-Regulated Dynamic Stereochemical Role of Lone-Pair Electrons in Layered BiOS.

Lone-pair electrons (LPEs) s in subvalent 14 and 15 groups lead to highly anharmonic lattice and strong distortion polarization, which are responsible for the groups' outstanding thermoelectric and optoelectronic properties. However, their dynamic stereochemical role in structural and physical properties is still unclear. Here, by introducing pressure to tune the behavior of LPEs, we systematically investigate the lone-pair stereochemical role in a BiOS. The gradually suppressed LPEs during compression show a nonlinear repulsive electrostatic force, resulting in two anisotropic structural transitions. An orthorhombic-to-tetragonal phase transition happens at 6.4 GPa, caused by the dynamic cation centering. This structural transformation effectively modulates the optoelectronic properties. Further compression beyond 13.2 GPa induces a 2D-to-3D structural transition due to the disappearance of the Bi-6s LPEs. Therefore, the pressure-induced LPE reconfiguration dominates these anomalous variations of lattice, electronic, and optical properties. Our findings provide new insights into the materials optimization by regulating the characters of LPEs.