Double lone pair electrons driving polar semiconductors and metals

The lone pair electrons are important to understand the origin of polarization in ferroelectrics such as PbTiO₃ and BiFeO₃. Here we investigate the mechanism of polarization driven by double lone pair electrons in I-IV-V compounds. Our theoretical results indicate that these compounds can crystallize in three polar structures with P6₃mc symmetry. We identify 174 polar semiconductors and 109 potential polar metals that simultaneously exhibit electrical conductivity and spontaneous polarization. Particularly noteworthy are select polar semiconductors demonstrating promising photocatalytic potential, attributable to their intrinsic spontaneous polarization and optimal band gap characteristics. The study further reveals the emergence of bulk Rashba splitting phenomena, Dirac point features, and topological insulating states within these materials. These diverse physical manifestations stem from symmetry-breaking mechanisms in I-IV-V compounds containing double lone-pair electrons, which enable unique electronic structure modifications.