Ferroelectric materials are fascinating for their non-volatile switchable electric polarizations induced by the spontaneous inversion-symmetry breaking. However, in all of the conventional ferroelectric compounds, at least two constituent ions are required to support the polarization switching^1,2. Here, we report the observation of a single-element ferroelectric state in a black phosphorus-like bismuth layer³, in which the ordered charge transfer and the regular atom distortion between sublattices happen simultaneously. Instead of a homogenous orbital configuration that ordinarily occurs in elementary substances, we found the Bi atoms in a black phosphorous-like Bi monolayer maintain a weak and anisotropic sp orbital hybridization, giving rise to the inversion-symmetry-broken buckled structure accompanied with charge redistribution in the unit cell. As a result, the in-plane electric polarization emerges in the Bi monolayer. Using the in-plane electric field produced by scanning probe microscopy, ferroelectric switching is further visualized experimentally. Owing to the conjugative locking between the charge transfer and atom displacement, we also observe the anomalous electric potential profile at the 180° tail-to-tail domain wall induced by competition between the electronic structure and electric polarization. This emergent single-element ferroelectricity broadens the mechanism of ferroelectrics and may enrich the applications of ferroelectronics in the future.

铁电材料因其自发反演对称破缺引起的非易失性可切换电极化而令人着迷。然而，在所有传统的铁电化合物中，需要至少两种组成离子来支持极化切换^1,2 。在这里，我们报告了在黑磷类铋层³中观察到的单元素铁电态，其中有序电荷转移和亚晶格之间的规则原子畸变同时发生。我们发现黑磷状 Bi 单层中的 Bi 原子保持着弱且各向异性的s p轨道杂化，而不是通常出现在基本物质中的同质轨道构型，从而产生了伴随电荷的反演对称性破缺的屈曲结构晶胞内的重新分布。结果，Bi单层中出现面内电极化。利用扫描探针显微镜产生的面内电场，铁电开关在实验上进一步可视化。由于电荷转移和原子位移之间的共轭锁定，我们还观察到电子结构和电极化之间的竞争引起的180°尾对尾畴壁处的异常电势分布。这种新兴的单元素铁电性拓宽了铁电体的机理，并可能丰富未来铁电子学的应用。