Surface ferroelectric phase transition triggered by s-p orbital interaction in two-dimensional monolayer perovskites

To accommodate the burgeoning growth of the electronic device industry and the evolving needs of human society, one avenue for device miniaturization is the utilization of multifunctional or multiferroic materials that integrate multiple physical properties within a single phase. In this study, we demonstrate that monolayer BiFeO₃ exhibits a surface-selective ferroelectric phase transition. The polarization direction is governed by the orientation-dependent tilting of the surface lone-pair electron density of states, which is originated from the s-p orbital coupling and the rotational/tilt distortion of incomplete oxygen octahedrons. That is to say, the ferroelectric polarization along the longer lattice constant side is nonzero, indicating a ferroelectric phase. Meanwhile, the ferroelectric polarization along the shorter lattice constant side is zero, indicating an antiferroelectric phase. More intriguingly, within the high-symmetric phase, the direction of ferroelectric polarization aligns along the z direction, which is distinct from that in the low-symmetric phase. Besides, the monolayer BiFeO₃ exhibits the characteristics of an altermagnet due to its rotated spin densities. Further computational analyses have elucidated that the surface ferroelectricity originates from the tilting of the half of the incomplete-octahedron in the positive direction of the shorter lattice constant side, while the remaining incomplete-octahedron tilt in the opposite direction, culminating in an antiferroelectric phase. Additionally, these structural units all align their tilt along the longer lattice constant side, which in turn induces a ferroelectric phase. The calculated partial density of states analysis revealed that the unique surface ferroelectric properties of the material stem from the coupling between the lone pair electrons of bismuth (Bi-s) and the surrounding 2p orbitals of oxygen atoms. Moreover, the tilted electron localization function and its surface-specific distribution clearly demonstrated the origin of the surface ferroelectric phase transition. Furthermore, our research has uncovered that the surfacespecific ferroelectric phase transition is prevalent among numerous monolayer two-dimensional perovskites. Remarkably, this characteristic persists even in bulk phases that inherently lack ferroelectric polarization. Our study discovered a new way to construct multifunctional materials and provide theoretical basis for designing new multifunctional devices.