Emergence of multiple ferroelectric states in multilayer black phosphorus

Sliding ferroelectricity is induced by the artificial stacking of two-dimensional materials, and with specific stacking sequences, it can even emerge in nonferroelectric layered systems. Here, based on first-principles calculations, we demonstrate that multilayer black phosphorus (BP) with layer number $N\phantom{\rule{4pt}{0ex}}\ensuremath{\ge}$ 3 exhibits ferroelectricity. The polarization originates from symmetry breaking induced by layer stacking and can be reversed through the relative sliding of adjacent layers. With an increasing number of layers, a greater diversity of polarization states emerges, highlighting the crucial role of layer number and interlayer dipole coupling in sliding ferroelectricity. Surface-layer displacement in BP slabs also generates spontaneous polarization, offering opportunities for experimental detection and manipulation. Our work establishes interlayer sliding as an effective approach to induce ferroelectricity in elemental multilayers, opening new possibilities for nonvolatile memory applications.