Quasi-one-dimensional sliding ferroelectricity in NbI₄

Sliding ferroelectricity was originally proposed to elucidate the out-of-plane polarization generated by a specific stacking arrangement of nonpolar van der Waals layers. However, the concept of sliding ferroelectricity can be generalized to more geometries. Here, the NbI₄ bulk is theoretically demonstrated as a quasi-one-dimensional sliding ferroelectric material, which exhibits a polarization of 0.11 µC/cm² perpendicular to the Nb's chains. The most possible ferroelectric switching path is found to be via the interchain sliding along the chain direction, while other paths such as Peierls dimerization of Nb pairs may also work. Moreover, its polarization can be augmented for 82% by hydrostatic pressure up to 10 GPa, beyond which NbI₄ becomes a polar metal. In addition, negative longitudinal piezoelectricity is also predicted.