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 ${\mathrm{NbI}}_4$ bulk is theoretically demonstrated as a quasi-one-dimensional sliding ferroelectric material, which exhibits a polarization of $0.11\mu \mathrm{C}/{\mathrm{cm}}^2$ 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 $10\mathrm{GPa}$, beyond which ${\mathrm{NbI}}_4$ becomes a polar metal. In addition, negative longitudinal piezoelectricity is also predicted.