Quasi-one-dimensional sliding ferroelectricity in NbI$_4$

TL;DR

This paper theoretically demonstrates that NbI4 is a quasi-one-dimensional sliding ferroelectric with pressure-tunable polarization and negative piezoelectricity, expanding the concept of sliding ferroelectricity to new geometries.

Contribution

It generalizes sliding ferroelectricity to quasi-one-dimensional structures and predicts pressure-enhanced polarization and negative piezoelectricity in NbI4.

Findings

NbI4 exhibits a polarization of 0.11 μC/cm² perpendicular to chains.

Hydrostatic pressure increases polarization by 82% up to 10 GPa.

NbI4 becomes a polar metal beyond 10 GPa.

Abstract

Sliding ferroelectricity was originally proposed to elucidate the out-of-plane polarization generated by a specific stacking arrangement of non-polar van der Waals layers. However, the concept of sliding ferroelectricity can be generalized to more geometries. Here, the NbI$_4$ bulk is theoretical demonstrated as a quasi-one-dimensional sliding ferroelectric material, which exhibits a polarization of $0.11$ $\mu$C/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 like 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 NbI$_4$ becomes a polar metal. In addition, the negative longitudinal piezoelectricity is also predicted.