Metallic Sliding Ferroelectricity in Trilayer Penta-NiN2

The experimental observation of metallic ferroelectricity has challenged the traditional understanding in recent years, as metallicity and ferroelectricity are historically considered mutually exclusive. This breakthrough has sparked significant interest in this field. However, the coexistence of metallicity and sliding ferroelectricity in 2D pentagonal materials has not been reported yet. Here, this is theoretically studied that the sliding ferroelectricity in metallic trilayer penta-NiN₂. By scanning the configuration space of stacking patterns, a pair of polarized configurations is identified with strong out-of-plane ferroelectric polarization of ± 5.39 × 10⁻¹³ C m⁻¹ and a low polarization switching barrier of 14.33 meV/atom together with a non-polarized configuration as the intermediate state for the ferroelectric transition. It is also found that there are substantial changes in the second harmonic generation of the stable configurations that would facilitate experimental observation. This work demonstrates the potential of pentagonal sheets as promising metallic sliding ferroelectrics, which would significantly expand the family of sliding ferroelectric materials.