Second-order Jahn-Teller effect induced high-temperature ferroelectricity in two-dimensional NbOX (X = I, Br).

Based on the first-principles calculations, we investigated the ferroelectric properties of two-dimensional (2D) materials NbOX (X = I, Br). Our cleavage energy analysis shows that exfoliating one NbOI monolayer from its existing bulk counterpart is feasible. The phonon spectrum and molecular dynamics simulations confirm the dynamic and thermal stability of the monolayer structures for both NbOI and NbOBr. Total energy calculations show that the ferroelectric phase is the ground state for both materials, with the calculated in-plane ferroelectric polarizations being 384.5 pC m and 375.2 pC m for monolayers NbOI and NbOBr, respectively. Moreover, the intrinsic Curie temperature of monolayer NbOI (NbOBr) is as high as 1700 K (1500 K) from Monte Carlo simulation. Furthermore, with the orbital selective external potential method, the origin of ferroelectricity in NbOX is revealed as the second-order Jahn-Teller effect. Our findings suggest that monolayers NbOI and NbOBr are promising candidate materials for practical ferroelectric applications.