Second-order Jahn-Teller effect induced high-temperature ferroelectricity in two-dimensional NbO 2 X (X = I, Br).

Based on the first-principles calculations, we investigated the ferroelectric properties of two-dimensional (2D) materials NbO 2 X (X = I, Br). Our cleavage energy analysis shows that exfoliating one NbO 2 I 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 NbO 2 I and NbO 2 Br. 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 -1 and 375.2 pC m -1 for monolayers NbO 2 I and NbO 2 Br, respectively. Moreover, the intrinsic Curie temperature T C of monolayer NbO 2 I (NbO 2 Br) 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 NbO 2 X is revealed as the second-order Jahn-Teller effect. Our findings suggest that monolayers NbO 2 I and NbO 2 Br are promising candidate materials for practical ferroelectric applications.