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.