Second-order Jahn-Teller effect induced high-temperature ferroelectricity in two-dimensional NbO2X (X = I, Br)

Based on the first-principles calculations, we investigated the ferroelectric properties of two-dimensional (2D) materials NbO2X (X = I, Br). Our cleavage energy analysis shows that exfoliating one NbO2I 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 NbO2I and NbO2Br. 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 NbO2I and NbO2Br, respectively. Moreover, the intrinsic Curie temperature T-C of monolayer NbO2I (NbO2Br) 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 NbO2X is revealed as the second-order Jahn-Teller effect. Our findings suggest that monolayers NbO2I and NbO2Br are promising candidate materials for practical ferroelectric applications.