Quasi–two-dimensional ferroelectricity with multiple switchable polarization states in N-H coinjected perovskite manganites

Topotactic transformation such as hydrogenation serves as a powerful approach to engineering material functionality. However, challenged by direct imaging of light elements and clarifying their role, dual ion-based topotactic tranformation has been poorly explored so far. Here, we report on NH₃ plasma–induced ferrodistortive phase transition in N_xH_y(La,Sr)MnO_3-δ films (0 < x < 0.2, 0.5 < y < 1.0, and δ ≈ 0.125, 0.25, and 0.5), where the injected H and N are resolved to enhance the polar order along with antisite defects by atomic-resolution electron microscopy. Besides unveiling the mediation of structural modulation and N-H competition by oxygen-vacancy ordering degree, our piezoresponse force microscopy unravels a unique quasi–two-dimensional (q2D) ferroelectricity in fourfold modulated brownmillerite phase (δ ≈ 0.25), which offers a series of switchable polarization states by an applied electric field. Unlike all-known ferroelectrics, the q2D ferroelectrics establishes a promising material platform for design of future electronic devices such as multistate information storage.