Ultralow-Barrier Antiferroelectric–Ferroelectric Transition Enabled by Competing Polar Distortion in Two-Dimensional Ruddlesden–Popper Nitride Perovskites

Two-dimensional Ruddlesden-Popper (2D RP) nitride perovskites, combining strong covalency with reduced dimensionality and weak interlayer coupling, are promising candidates for low-energy polarization control. Here, we study 2D RP nitride perovskite La₂WN₄ using first-principles calculations in combination with symmetry analysis and mode decomposition. La₂WN₄ hosts a competing semiconducting ferroelectric (FE, Aba2) phase and antiferroelectric (AFE, Pna2₁) phase, where polarization originates from WN₆ octahedral distortions coupled to nitrogen displacements. Both the AFE-FE interconversion and polarization reversal proceed over low barriers, enabling fast, energy-efficient switching. External stimuli provide effective control, given that 1% biaxial compressive strain or an in-plane electric field of 0.025 V/Å can reversibly toggle between AFE and FE states. Moreover, La₂WN₄ can be exfoliated into a stable semiconducting monolayer that retains sizable in-plane polarization with a low switching barrier, highlighting its potential for low-power-programmable AFE/FE functionalities.