The competition between interlayer rumpling and rotations is crucial for understanding the origin of hybrid improper ferroelectricity for Ruddlesden–Popper (RP) oxides, but the corresponding relationship is still unclear in the RP nitrides. Inspired by the recent experimental synthesis of RP layered nitrides, we explore competitive relationships of distortion modes in La3W2N7 and the strategies for enhancing the polar modes. The results show that although the potential energy surface of the rumpling mode presents a single-well shape, it can enhance the stability of both the polar and antipolar coupling modes. The antipolar coupling mode is more energy preferable, so the stable La3W2N7 is a centrosymmetric structure with the space group Pnma. We further enhance the rumpling distortion through the interface atomic substitution strategy, making the polar coupling mode more dominant in the (LaY2)W2N7, resulting in a non-centrosymmetric ground state structure. The analysis of electronic structure and electrostatic potential indicates that the electrostatic potential differences between the rock salt and perovskite layers strengthen the amplitude of the rumpling mode and the Jahn–Teller distortion of the WN6 octahedron. Our study provides a viable strategy for designing non-centrosymmetric n = 2 RP layered nitrides and offers a promising pathway toward the future development of nitride-based ferroelectric and multiferroic materials.
Deng et al. (Mon,) studied this question.