Improper antiferroelectricity in NaNbO₃-based perovskites driven by antiferrodistortive modulation

Perovskite materials exhibit a wide array of fascinating properties arising from various structural instabilities and the interplay between them. Probing such instabilities demands the use of high-resolution, high-sensitivity characterization techniques to prototypical materials with minimized complexity. Here we present the discovery of unconventional improper antiferroelectricity driven by antiferrodistortive modulation in NaNbO₃-based perovskites, using advanced scanning transmission electron microscopy conducted on compositionally engineered samples, with a focus on Mn-doped (Na_0.65Ag_0.20Ca_0.15)(Nb_0.85Ti_0.15)O₃. Contrary to the prevailing understanding that such octahedral-rotation-driven improper polarization requires symmetry breaking at the interfaces in layered perovskites, our observation indicates that it can also be enabled in non-layered perovskites, by modulated octahedral rotations following an alternating sequence of (a^-b^-c⁺)_m (m = integer) and a^-b⁺c⁺ that is tunable via chemical doping. Combining with first-principles calculations and group theoretical analysis, we reveal a multimode interaction picture to generate the unique dipole order, resolving its long-standing structural ambiguity. The identified mechanism for octahedral-rotation-driven improper polarization represents a new design freedom to tailor the interplay of instabilities for coupled functionalities in perovskite oxides.