Improper antiferroelectricity in NaNbO-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 NaNbO-based perovskites, using advanced scanning transmission electron microscopy conducted on compositionally engineered samples, with a focus on Mn-doped (NaAgCa)(NbTi)O. 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 (abc) (m = integer) and abc 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.