Asymmetric oxygen displacement-induced contrast modulation and multi-state ferroelectricity in distorted perovskite oxides

Distinct from the phase-contrast annular bright field in scanning transmission electron microscopy, where the specimen tilt and aberration coefficients may introduce atomic off-center artifacts, the high-angle annular dark field (HAADF), largely immune to them, is widely adopted for its easy Z-contrast interpretation. However, the impact of light-element occupancy on HAADF contrast is rarely explored, which impedes understanding of the material properties. Here, we observe an oxygen-vacancy (Vo) order induced periodic A-site intensity modulation in HAADF images of La0.7Sr0.3MnO2.75 thin films. Linking closely with the regular stacking of one tetrahedral (1T) and three octahedral (3O) layers, the manganite is found to exhibit a switchable multi-state ferroelectricity by piezoresponse force microscopy. In combination with integrated differential phase contrast microscopy, our multi-slice HAADF image simulations and extended studies on ABO2.75 (A = Sr, La; B = Ti, Co, Mn) reveal that the intensity modulation is attributed to (1) T-layer-based interlayer expansion, (2) polarity of the adjacent AO plane, and (3) oxygen octahedral rotation in Mn- and Co-containing oxides. While for Ti-containing ABO2.75 oxides, the intensity modulation is only governed by the former two factors. Our findings point out a facile method to disclose the ferroelectric ABO2.75 compounds that can potentially be used for multi-state information storage.