Unconventional Ferroelectric‐Ferroelastic Switching Mediated by Non‐Polar Phase in Fluorite Oxides

Abstract HfO 2 /ZrO 2 ‐based ferroelectrics present tremendous potential for next‐generation non‐volatile memory due to their high scalability and compatibility with silicon technology. Unlike the continuous polar layers in perovskite ferroelectrics, HfO 2 /ZrO 2 ‐based ferroelectrics are composed of alternating polar layers with oxygen shifts and non‐polar spacers, which leads to a distinct ferroelectric switching mechanism. However, directly observing the switching process has been a big challenge due to the polymorph feature of nanoscale fluorites and the difficulty in in situ imaging on light elements. Here, the ferroelectric‐ferroelastic coupled switching process in freestanding ZrO 2 thin films is directly visualized by in situ imaging on oxygen motions. A multi‐step 90‐degree polarization switching mechanism is uncovered that challenges the conventional one‐step 180‐degree switching paradigms in fluorite oxides, which is highly consistent with the interlocked nature of ferroelectricity and ferroelasticity. A non‐polar tetragonal (T) phase is discovered as a crucial intermediate state, lowering the energy barrier for polarization switching by 35%. More importantly, the T phase prevents irreversible transitions to the non‐polar ground state and facilitates stable ferroelectric switching. These findings are fundamental to understanding nanoscale polarization switching mechanisms in fluorite ferroelectrics, paving the way for advanced high‐durability devices.