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

HfO₂/ZrO₂-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, HfO₂/ZrO₂-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 ZrO₂ 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.