Polarization Switching and Correlated Phase Transitions in Fluorite-Structure ZrO2 Nanocrystals

Unconventional ferroelectricity in fluorite-structure oxides enables tremendous opportunities in nanoelectronics owing to their superior scalability and silicon compatibility. However, their polarization order and switching process remain elusive due to the challenges of visualizing oxygen ions in nanocrystalline films. In this work, the oxygen shifting during polarization switching and correlated polar-nonpolar phase transitions are directly captured among multiple metastable phases in freestanding ZrO2 thin films by low-dose integrated differential phase-contrast scanning transmission electron microscopy (iDPC-STEM). Bidirectional transitions between antiferroelectric and ferroelectric orders and interfacial polarization relaxation are clarified at unit-cell scale. Meanwhile, polarization switching is strongly correlated with Zr-O displacement in reversible martensitic transformation between monoclinic and orthorhombic phases and two-step tetrahedral-to-orthorhombic phase transition. These findings provide atomic insights into the transition pathways between metastable polymorphs and unravel the evolution of polarization orders in (anti)ferroelectric fluorite oxides.

Automated summary

In this work, the researchers used low-dose integrated differential phase-contrast scanning transmission electron microscopy (iDPC-STEM) to directly observe the oxygen shifting and polar-nonpolar phase transitions in freestanding ZrO2 thin films. They clarified bidirectional transitions between antiferroelectric and ferroelectric orders and explained the mechanism of interfacial polarization relaxation at the unit-cell scale. The polarization switching was found to be strongly correlated with Zr-O displacement and two-step tetrahedral-to-orthorhombic phase transition.