Grain boundary stabilization of fluorite ferroelectrics

Grain boundaries (GBs) can tailor the macroscopic properties of polycrystalline materials via their intrinsic structural and electronic states. However, as independent heterointerfaces, their role in stabilizing grain phases remains largely unexplored, especially at the atomic scale. Here we report that chemically ordered heterogeneous GBs in ZrO₂ thin films act as active stabilizers of a metastable polar phase. The atomically sharp and ordered La(Sr)-Mn-O configurations at GBs are identified at the atomic scale. The resultant charge ordering and bond covalency of the GBs are validated by four-dimensional scanning transmission electron microscopy. This structural motif induces e_g/t_2g orbital ordering of Mn ions at GBs, modulating Zr-O bond strength to stabilize the polar phase. This work establishes a GB-centric paradigm for engineering nanoscale phase diagrams, offering a promising strategy for designing metastable functional materials via GB chemistry.