Atomic‐Scale Insights into Yttrium‐Induced Grain Boundary Structure Modification in Al ₂ O ₃

Abstract Impurity segregation at grain boundaries (GBs) often induces structural transformations at the atomic level and significantly influences materials’ properties, underscoring the importance of understanding the underlying mechanisms of GB segregation at the atomic scale. Here, the atomic structure of a Y‐segregated ∑13(104)/[20] GB is thoroughly studied in α‐Al ₂ O ₃ via scanning transmission electron microscopy and Monte Carlo and molecular dynamics simulations based on a neural network potential. It is found that the Y segregation at the GB involves not only the simple substitution of Y for Al atoms but also a structural adaptation with a change in GB atomic density. Such a change alters the local bonding environment at the GB so that the absolute excess volume is minimized for the lowest‐energy structure. This study offers a new insight into the atomic‐scale mechanism of GB structural transformation.