Highly Coherent Grain Boundaries Induced by Local Pseudomirror Symmetry in β-Ga₂O₃

Grain boundaries have an extensive influence on the performance of crystal materials. However, the atomic-scale structures and their relations to local and crystallographic symmetries remain elusive in low-symmetry crystals. Herein, we find that the local pseudomirror-symmetric atomic layer is the common physical origin of a series of highly coherent grain boundaries in the low-symmetry β-Ga2O3 crystal. These include the (100) twin boundary and an emerging series of (h-1′0′2)/(h+1′0′2̅) coherent asymmetric grain boundaries (CAGBs). Owing to the local pseudomirror symmetry and the special geometric relation of the β-Ga2O3 conventional cell, these CAGBs place 80% of the boundary atoms in pseudocoincident sites, exhibiting high coherence under the coincident-site lattice model. With a combination of density functional theory calculations, Czochralski growth experiment, and atomic-scale characterizations, the structure and stability of the (002)/(202̅)-A CAGB are confirmed, with an interface energy density as low as 0.36 J m–2. This CAGB is responsible for the spontaneous formation of a twinned defect facet at the surface steps during the epitaxy growth of β-Ga2O3, warranting a substrate orientation selection rule for β-Ga2O3. Through this study, we provide insights into the grain boundary physics in the low-symmetry β-Ga2O3 crystal while emphasizing the importance of the local pseudosymmetries in the low-symmetry crystals.