Figure 1. Structures of interstitial clusters within the Σ11 symmetric tilt GB for cluster sizes ranging from (a) 1 to (e) 5 interstitials.

The color scheme is large (green) spheres for interstitials and (red) squares for apparent vacancies, where defects are defined using the refence lattice method²⁵,²⁶ with a cutoff of 0.8Å. In this scheme, the “vacancies” indicate atoms in the original GB structure that were displaced significantly upon introduction of interstitials; in each case, the net number of defects (interstitials minus vacancies) is equal to the number of extra atoms inserted into the GB (see Supplementary Information for details). The rest of the atoms, indicated by the small (white) spheres, are in their position within the undefective GB structure. The orientation of each view is given as insets in the first frame, to be compared with the orientation of the grains given in Table S1.