Two-dimensional defects in materials

Defects and imperfections can also exist in 2D and 3D beyond 0D and 1D. For example, grain boundaries are regions where two different crystallographic orientations come together. The grain boundary is only a few atoms thick but has many imperfections, missing bonds, crowded or overly spaced out atoms. The grain boundary surface contains a surface energy due to these imperfections. The surface energy is the reason why impurities tend to segregate along grain boundaries. Other defects include twin boundaries which are special versions of grain boundaries that preserve mirror symmetry across the boundary. There can also be stacking faults where the order of stacking layers gets interrupted. Domain boundaries are like grain boundaries except instead of mismatched crystal orientation, the orientation is preserved but there can be different regions of dipole or magnetic moment.

[abdullahalharthi]

Are there atoms in the grain boundaries, or is it an empty space?