**2.2 Extended defects: Grain boundaries**

Grain boundaries are the interfaces between like crystals, at which atomic planes are always disrupted to some extent. The atomic order of the lattice is preserved up to within approximately a unit cell of the dividing plane. Thus, the disordered region of a grain boundary is typically only 0.5—1 nm wide, although it does vary somewhat with the type of boundary and the crystal lattice periodicity. Grain boundaries provide segregation sites for impurities and defects.
