The two-dimensional defects that appear in crystals can be usefully divided into

three types: free surfaces, which are the external surfaces at which the solid terminates at

a vapor or liquid, intercrystalline boundaries, which separate grains or distinct phases

within the solid, and internal defects that disrupt the crystalline pattern over a surface

within a crystal. All of these defects have two important characteristics. First, since they

are surfaces in a crystal they have particular atomic structures that depend on orientation.

Second, they have a positive energy. The energy per unit area is ordinarily equal to the

surface tension, ß, of the interface.