A line defect involves a series of structural imperfections that produces a linear feature and, commonly, offsets or “dislocations”, within an atomic structure. An “edge dislocation” forms where a plane of atoms or ions that would normally extend throughout the crystal terminates along a line within the crystal. Distortions are at a maximum along the line of termination. A “screw dislocation” is a localized screw axis involving atoms or ions (or blocks of atoms or ions) to form a spiral, much like a spiral staircase, with a step at the outer surface of the crystal. Because the screw dislocation is a local feature, the screw axis is not related to the space group (overall symmetry) of the structure, and the crystal regains its overall atomic periodicity at distances further from the central line of the dislocation. The step at the surface of the crystal is believed to enhance crystal growth because atoms or ions can better adhere and bond to sites associated with greater surface area at the step.
Cf., point defect, screw axis