 Crystallographic defect

A crystallographic defect is a departure from ideal repeated structure. In an ionic crystal, a missing ion can provide an available site for another ion to move into; movement still requires overcoming an activation energy. This is why an ideal immobile-site model is a limit rather than a universal law for solid electrical conductivity.^[ref]

Crystallographic defects, also known as crystal defects, refer to imperfections in the regular geometric arrangement of atoms or molecules in a crystalline structure. These defects play a crucial role in determining the physical, chemical, and mechanical properties of materials.