Crystallographic defect
![Electron microscopy of antisites (a, Mo substitutes for S) and vacancies (b, missing S atoms) in a monolayer of molybdenum disulfide. Scale bar: 1 nm.[1]](/Images/godic/202412/21/MoS2_antisites&vacancies2453.jpg")
![Schematic illustration of some simple point defect types in a monatomic solid](/Images/godic/202412/21/Defecttypes2453.png")
![Schematic illustration of defects in a compound solid, using GaAs as an example.](/Images/godic/202412/21/Compounddefects2453.png")
![An edge dislocation is shown. The dislocation line is presented in blue, the Burgers vector b in black.](/Images/godic/202412/21/Dislocation_edge_d2.svg2453.png")
Crystalline solids exhibit a periodic crystal structure. The positions of atoms or molecules occur on repeating fixed distances, determined by the unit cell parameters. However, the arrangement of atoms or molecules in most crystalline materials is not perfect. The regular patterns are interrupted by crystallographic defects.