Vacancy

Unoccupied sites within a crystal that are usually occupied by an atom. A vacancy may move when a neighboring atom or ion moves to occupy the vacant site (the vacancy shift in the opposite direction to the site that was the source of the atom). The surrounding bonded crystal structure ensures that the neighboring atoms do not collapse around the vacancy. In some materials, neighboring atoms actually move away from a vacancy, because they can better form bonds with atoms in the other directions.