When a pair of cation and anion are missing in a crystal, it is called?

Question

RRB Paper · Accepted Answer

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Vacancy defect

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When a pair of cation and anion are missing in a crystal, it is called Schottky’s defect.

Schottky’s defect forms when two atoms of opposite charge (anion and cation) leaves their regular atomic positions thus creating two vacancies.
As two atoms of opposite charges are leaving, there is no change in the overall charge of the material with Schottky’s defect.
Schottky’s defect is known for its presence in ionic crystals.