Step 1: Understanding the Question:
The question defines a specific type of point defect in ionic crystals where a pair of oppositely charged ions is missing from their lattice sites.
Step 2: Detailed Explanation:
Schottky Defect: This occurs when an equal number of cations and anions are missing from their regular lattice positions in an ionic crystal.
This pair of vacancies ensures that the electrical neutrality of the crystal is maintained.
It is common in compounds with high coordination numbers where cations and anions are of similar size (e.g., NaCl, KCl, CsCl).
A key consequence of Schottky defects is that they decrease the density of the crystal.
Comparison with Frenkel Defect: A Frenkel defect occurs when an ion (usually the smaller cation) leaves its lattice site and occupies an interstitial position. The density remains the same.
Step 3: Final Answer:
The missing cation-anion pair in an AB-type crystal is known as a Schottky defect.