Concept:A Schottky defect occurs when equal numbers of cations and anions are missing from their regular lattice positions in an ionic crystal.
Explanation:In an ionic solid, when one cation and one anion are simultaneously absent from their lattice sites, a pair of vacancies is created.
This type of defect is called a Schottky defect.
The crystal remains electrically neutral because equal numbers of positive and negative charges are removed from the lattice.
As a result of these missing ions, the density of the crystal decreases.
Schottky defects are commonly found in ionic compounds where the cations and anions have similar sizes and the coordination number is high, such as
NaCl and
KCl.
In contrast, a Frenkel defect involves an ion being displaced from its lattice site to an interstitial position, while a simple vacancy defect refers to missing atoms without the condition of equal cation–anion absence.
Since the question specifically states that equal numbers of cations and anions are missing, the defect is the Schottky defect.
Answer:C. Schottky defect