Schottky defect is a type of point defect that occurs in ionic crystals. It involves the simultaneous absence of a cation and an anion from their respective lattice sites. The correct answer is option 'D' - an equal number of cations and anions are missing from the lattice.

Explanation:
When a crystal lattice has a Schottky defect, an equal number of cations and anions are missing from their normal lattice positions. This leads to the formation of vacancies in the crystal lattice structure. Here is a detailed explanation of why option 'D' is the correct answer:

1. Schottky defect and missing ions:
In an ionic crystal lattice, the positively charged cations and negatively charged anions are held together by strong electrostatic forces. In a perfect crystal lattice, each cation is surrounded by anions and vice versa. However, in a Schottky defect, some cations and anions are missing from their lattice positions.

2. Equal number of missing cations and anions:
A Schottky defect involves the simultaneous absence of a cation and an anion from their respective lattice sites. This means that for every missing cation, there is a corresponding missing anion. The number of missing cations and anions is equal, ensuring electrical neutrality of the crystal.

3. Formation of vacancies:
The absence of cations and anions creates vacancies in the crystal lattice. These vacancies are essentially empty spaces where the ions should be located. The presence of vacancies affects the overall structure and properties of the crystal.

4. Characteristics of Schottky defect:
- Schottky defects are common in ionic crystals with high coordination numbers and high lattice energies.
- Schottky defects reduce the density of the crystal since some ions are missing.
- Schottky defects do not introduce any charge imbalance in the crystal because the number of missing cations is equal to the number of missing anions.

In conclusion, the Schottky defect in crystals is observed when an equal number of cations and anions are missing from the lattice, leading to the formation of vacancies. This defect does not introduce any charge imbalance in the crystal and affects its density and overall structure.

Equal number of cations and anions are missing from
the lattice