Stoichiometric defects are the deviations from the ideal stoichiometry of a compound due to the presence of vacancies or interstitials in the crystal lattice. These defects can occur in ionic compounds, such as ZnS, which has a zinc-blende crystal structure.

The given options for the type of stoichiometric defect shown by ZnS are:

a) Schottky defect
b) Frenkel defect
c) Interstitial defect
d) Both a and b

The correct answer is option 'B' - Frenkel defect.

Explanation:
Frenkel defect is a type of point defect that occurs when an ion leaves its normal lattice site and occupies an interstitial position within the crystal lattice. In the case of ZnS, the zinc ions (Zn2+) can leave their lattice sites and occupy interstitial positions, creating Frenkel defects.

Now, let's break down the answer into headings and bullet points for better organization and readability:

Frenkel Defect in ZnS:

1. Definition of Frenkel defect:
- Frenkel defect is a type of point defect in a crystal lattice.
- It occurs when an ion leaves its normal lattice site and occupies an interstitial position within the crystal lattice.

2. ZnS crystal structure:
- ZnS has a zinc-blende crystal structure.
- In this structure, Zn2+ ions occupy the face-centered cubic (FCC) lattice sites, while S2- ions occupy the tetrahedral interstitial sites.

3. Frenkel defects in ZnS:
- In ZnS, Frenkel defects occur when Zn2+ ions leave their lattice sites and occupy interstitial positions.
- These defects do not affect the overall stoichiometry of the compound because the number of Zn2+ and S2- ions remains the same.
- The crystal lattice maintains charge neutrality.

4. Comparison with other types of defects:
- Schottky defect:
- Schottky defect occurs when equal numbers of cations and anions are missing from the lattice, creating vacancies.
- It leads to a decrease in the density of the crystal.
- Interstitial defect:
- Interstitial defect occurs when foreign atoms or ions occupy the interstitial positions in the crystal lattice.
- It can affect the stoichiometry of the compound if the foreign atoms have a different valence or charge.

In conclusion, the type of stoichiometric defect shown by ZnS is the Frenkel defect, where Zn2+ ions leave their lattice sites and occupy interstitial positions within the crystal lattice.