In which of the following point defects, density is not affected?

Frenkel defect

The correct answer is option 2. Frenkel defect.

Defects in crystals can alter the arrangement of atoms and potentially affect the material's properties, including density. Let us explore each defect and its impact on density:

1. Schottky defect: Imagine an ionic solid like sodium chloride \((NaCl)\). In a Schottky defect, a cation \((Na^+)\) or an anion \((Cl^-)\) is missing from its regular lattice site. This vacancy creates a hole, reducing the overall mass of the crystal. Since the volume remains relatively unchanged, the density decreases.

3. Interstitial defect: Here, an extra atom or ion squeezes itself into a space between the normal lattice sites. This additional atom adds mass without significantly altering the crystal's overall volume. As a result, the density increases.

4. Metal vacancy defect: This defect is common in alloys. Imagine a metal alloy where a metal atom is missing from its designated position. Similar to a Schottky defect, this vacancy reduces the total mass of the material, leading to a decrease in density.

2. Frenkel defect: This defect occurs primarily in ionic crystals where the cation (positively charged ion) is smaller than the anion (negatively charged ion). In a Frenkel defect, the smaller cation displaces itself from its regular lattice site and occupies an empty space between the normal lattice sites (an interstitial position). This creates two key changes:

• A vacancy at the original cation site.
• An interstitial cation at the new location.

Why doesn't density change in a Frenkel defect?

The crucial point here is that the missing cation ends up occupying a previously empty space within the crystal. There's a rearrangement of ions, but the overall occupied volume doesn't change significantly. The mass of the material remains nearly constant because the displaced cation hasn't left the crystal; it's just relocated. Therefore, the density is not affected by a Frenkel defect.

In essence, a Frenkel defect redistributes the mass within the crystal without a significant change in the overall volume, resulting in no net change in density.