In which of the following defects, density of crystal decreases?

The correct answer is option 1. Schottky defect.

To understand which defect causes a decrease in the density of a crystal, let us review each type of defect:

1. Schottky Defect:

A Schottky defect occurs when an equal number of cations and anions are missing from their lattice sites. This type of defect maintains electrical neutrality. Since atoms or ions are missing from the crystal lattice, the overall mass of the crystal decreases while the volume remains unchanged, leading to a decrease in density.

2. Frenkel Defect:

A Frenkel defect occurs when an ion (usually a cation) leaves its lattice site and occupies an interstitial site. The total number of atoms remains the same, but their positions change. The overall mass and volume of the crystal do not change significantly, so the density remains essentially unchanged.

3. Interstitial Defect:

An interstitial defect occurs when extra atoms or ions occupy interstitial sites in the crystal lattice. The addition of extra atoms increases the mass of the crystal without a significant change in volume, leading to an increase in density.

4. Metal Excess Defect: A metal excess defect typically occurs due to an anion vacancy or the presence of extra cations, leading to an excess of metal ions in the crystal lattice. This defect usually results in a slight increase in the mass of the crystal, which could lead to an increase in density. Among the given defects, the Schottky defect is the one that leads to a decrease in the density of the crystal. This is because the removal of an equal number of cations and anions reduces the overall mass of the crystal while keeping the volume constant.

Therefore, the correct answer is Schottky defect.