Although crystalline solids have short range as well as long range order in the arrangement of their constituent particles, yet crystals are not perfect. Usually, a solid consists of an aggregate of large number of small crystals. These small crystals have defects in them. This happens when crystallization process occurs at fast or moderate rate. Single crystals are formed when the process of crystallization occurs at extremely slow rate. Even these crystals are not free of defects. The defects are basically irregularities in the arrangement of constituent particles. Broadly speaking, the defects are of two types, namely, point defects and line defects. Point defects are the irregularities or deviations from ideal arrangement around a point or an atom in a crystalline substance, whereas the line defects are the irregularities or deviations from ideal arrangement in entire rows of lattice points. These irregularities are called crystal defects.

Which of the following point defects is also known as dislocation defect?

Frenkel defect

The correct answer is option 2. Frenkel defect.

The point defect that is also known as a dislocation defect is the Frenkel defect.

Let us look at each of the given options:

1. Schottky defect: This defect involves the absence of an equal number of cations and anions from their lattice sites, maintaining electrical neutrality. It is a type of vacancy defect.

2. Frenkel defect: This defect occurs when an ion (usually a smaller cation) is displaced from its lattice site to an interstitial site, creating a vacancy at the original site and an interstitial defect at the new location. It involves a cation leaving its place in the lattice and moving to an interstitial site, thus creating a vacancy-interstitial pair. This is often referred to as a dislocation defect because it involves the displacement or "dislocation" of ions within the crystal lattice.

3. Impurity defect: This defect occurs when foreign atoms are present in the crystal lattice. These can either substitute for the host atoms or occupy interstitial sites.

4. Metal excess defect: This defect occurs due to an excess of metal ions in the lattice. This can happen through anionic vacancies or the presence of extra cations at interstitial sites.

Thus, the Frenkel defect is the one that involves the dislocation of ions within the crystal lattice, making it synonymous with a dislocation defect.