Question

Why is the Frenkel defect not found in pure alkali metal halides?

Answer 1

Hint: Alkali metal halides (also known as alkali halides) are an inorganic compound family having the chemical formula MX, where M is an alkali metal and X is a halogen. Certain compounds are frequently the most important commercial sources of these metals and halides. Sodium chloride, also known as table salt, is the most well-known of these substances.

Complete answer:
Face-centered cubic lattices form in the majority of alkali metal halides. The metals and halides in this structure have octahedral coordination geometry, with each ion having a coordination number of six. CsCl, CsBr, CsI crystallise in a body-centered cubic lattice that accommodates the bigger metal cation's coordination number of eight (and the anion also).
Schottky defects are vacancies in ionic solids that develop when atoms or ions are absent from the lattice point. The quantity of missing cations and anions must be equal to preserve electrical neutrality. The density of the material is reduced as a result of this flaw. Ionic substances with comparable cation and anion sizes, such as NaCl, KCl, and CsCl, demonstrate this.
In pure alkali metal halides, the Frenkel defect does not exist. This is owing to the greater size of the cations of alkali metals, which prevents them from fitting into the interstitial spaces.

Note:
The alkali metal halides are colourless crystalline solids that look white as finely crushed powders. They turn into colourless liquids when heated to high temperatures, generally several hundred degrees. Their high lattice energy is reflected in their high melting point. These liquids evaporate at even greater temperatures, resulting in gases made up of diatomic molecules.