Question

What is the schottky defect in a solid?

Answer 1

Hint: Metal defects are irregularities in the crystal lattice of metal. These irregularities are due to irregularities in the arrangement of constituent particles.

Complete answer:
Metal defects are classified into point defect and line defect. Solids have stoichiometric defects, non-stoichiometric defects and impurity defects. Stoichiometric defects are of two types, vacancy defect and interstitial defect. Non-stoichiometric defects include metal excess defect and metal deficiency defect. Metal excess defect is of two types, one is due to anionic vacancy and other is due to presence of interstitial cation.
In schottky defect, equal numbers of cation and anion are missing from the lattice. To maintain electrical neutrality, the number of positive missing should be missing from the number of negative charges.
This defect is observed in compounds with high coordination numbers. Both cation and anion are small sized.
Since some ions are missing from the crystal, the density of crystals decreases. This is because the total volume of crystal remains the same as the dimensions of crystal lattice remains the same. But the mass of the crystal decreases as few ions are missing. Density being the ratio of mass to volume, decreases.
Potassium chloride and sodium chloride are some of the crystals that have schottky defects.
A diagrammatic representation of crystal lattice having schottky defect is as shown below:

In the above diagram, two cations and two anions are missing from the crystal lattice.
On the other hand, in the Frienkel defect, an ion is missing from its position in the lattice, but it occupies interstitial position. Due to this defect, the density of crystal is not affected.
Note: Schottky defect is a stoichiometric defect as the stoichiometry of the crystal lattice is not disturbed. Here stoichiometry refers to the ratio of the number of cations and anions. The stoichiometry is not disturbed as an equal number of cations and anions are missing.