Question

Which of the following is/are correct regarding defects in solids?

• A. Stoichiometric defect is also called as intrinsic defect
• B. Metal excess defect is a stoichiometric defect
• C. Defects in crystal increase thermodynamic stability
• D. All point defects results in the decrease in the density of the solid

Answer 1

Answer: (A), (C)

Defects in crystal increase thermodynamic stability

Defect is a disruption in the periodic order of a crystalline material. It occurs in crystals to lower their energy and increase thermodynamic stability.

Point defect is of three types,
1. Stoichiometric defect
2. Non-Stoichiometric defect
3. Impurity defects

Point defects that do not disturb the stoichiometry of the solid is called stoichiometric defect. The tendency to show this defects depends on the temperature arrangement. This defect is also known as thermodynamic or intrinsic defects.

Non-Stoichiometric defects occur in solids which contain constituent elements in non-stoichiometric ratio due to defects in their crystal structures.
Metal excess defect is a non-stoichiometric defect because it will have an excess metal ion.

As density is the ratio of mass to volume, only defects where there is loss of mass of both cations and anions leaving both cationic and anionic vacancies (Schottky defect) show decrease in density. Others defects like Frenkel defect defects does not show decrease in density.
$\therefore$ Options a and c are correct.