Question

Zinc Oxide ($ZnO$) is white when cold and yellow when heated. It is due to the development of:
A.Frenkel defect
B.Metal excess defect
C.Schottky defect
D.Metal deficiency defects.

Answer 1

Metal deficiency defects: In this defect cation (positively charged atom) is missing from the lattice.
Metal excess defect: It is defined as when lattice has more number of cations (positively charged atoms) as compared to anions (negatively charged atoms).
Frankel defect: In this defect atom moves to a new position between other atoms of the solid.
Schottky defect: In this defect atoms move from inside of the lattice to the surface of the lattice.

Complete step by step solution:
Unit of crystal: It is defined as the repeating unit in the crystal. They are of three types: (I) primitive cubic unit cell, (II) Body-centered cubic unit cell (III) Face-centered cubic unit cell.
(I)Primitive cubic unit cell: In this unit cell, atoms are present only at the corners of the cube. We know that there are eight corners in a cube. So one atom is shared by right corners. So the contribution of an atom in one corner is $\dfrac{1}{8}$. The number of atoms present in a primitive cubic unit cell is one.
(II)Body-centered cubic unit cell: In this unit cell, atoms are present at the centre and corners of the cube. We know that there is only one centre in a cube. So one atom is completely present there. And one atom is contributing to the eight corners. So the total number of atoms present in a Body-centered cubic unit cell is $2$.
(III)Face-centered cubic unit cell: In this unit cell, atoms are present at corners and at the face centres of the cube. There are a total six faces in a cube. So one atom contributes to two face centres i.e. contribution is one atom is half. And one atom contributing to all eight corners of the cube. So there are four atoms present in the face-centered cubic unit cell.
Now, in a crystal there are some defects which are known as defects in the lattice. Some defects are as follows: Frankel defect, Metal excess defect, Schottky defect, Metal deficiency defects.
These are defined as follows:
Metal deficiency defects: In this defect cation (positively charged atom) is missing from the lattice.
Metal excess defect: It is defined as when lattice has more number of cations (positively charged atoms) as compared to anions (negatively charged atoms).
Frankel defect: In this defect atom moves to a new position between other atoms of the solid.
Schottky defect: In this defect atoms move from inside of the lattice to the surface of the lattice.
When zinc oxide is heated then zinc ions move to the interstitial sites and electrons are also entrapped in nearby interstitial sites to balance the charge. Since the number of metal ions are more in the crystal lattice. So the defect is of metal excess type. Hence option B i.e. $ZnO$ is white when cold and yellow when heated. It is due to the development of metal excess defects.

So the correct answer is option B.

Note: $AgBr$ shows two types of defects i.e. Frankel as well as Schottky defect. Because silver ions are smaller in size than that of bromide ions and hence when silver ions are removed from the lattice then they can occupy interstitial sites. Hence silver bromide shows both the defects.