Metal Excess Defect in Zinc Oxide ZnO

The video tutorial explains the concept of metal excess defect, which occurs when excess cations and free electrons occupy interstitial positions in a crystal to maintain electrical neutrality. This defect imparts color to the crystal and enhances its electrical conductivity. The tutorial uses zinc oxide as an example, describing how it changes color from white to yellow upon heating due to the movement of zinc ions and free electrons. When light falls on these heated crystals, the free electrons absorb and emit yellow light, causing the yellow coloration.

What role do free electrons play in metal excess defects?

• They replace cations in the crystal lattice.

• They cause the crystal to become brittle.

• They occupy interstitial positions to maintain electrical neutrality.

• They are responsible for the crystal's hardness.

How does the presence of metal excess defects affect the electrical conductivity of a crystal?

• It has no effect on conductivity.

• It decreases the conductivity.

• It makes the crystal non-conductive.

• It increases the conductivity.

What happens to zinc oxide crystals when they are heated?

• They turn yellow.

• They remain white.

• They become transparent.

• They turn blue.

What causes the yellow coloration in heated zinc oxide crystals?

• Emission of yellow light by free electrons.

• Absorption of ultraviolet light by zinc ions.

• Reflection of light by the crystal surface.

• Presence of impurities in the crystal.

What happens to the free electrons in zinc oxide crystals when light falls on them?

• They get excited and emit yellow light.

• They emit blue light.

• They are absorbed by the crystal lattice.

• They cause the crystal to expand.