ZnO crystal on heating acquires the formula Zn_{1 + x} O. Give reason.

OR

There is an increase in conductivity when Silicon is doped with Phosphorous. Give reason

ZnO crystal on heating acquires the formula Zn_{1 + x}O due to a metal excess defect. This defect is caused due to the presence of extra cations in interstitial sites.

Explanation:

The metal excess defect is a non - stoichiometric defect that occurs in crystals. A non-stoichiometric defect is a defect that does not affect the stoichiometry of the crystalline substance.

As we know ZnO crystals are white. When they are heated, they lose one oxygen atom and turn yellow.

ZnO → Zn^{2 +} + 1/2 O₂ + 2e ^-

Since the crystal has lost one oxygen atom, it now has an excess of zinc in it. The formula becomes Zn_{1 + x}O, and the excess Zn^{2 +} ions formed to occupy the interstitial (vacant or empty) sites in the crystal.

OR

This is due to the availability of delocalized electrons after doping Silicon with Phosphorous.

Explanation:

Doping is the process of adding an impurity to an intrinsic semiconductor like silicon to increase its conductivity.

Silicon belongs to group 14 and has four valence electrons. It is doped with an element like Phosphorous of group 15 which has five valence electrons. 4 out of 5 electrons of Phosphorous form covalent bonds with the 4 electrons of Silicon. The 5^th electron becomes delocalized. Electrons that are free to move in the entire structure are called a delocalized electron. These delocalized electrons increase the conductivity of the semiconductor. Such semiconductors are thus called n-type semiconductor.