Question

When ZnO is heated it turns yellow and returns back to original white colour on cooling. What could be the reason ?

Answer 1

When colourless ZnO is strongly heated, the metal atoms are deposited on crystal surface and anions O^2- migrate to the surface producing vacancies at anion lattice points.

These anions combine with Zn atoms forming

ZnO and release electrons.

Zn + O^2- \(\rightarrow\) ZnO + 2^e-

These released electrons diffuse into the crystal and occupy vacant sites of anions and produce F- centres. Due to these colour centres, ZnO turns yellow.

Let a small quantity of phosphorus be doped into pure silicon.