When ZnO is heated it turns yellow and returns back to original white

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

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

Answer»

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.

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

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