Explain the metal excess defect.

(N/A) Metal excess defect is of two types:
$(i)$ Metal excess defect due to anionic vacancies: Alkali halides like $NaCl$ and $KCl$ show this type of defect. When crystals of $NaCl$ are heated in an atmosphere of $Na$ vapor,$Na$ atoms are deposited on the surface of the crystal. $Cl^-$ ions diffuse to the surface of the crystal and combine with $Na$ atoms to give $NaCl$.
This happens by loss of electrons by $Na$ atoms to form $Na^+$ ions. The released electrons diffuse into the crystal and occupy anionic sites. As a result,the crystal now has an excess of $Na$.
Anionic sites occupied by unpaired electrons are called $F$-centers (from the German word $Farbenzenter$ meaning color center). They impart yellow color to the crystals of $NaCl$.
This color results from the excitation of these electrons when they absorb energy from the incident visible light. Similarly,excess $Li$ makes $LiCl$ crystals pink and excess $K$ makes $KCl$ crystals violet (or lilac).
$(ii)$ Metal excess defect due to the presence of extra cations at interstitial sites: This defect arises due to the presence of extra metal ions in the interstitial sites.
$ZnO$ is white in color at room temperature. On heating,it loses oxygen and turns yellow. The reaction is: $ZnO \xrightarrow{\Delta} Zn^{2+} + \frac{1}{2}O_2 + 2e^-$.
Now,there is an excess of zinc in the crystal and its formula becomes $Zn_{1+x}O$. The extra $Zn^{2+}$ ions move to interstitial sites and the electrons move to neighboring interstitial sites.