8.1 Metal Excess Defect

1.1 Difference between Crystalline and Amorphous Solids
1.2 Space Lattice and Unit Cell

2.1 Seven Crystal Systems and Fourteen Bravais Lattice

5.1 Radius Ratio Rules

8.1 Metal Excess Defect
8.2 Metal Deficiency Defect

9.1 Amorphous Solids

10.1 Types of Silicates

This may occur in two different ways:

$1.$ $F-$Centres:

A negative ion may be absent from its lattice site leaving a hole which is occupied by an electron, thereby maintaining the electrical balance. This type of defect is formed by crystals which would be expected to form Schottky defects.

When compounds such as $NaCl$, $KCl$, are heated with excess of their constituent metal vapours, or treated with high energy radiation, they become deficient in the negative ions and their formulae may be represented by $AX_{1–\delta}$, where $\delta $ is a small fraction.

The crystal lattice has vacant anion sites which are occupied by electrons. Anion sites occupied by electrons in this way are called $F$ centres ($F$ is an abbreviation Farbe, the German word for colour).

$2.$ Interstitial ions and electrons:

Metal excess defects also occur when an extra positive ion occupies an interstitial position in the lattice and electrical neutrality is maintained by the inclusion of an interstitial electron. Their composition may be represented by general formula ${A_{1 + \delta }}X$.

This kind of metal excess defect is much more common than the first and is formed in crystals which would be expected to form Frenkel defects. Examples include $ZnO$, $CdO$, $Fe_2O_3$.

Crystals with either type of metal excess defect contain free electrons, and if these migrate they conduct an electric current. These free electrons may be excited to higher energy levels, giving absorption spectra and in consequence their compounds are often coloured e.g. non-stoichiometric $NaCl$ is yellow, non-stoichiometric $KCl$ is lilac.