Question

Write the defect of non-stoichiometric compounds.

Answer 1

To answer this question, we have to know about the different types of defect in compounds. It is broadly divided into stoichiometric and non-stoichiometric defects. The defects which disturb the stoichiometry of the compounds are called non-stoichiometric defects. These defects are either due to presence of excess metal ions or deficiency of metal ions.

Complete step by step answer:
There are mainly four different types of non-stoichiometric defects. They all cause changes to the stoichiometric formula of the compound. They are:
1.Metal excess defect due to anion vacancy: A compound may have excess metal ions if an anion if missing from its lattice site, leaving a positive charged hole behind. It is sometimes occupied by an electron to maintain electrical neutrality. The holes occupied by electrons are called F-centres. They occur in compounds with Schottky defects. Ex – alkali metal halide
2.Metal excess defect due to interstitial cation: A compound may also have excess metal ions if an extra positive ion is present in the interstitial site. Electrical neutrality is maintained by the presence of electrons in an interstitial site. This defect occurs mainly in compounds with Frankel defect. Ex – zinc oxide.
3.Metal deficiency by cation vacancy: In this type of defect, the cation is missing from its lattice site. Electrical neutrality is maintained by the nearest metal ion acquiring extra positive charge. They occur in compounds where the metal can exhibit variable oxidation state. Ex – nickel oxide.
4.Metal deficiency due to extra anion in interstitial site: In this type of defect, an anion is present at the interstitial position. The electrical neutrality is maintained by one extra positive charge on the adjacent metal ion. This defect mainly occurs when the anion is smaller than the cation, which is very rare.

Note:
In the case of stoichiometric defects, electrical and stoichiometric neutrality is maintained. This means that if an anion is missing, then a cation will also be missing. In this way, equal amounts of cation and anion will be missing in the lattice. In case of non-stoichiometric defects, it is not so. If a cation is missing, then the adjacent cation will increase its charge to maintain neutrality, but there will be no change in the number of anions. This affects the stoichiometry of the compound. The free electrons or holes present are used to conduct electricity and thus compounds with non-stoichiometric defects act as semiconductors (n-type and p-type semiconductors respectively).