Metal Deficiency Defect

Point defects are called zero dimensional defects, termed this way as they are considered dimensionless and unit less quantities. Point defects occur when one or more than one atom of an ionic compound leaves their original lattice position or another atom binds in that space of the crystal. It is further divided into stoichiometric and non-stoichiometric defects. Stoichiometric defects are those that do not change the ratio existing between anions and cations in the structure. In non-stoichiometric defects, the ratio between anions and cations are altered by removing or adding ions in a compound. The metal deficiency defect comes under the non-stoichiometric defect.

Classification of Point Defect:

Point defects are divided into stoichiometric and non-stoichiometric defects which are further divided.

• Stoichiometric Defect 

• Schottky Defect
• Frenkel Defect 

•  Non-Stoichiometric Defect

• Metal Excess Defect
• Metal Deficiency Defect

In this article, we will learn about the metal deficiency defect.

Metal Deficiency Defect

This comes under the non-stoichiometric point defect. As we know, in non-stoichiometric defects, the ratio between anions and cations are altered by the removal or addition of ions in a compound. 

The metal deficiency defect is seen when variable valencies are shown by a metal. Some cations leave from the lattice site. When this happens, cations increase their valency in the compound to maintain its electrical neutrality. 

Transition metals like nickel oxide, iron sulphide, and ferrous oxide are some compounds which exhibit variable valencies. Due to the ability to accept excess positive charges, transition metals display the metal deficiency defect.

Types of Metal Deficiency Defect

There are two types under metal deficiency defects:

• Cation Vacancies
• Extra Anions Occupying the Interstitial Sites

Cation Vacancies

This happens when one of the positive ions leaves from a lattice site, and the formed additional negative charge is balanced through some close metal ions that cause it to receive extra charges in the place of the real charge. This happens to maintain electrical neutrality in the compound. Usually cation vacancies are seen in transition metals like Nickel oxide, iron sulphide, and ferrous oxide. These compounds have variable valencies.

Ferrous Oxide (FeO) shows metal shortage. This metal shortage happens because the constructive ion is smaller in size than the negative ions present in the crystal. FeO seems to have black coloured crystals which are impenetrable in alcohol, alkali, and water. It can be fully dissolved in acid. In this metal, there is a lack of a cation from its lattice site.

In some other cases, the positive ions can be missing from their lattice site. Positive ions will be obtained by one of the adjacent metal ions to hold electric neutrality. The explanation for variable oxidation states is provided by this kind of deficiency. From the above example, it is easy to understand that all the point defects affect the formation of vacancies in the crystal lattice of a compound. More vacancies lower the density of the compound and serve as the reason for the fractional fall down of the lattice.

Extra Anions Occupation

Under this defect, we can observe that the additional positive charges are in interstitial sites. The additional negative charge is balanced through some close metal ions that cause it to receive extra charges in the place of the real charge. The electrical neutrality of the compound is maintained in this way. 

As we said before, this defect is not common. This is because when comparing the size of negative ions and size of interstitial sites, negative ions are normally big, and these negative ions can’t be placed in the interstitial sites.

When talking about the metal deficiency defect, then we also have to give a brief description of the metal excess defect. 

Metal Excess Defect

The metal excess defect is also a non-stoichiometric defect. Due to anionic vacancies, the presence of cations are seen in the interstitial sites. This is termed the metal excess defect. 

Differences between Metal Deficiency Defect and Metal Excess Defect

Conclusion

In this article, we clearly described the meaning of the metal deficiency defect. We also explained the classification of point defects, learning that they are divided into stoichiometric and non-stoichiometric defects. The metal deficiency defect falls under the non-stoichiometric defect, which is further divided into cation vacancies and extra anions occupying the interstitial sites. Nickel oxide, iron sulphide, and ferrous oxide are some compounds which exhibit variable valencies and are examples of the metal deficiency defect. The difference between the metal deficiency defect and the metal excess defect was also discussed.