The key difference between metal excess defect and metal deficiency defect is that metal excess defect is caused by anionic vacancies and extra cations in the interstitial sites whereas metal deficiency defect is caused by cationic vacancies and extra anions in the interstitial sites.
Metal excess defect and metal deficiency defect are two types of defects we can observe in crystal lattices of some substances. These defects arise due to the presence or absence of cations or anions in the crystal lattices.
CONTENTS
1. Overview and Key Difference
2. What is Metal Excess Defect
3. What is Metal Deficiency Defect
4. Side by Side Comparison – Metal Excess Defect vs Metal Deficiency Defect in Tabular Form
5. Summary
What is Metal Excess Defect?
A metal excess defect is a type of crystal defect that occurs in crystal lattices. Either an anionic vacancy or an extra cation causes this defect. Due to the presence of these defects in non-stoichiometric inorganic solids, these solids contain constituent elements in a non-stoichiometric ration.
When we heat alkali metal halides that are placed in an atmosphere containing the vapour of the alkali metal, it causes the formation of anion vacancies. Then these anions tend to diffuse to the surface of the crystal and combine with newly generated metal cations. Here, an electron is lost from the metal atom, followed by the diffusion of the atom from the crystal to occupy the anionic vacancy site, forming an F-center inside the crystal. The F-centers that form in the crystal can give different colours to the crystal lattice. E.g. sodium chloride – yellow colour.
There are two different types of metal excess defects:
- Metal Excess Defect due to Anionic Vacancy
We can find this type of metal excess defects in alkali halides such as sodium chloride and potassium chloride. These defects include the loss of negative ions from the lattice site, which leaves behind a hole that is occupied by an electron in order to maintain the electric balance of the crystal lattice. These electrons then tend to trap in the anionic vacant sites of the crystal.
- Metal Excess Defect due to Extra Cations
This type of metal excess defects forms when heating crystal compounds, where they release extra cations. These cations tend to occupy the interstitial sites of the crystal lattice. Along with this cation formation, the released electrons (from cations) go to the neighbouring interstitial sites. An example of a substance that can bear this type of defect is ZnO, zinc oxide.
What is Metal Deficiency Defect?
Metal deficiency defect is a type of crystal defect that occurs in crystal lattices in which either a cation vacancy or an extra anion causes the defect. This type of metal defects can be observed in metal complexes having a variable valency. There are two types:
- Metal Deficiency Defect due to Cation Vacancy
In this type of defects, a cation is missing from its lattice site; therefore, the extra negative charge is balanced by getting two positive charges instead of one charge. These defects occur mainly in compounds having variable oxidation states. E.g. nickel oxide.
- Metal Deficiency Defect due to Presence of Extra Cations
In these crystal lattices, the extra anions occur at the interstitial sites, and the adjacent ions at another interstitial site help to maintain the electrical neutrality of the lattice. This type of very rare.
What is the Difference Between Metal Excess Defect and Metal Deficiency Defect?
Metal excess defect and metal deficiency defect are two types of defects we can observe in crystal lattices of some substances. The key difference between metal excess defect and metal deficiency defect is that metal excess defect is caused by anionic vacancies and by extra cations in the interstitial sites whereas metal deficiency defect is caused by cationic vacancies and by extra anions in the interstitial sites.
Below is a summary tabulation of the difference between metal excess defect and metal deficiency defect.
Summary – Metal Excess Defect vs Metal Deficiency Defect
Metal defects arise due to the presence or absence of cations or anions in the crystal lattices. The key difference between metal excess defect and metal deficiency defect is that metal excess defect is caused by the anionic vacancies and by the extra cations in the interstitial sites whereas metal deficiency defect is caused by the cationic vacancies and by the extra anions in the interstitial sites.
Reference:
1.“What Metal Excess Defects Explain With Example.” BYJUS, 23 Nov. 2020, Available here.
Image Courtesy:
1. “Point defects in crystal structures” By DaniFeri – Own work (CC BY-SA 3.0) via Commons Wikimedia