Metal Excess Defect: Explanation, Types and Sample Questions

Metal Excess Defect is one of the flaws that is encountered in the process of crystal formations. Metal deficit defect is another such flaw. Due to flaws or defects in their crystal structures, these non-stoichiometric inorganic solids contain non-stoichiometric ratios of constituent elements. Metal Excess Defect are of two types: due to anionic vacancies and due to extra cations present. In this article, we will discuss types, examples, and sample questions related to the metal excess defect.

Keyterms: Metal Deficiency defect, Solids, Metal, Stoichiometry, Anions, Cations, Electron, Crystalline, Alkali metal, Frenkel defect, Ionic solid, Stoichiometry, Point defect

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What is Metal Excess Defect?

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A metal excess defect is a form of point defect that can disrupt an ionic solid's stoichiometry. Due to the absence of anions at specified places or the inclusion of extra cations, the ideal crystalline structure undergoes a non-stoichiometric distortion.

• Anionic vacancies and the presence of additional cations in the interstitial sites induce this problem.
• Anion vacancies are formed when alkali metal halides are heated in an alkali metal vapor environment. The anion that is formed then diffuses into the surface of the crystal, where it further joins metal cations that are newly formed.
• The metal atom loses one electron, which diffuses through the crystal, occupying the anionic vacancy site and forming F-centres.
• These F-centres produce distinct colors, such as the yellow color produced by NaCl. KCl produces a violet color, while HCl produces a pink color.
• Due to the existence of unpaired electrons at lattice locations, metal excess defects crystals are often paramagnetic.
• The trapped electron absorbs some component of white light for excitation from the ground state to the excited state when the crystal is bombarded with white light. Color is created as a result of this. F-centers are the name given to such sites. 
• Excess ions are accompanied by positive ion vacancies (the German word Farbe meaning "color"). These vacancies trap holes in the same way that anion vacancies capture electrons. V-centers are the color centers that are formed as a result of this process.
• Due to the presence of the free electrons in the structure, the crystals showing the metal excess defect conduct electricity and are known as semiconductors.

The video below explains this:

Imperfections in Electrical and Magnetic properties of Solids Detailed Video Explanation:

Also Read:

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Types of Metal Excess Defect

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Metal excess defects can be divided into two categories which are:

• Metal excess defect caused due to anionic vacancies

Alkali halides, such as NaCl and KCl, have anionic vacancies. The negative ion is missing from the lattice site in this defect, leaving behind a hole that is filled by an electron to preserve the electric balance. In the anionic vacancies, these electrons are then trapped.

For example - When NaCl is heated in a sodium (Na) vapor environment, excess sodium (Na) atoms deposit on the surface of the NaCl crystal, causing Cl– ions to diffuse into the surface and interact with the Na+ ions, resulting eventually in the loss of an electron.

• Extra cations at interstitial sites also result in a metal excess defect

When a compound is heated, it releases additional cations, which results in a fault or a defect. In crystals, these cations occupy interstitial sites, and an equal number of electrons are transferred to neighboring interstitial sites. Crystals with the Frenkel Defect, such as ZnO, might have extra cations present in the interstitial location.

For example- At room temperature, zinc oxide appears white; however, when it loses oxygen, it turns yellow. 

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Things to Remember

• Non-stoichiometric flaws are imperfections that disrupt the stoichiometry of the compounds. 
• These flaws are caused by either an excess of metal ions, metal excess defect, or a lack of metal ions, metal deficit defect.
• A metal excess defect is a form of point defect that can disrupt an ionic solid's stoichiometry. 
• Due to the absence of anions at specified places or the inclusion of extra cations, the ideal crystalline structure undergoes a non-stoichiometric distortion.
• Due to the existence of unpaired electrons at lattice locations, metal excess defects crystals are often paramagnetic.
• The crystals showing the metal excess defect conduct electricity and are known as semiconductors.

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Previous Year Questions

1. An element has a body centered cubic (bcc) structure with a cell edge of 288 pm. The atomic radiusis:...[NEET 2020]
2. The vacant space in bcc lattice unit cell is:...[NEET 2015]
3. The anions form hexagonal close packed (hep) lattice and the cations occupy 75% of octahedral voids. The formula of the compound is :...[NEET 2019]
4. In crystals of which of the following ionic compounds would you expect maximum distance between centres of cations and anions?..[NEET 1998]
5. The formula of the compound is...[NEET 2000]
6. The edge of the unit cell is 408pm. The diameter of the metal atom is….[NEET 2012]
7. The molar mass of the metal is :...[NEET 2013]
8. If the radius of the cation is 100pm, the radius of the anion (Y−) will be -...[NEET 2011]
9. The distance between two oppositely charged ions in the lattices is...[NEET 2010]
10. The density of element is (No. of atom in BCC ( Z ) = 2....[NEET 1996]
11. The formula of the compound is :-...[NEET 2004]
12. Cation and anion combines in a crystal to form following type of compound….[NEET 2000]
13. Avogadro number being 6.023×1023 mol−1, the density of CsBr is :…..[NEET 2006]
14. For orthorhombic system axial ratios are a=b=c and the axial angles are….[NEET 1991]
15. Identify the wrong statement among the following….[NEET 1997]