What is metal excess defects? Explain with example.

Answer: A metal excess defect is a form of point defect that can disrupt an ionic solid 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.

The nature of point defects might be stoichiometric or non-stoichiometric. The former does not affect the stoichiometry of ideal arrangement in crystal lattices, whereas the latter is known to distort both the stoichiometry and the ideal arrangement.

The metal surplus defect is an example of a non-stoichiometric defect. Which can happen for a variety of reasons, including the following:

1.Anionic vacancies cause metal excess defects: Alkali halides are ionic compounds with an ordered lattice. This flaw is caused by heating these solids in an environment of metallic vapour from the same metal that creates the alkali halide.

When NaCl is surrounded by Na vapours, for example, a deposition of these sodium atoms occurs on the crystal lattice’s surface, leading the Cl ions to escape and leave their respective positions inside the lattice and diffuse to the surface, resulting in an excess of cations inside the lattice. This diffusion causes chloride anions to combine with Na+ cations on the surface, as well as the simultaneous loss of electrons from sodium atoms, which then diffuse into the lattice and fill the empty sites left by Cl ions. F- centres are anionic sites that have been taken up by unpaired electrons and are responsible for colour transmission due to the stimulation of these free electrons and their subsequent fall back to lower energies in the visible spectrum.

Similar results can be seen with KCl and LiCl.

2.Excess metal in the crystal lattice due to the presence of additional cations at interstitial sites: Interstitial sites are spaces inside the crystal lattice that are not occupied by cations or anions. However, an excess of cations in the vicinity of an ionic solid causes the addition of a few cations to occupy interstitial spaces, resulting in metal overload.

Zinc oxide, for example, turns yellow from white when heated because oxygen is lost, resulting in an excess of zinc cations in the lattice.