Question

What is a metal excess defect? Explain with examples

Answer 1

Metal excess defect is a type of point defect that is capable of disturbing the stoichiometry of an ionic solid. The perfect crystalline structure undergoes a non-stoichiometric distortion due to the absence of anions at specific positions or addition of extra cations.

Complete answer: Crystalline solids tend to have a perfectly ordered arrangement of their constituent particles which is why they are said to have a long range as well as a short range order. However various factors can contribute to bringing irregularities to their otherwise perfect lattices.
Point defects are the deviations or irregularities from a perfect and ordered arrangement due to certain atoms (points of the lattice) being displaced, removed or added at specific positions.
Point defects can be either stoichiometric or non-stoichiometric in nature. While the former does not disturb the stoichiometry of ideal arrangement present in crystal lattices, the latter is known to distort the stoichiometry as well as the ideal arrangement.
One such non-stoichiometric defect is the metal excess defect. Which can occur due to two possible reasons given as follows:
1.Metal excess defect due to anionic vacancies: Alkali halides are ionic compounds with an ordered lattice. However heating of these solids in an atmosphere of metallic vapour of the same metal that forms the alkali halide causes this defect.
For example, when $NaCl$ gets surrounded by $Na$ vapours , then there comes a point at which a deposition of these sodium atoms is observed on the surface of the crystal lattice which causes the $C{l^ - }$ ions to escape and vacate their respective position inside the lattice and diffuse to the surface causing an excess of cations inside the lattice. This diffusion leads to the combining of chloride anions with $N{a^ + }$ cations on the surface and the simultaneous loss of electrons from sodium atoms which subsequently diffuse into the lattice and occupy the vacant positions of $C{l^ - }$ ions. These anionic sites taken over by unpaired electrons are known as F- centres which are responsible for imparting colours due the excitation of these free electrons and falling back to lower energies in the visible spectrum.

Similar effects are also observed in $KCl$ and $LiCl$.

2.Metal excess defect due to the presence of extra cations at interstitial sites: Interstitial sites are gaps inside the crystal lattice that are not occupied by either cations or anions. But the presence of an excess of cations in the vicinity of an ionic solid leads to the addition of a few cations that occupy interstitial sites and lead to metal excess.
For example, Zinc oxide upon heating turns yellow from white due to loss of oxygen resulting in an excess of zinc cations in the lattice.
$ZnO\xrightarrow{{heating}}Z{n^{2 + }} + \dfrac{1}{2}{O_2} + 2{e^ - }$
Here the formula due to excess of zinc becomes $Z{n_{1 + x}}O$.

Note:
Metal excess does not necessarily mean that the metal needs to be added inside the lattice to result in an irregular arrangement. This defect can also be brought about by creating vacant sites by the removal of atoms or ions other than the metal itself.