Introducing impurity defect in ionic solids

If an imperfection causes the ratio of cations to anions to become different from that indicated by the ideal chemical formula, the defect is called non-stoichiometric. There are three types:

Metal excess

Metal deficiency

Impurity defects.

Metal excess

This may occur in either of the following two ways

1. By anion vacancies

A negative ion may be missing from its lattice site, leaving a hole, which is occupied by an electron thereby maintaining an electrical balance. The trapped electrons are called F-centers or color centers because they are responsible for imparting color to the crystal.
Example: NaCl when heated in Na vapor, the excess Na deposits on the surface. Cl^- diffuse to the surface where they combine with the Na atoms, which lose the electrons. The electrons diffuse into the vacant site created. The electrons absorb some energy from the white light and re-emit the yellow color. Excess of Li in LiCl gives a pink color. Excess of K in KCl makes it violet. This defect is found in crystals having Schottky defects.

fig 2.32 - Metal excess defect due to anion vacancy

fig 2.33 - Metal excess defect
caused by extra cation in interstitial position

2. By presence of extra cations in the interstitial sites

Extra cations occupying interstitial sites with electrons present in another interstitial site to maintain electrical neutrality can cause metal excess. This defect is similar to Frenkel defect and is formed in crystals having Frenkel defects.
Example: If ZnO is heated, it loses oxygen and turns yellow.

The excess Zn²⁺ ions thus formed get trapped into the vacant interstitial sites and the electrons in the neighboring interstitial sites. It turns yellow when hot and white when cold. Crystals with either type of metal excess act as semiconductors.

Metal deficiency

fig 2.34 - Metal deficiency defect due to missing of a cation of lower valency and presence of a cation of higher valency Occurs when metals show variable valency.
Example: Transition elements It occurs due to the missing of a cation from its lattice site and presence of the cation with higher charge (e.g., +2 instead of +1) in the adjacent site.
Examples: FeO, FeS and NiO.These defects arise when foreign atoms are present at the lattice site (in place of host atoms) or at the vacant interstitial sites. The formation of former depends upon the electronic structure of the impurity while that of the latter on the size of the impurity.

Introducing impurity defect in covalent solids

Group 13 elements such as Ga and Al and group 15 elements such as P and As can enter the crystal structure of group 14 elements Ge or Si substitutionally. Group 15 elements have one excess valence electron as compared to Group 14 elements (Si or Ge). Therefore after forming four covalent bonds, one electron remains excess which give rise to electrical conduction.
Group 13 elements have one valence electron less compared to group 14 elements leading to electron deficient bond or a hole. Such holes can move across the crystal giving rise to electrical conductivity. Group 14 doped with group 15 elements are called n-type semiconductors - (n) - negative charge flow. Group 14 doped with group 13 elements are called p - type semiconductors - (P) - Positive hole movement.

Introducing impurity defect in ionic solids

In case of ionic solids, the impurities are introduced by adding impurity of ions. If the impurity ions are in a different oxidation state from that of the host ions, vacancies are created.
For e.g., If molten NaCl, containing a little of SrCl₂ as impurity is allowed to cool, in the crystals of NaCl formed, at some lattice sites Na⁺ ions are substituted by Sr²⁺ ion. For every Sr²⁺ thus introduced two Na⁺ ions are removed to maintain electrical neutrality.