Defects in Solids

Atomic imperfections / point defect

When deviations exist from the regular arrangement around an atom or a group of atom in a crystalline substance the defects are called point defects.

Types of point defects

(A) Stoichiometric defects If imperfection in the crystal are such that the ratio between the cations and the ratio between the cations and anions remains the same as represented by the molecular formula, the defects are called Stoichiometric defects (for CSIR-NET Coaching in Chemistry, JAM Coaching in Chemistry

These are 2 types

1. Schottky defects In an ionic crystal of A⁺ B^- types equal number of cations and anions are missing from their lattice site so that the electrical neutrality is maintained it is called schottky defects. The schottky defects containing one pair of holes due to missing of one cation and one anion is shown in Fig 

Compound exhibiting schottky defects have

• High coordination number
• Small difference in size of cation and anions

Examples are NaCl, KCl, KBr, and CsCl

As a result of this defect, the density of solid decreases

2. Frenkel defect if an ion is missing from its lattice site (causing a vacancy or a hole these) and it occupies the interstitial site electrical neutrality as well as stoichiometry of the compound are maintained. This type of defect is called Frenkel defect

→ Frenkel defects is a hybrid type of defect as it a combination of schottky defect and interstitial defects.

→ The Frenkel defects in which one cation is missing from the lattice site and is occupying the interstitial site is shown in fig

Types of compounds exhibiting

compounds which have Low coordination number

Frenkel defects are found in silver halides due to small size of Ag⁺ ion.

Effect on Density

Since no ions are missing from the crystal as a whole, therefore density of the solid remains unchanged.

Non-Stoichiometric defects

If, as a result of the imperfections in the crystal, the ratio of the cations to the anion becomes different from that indicated by the ideal chemical formula, the defects are called non-stoichiometric defects.

E.g. It is difficult to prepare ferrous oxide with the ideal composition, FeO what is actually obtain is Feo, ₉₅O or Fe_xO where x = 0.93 to 0.96 These are 2 types

(A) Metal excess

(i) By anion Vacancies 

A negative ion may be missing from its lattice site, leaving a hole which is occupied by an electron for maintain the electrical balance (electrical neutrality)

The electrons thus trapped in the anion vacancies are called F-centers because they are responsible for imparting colour in the crystals (F- farbe which is a German word for colour) 

Eg. When NaCl is heated in an of Na vapour, the excess of Na atoms deposit on the surface of NaCl crystal. Cl^- ion then diffuse to the surface where they combine with Na atoms which become ionized by losing electrons. These electron defuse back into the crystal and occupy the vacant sites created by the Cl^- ions.

These electrons absorb some energy of the white light giving yellow colour to NaCl

Similarly excess of Li in LiCl makes it pink and excess of K in KCl makes it violet.

This defect is similar to schottky defect and is found in crystals having schottky defects.

(ii) By the presence of extra cation in the interstitial sites

Metal excess may also be caused by an extra cation occupying the interstitial site. Electrical neutrality is maintained by an electron present in another interstitial site. This defect is similar to frenkel defect and is found in crystal having frenkel defects.

Eg. When Zno is heated, it loses oxygen and turns yellow due to the following reach

Zno→Zn²⁺ + O₂ + ze^-

The excess of Zn²⁺ ion thus formed get trapped into the vacant interstitial sites and the electrons in the neighboring interstitial sites

Crystals with either type of metal excess defect contain some free electrons. Hence such materials acts as semiconductors.

(B) Metal deficiency

This defect occurs when the metal shows variable valiancy i.e. Transition metals. The defects usually occurs due to the missing of a cation from its lattice site and presence of cation having higher charge (eg. +2 instead of 1) in the adjacent lattice site.

Ex. FeO , Fes and NiO

The above mentioned topic is essential for CSIR-NET Coaching in Chemistry. It is also a hot topic for JAM Coaching in Chemistry