Imperfections in Solids

In a crystalline solid, the atoms, ions and molecules are arranged in a definite repeating pattern, but some defects may occur in the pattern. Derivations from perfect arrangement may occur due to rapid cooling or presence of additional particles.

The defects are of two types, namely point defects and line defects

Point Defects:

Point defects are the irregularities or deviations from ideal arrangement around a point or an atom in a crystalline substance Point defects can be classified into three types:

· Stoichiometric defects

· Impurity defects

· Non–stoichiometric defects

Stoichiometric Defect

These are point defects that do not disturb the stoichiometric of the solid. They are also called intrinsic or thermodynamic defects.

In non-ionic solids, these are of two types,

· Vacancy Defect

· Interstitial Defect

In ionic solids, these are of two types,

· Frankel defect

· Schottky defect

Vacancy Defect:

When some of the lattice sites are vacant, the crystal is said to have vacancy defect. This results in decrease in density of the substance. This defect can also develop when a substance is heated.

Vacancy defects

Interstitial Defect:

When some constituent particles (atoms or molecules) occupy an interstitial site, the crystal is said to have interstitial defect. This defect increases the density of the substance.

Frenkel Defect:

This defect is shown by ionic solids. The smaller ion (usually cation) is dislocated from its normal site to an interstitial site. It creates a vacancy defect at its original site and an interstitial defect at its new location.

Frenkel defect is also called dislocation defect. It does not change the density of the solid. Frenkel defect is shown by ionic substance in which there is a large difference in the size of ions, for example, ZnS, AgCl, AgBr and AgI due to small size of Zn²⁺ and Ag⁺ ions.

Frenkel Defect

Schottky Defect:

It is basically a vacancy defect in ionic solids. In order to maintain electrical neutrality, the number of missing cations and anions are equal.

Like simple vacancy defect, Schottky defect also decreases the density of the substance. Number of such defects in ionic solids is quite significant. For example, in NaCl there are approximately 10⁶ Schottky pairs per cm³ at room temperature. In 1 cm³ there are about 10²² ions. Thus, there is one Schottky defect per 10¹⁶ ions. Schottky defect is shown by ionic substances in which the cation and anion are of almost similar sizes. For example, NaCl, KCl, CsCl and AgBr.

Schottky Defect

Impurity Defect

It arises when foreign atoms or ions aloe present in the lattice. In case of ionic compounds, the impurity 1S also ionic in nature. When the impurity has the same charge as the host ion. It just substitutes some of the host ions.

Impurity defects can also be introduced by adding impurity ions having different charge than host ions. E.g., molten NaCl containing a little amount of SrCl₂ is crystallised. In such cases,

Cationic vacancies produced = [number of cations of higher valence × Difference in valence of the host cation and cation of higher valence]

Non-Stoichiometric Defect

Non-stoichiometric crystals are those which do not obey the law of constant proportions.

The numbers of positive and negative ions present in such compounds are different from those expected from their ideal chemical formulae. However, the crystal as a whole in neutral. These defects are of two types:

· Metal excess defect

· Metal deficiency defect

Metal excess defect

Metal excess defect due to anionic vacancies: Alkyl halides like NaCl and KCl show this type of defect.

When crystals of NaCl are heated in an atmosphere of sodium vapour, the sodium atoms are deposited on the surface of the crystal. The Cl^– ions diffuse to the surface of the crystal and combine with Na atoms to give NaCl. This happens by loss of electron by sodium atoms to form Na⁺ ions. The released electrons diffuse into the crystal and occupy anionic sites. As a result the crystal now has an excess of sodium.

The anionic sites occupied by unpaired electrons are called F-centres (from the German word Farbenzenter for colour centre). They impart yellow colour to the crystals of NaCl. The colour results by excitation of these electrons when they absorb energy from the visible light falling on the crystals.

Metal deficiency defect due to cation vacancy: Metal excess defect due to presence of extra cations at interstitial sites, e.g., zinc oxide is white in colour at room temperature. On beating, it loses oxygen and turns yellow.

Now there is excess of zinc in the crystal and its formula becomes Zn_1+xO. The excess Zn²⁺ ions move to interstitial sites and the electrons to neighbouring interstitial sites.

Metal deficiency defect

· It is due to the absence of a metal ion from its lattice site and charge is balanced by ion having higher positive charge.

· Transition metals exhibit this defect, e.g., FeO, which is found in the composition range from Fe_0.93O to Fe_0.96O.

· In crystal of FeO, some Fe²⁺ cations are missing and the loss of positive charge is made up by the presence of required number of Fe³⁺ ions.