Imperfections in Solids: Defects in Crystals

Atomic imperfections / point defects:

When deviations exist from the regular (or periodic) arrangements around an atom or a group of atoms in a crystalline substance, the defects are called point defects.

Type of point defects – point defects in a crystal may be classified into three types:

(a) Stoichiometric defects

(b) Non – stoichiometry defects

(c) Impurity defects

Stoichiometry defect:

The compounds in which the number of cation and anions are exactly in the same ratio as represented by their chemical formula are called stoichiometric compounds. The defects that do not disturb the ratio of cations and anions are called stoichiometric defect.

These are of two types:

1. Schottky defect

If in an ionic crystal of the type A+ B-, equal number of cations and anions are missing from their lattice. It is called Schottky defect.

This type of defect is shown by highly ionic compounds which have

(i) High Co–ordination number and

(ii) Small difference in the sizes of cations and anions

A few examples of ionic compounds exhibiting Schottky defect are NaCl, KCl, KBr and CsCl

Consequences of schottky defect

(a) As the number of ions decreases as a result of this defect, the mass decreases whereas the volume remains the same. Hence density of the solid decreases

(b) The crystal begins to conduct electricity to a small extent by ionic mechanism

(c) The presence of too many voids lowers lattice energy and the stability of the crystal

2. Frenkel defect

If an ion is missing from its correct lattice sites (causing a vacancy or a hole) and occupies an interstitial site, electrical neutrality as well as stoichiometry of the compounds are maintained. This type of defect is called Frenkel defect. Since cations are usually smaller it is more common to find the cations occupying interstitial sites.

This type of defect is present in ionic compounds which have

(i) Low co ordinations number

(ii) Larger difference in size of cation and anions

(iii) Compounds having highly polarising cation and easily polarisable anion. A few examples of ionic compounds exhibiting this defect are AgCl, AgBr, AgI, ZnS etc.

Consequences of Frenkel defect

(a) As no ions are missing from the crystal lattice as a whole, therefore density of the solid remains the same

(b) The closeness of like charges tends to increases the dielectric constant of the crystal

(c) The crystal conducts electricity to a small extent by ionic mechanism

Non – stoichiometric defects:

If as a result of imperfection, the ratio of number of cation to anion becomes different from that indicated by the ideal chemical formula, the defects are called non – stoichiometric defects. These defects arise either due to excess of metal atoms or non metal atom or presence of impurities / foreign particle.

(a) Metal excess defects due to anion vacancies: -

A compound may have excess metal ion if a negative ion is absent from its lattice site leaving a hole which is occupied by electron to maintain neutrality.

The holes that are occupied by electrons are called ‘F’ centres (or colour centres) and are responsible for the colour of the compound and many interesting properties.

(i) Metal excess defects due to interstitial cations

Metal excess may also be caused by an extra cation (positive ion) present in an interstitial site. Electrical neutrality is maintained by presence of an electron in another interstitial site. This defect is similar to Frenkel defect and is found in crystals having Frenkel defects.

(ii) Metal deficiency due to cation vacancies

The non-stoichiometric compounds may have metal deficiency due to the absence of a metal from its lattice site. The charge is balanced by an adjacent ion having higher positive charge. This type of defects are generally shown by compounds of transition elements.

(b) Point defects due to the presence of foreign atoms:-

These defects arise when foreign atoms are present at the lattice site (in place of host atoms) or at the vacant interstitial sites. In the former case, we get substitutional solid solutions. The formation of former depends upon the electronic structure of impurity while that of later on the size of impurity.