Chemistry Imperfections in Solids

Topics to be covered :

This lecture covers following topics :

● Imperfections in Solids
● Types of Point Defects
● Stoichiometric Defects
● Frenkel and Schottky Defects
● Impurity Defects
● Non-Stoichiometric Defects

Imperfections in Solids :

Defects : These are basically irregularities in the arrangement of constituent particles. Defects are of two types which are point defects and line defects.

Note : Defects in crystals are due to the fast or moderate speed of crystallisation.

Point Defects : This is the deviation from ideal structure around a point or atom in a solid.

Line Defects : This is the deviation from ideal structure in entire rows or lattice points. These irregularities are called crystal defects.

Types of Point Defects :

There are three types of defects :

(i) Stoichiometric Defects

(ii) Impurity Defects

(iii) Non-Stoichiometric Defects

Stoichiometric Defects :

In this, Stoichiometry of the solid does not change. They are called intrinsic or thermodynamic defects.

Types of Stoichiometric Defects :

(i) Vacancy Defect : This is due to the vacancy present in the crystal. Due to this, density of crystal decreases. This defect can develop when a substance is heated.

(ii) Interstitial Defect : In this, some constituent particles occupy an interstitial site. Due to this defect, density of the substance increases.

Note : (i) Both these defects are shown by non-ionic solids.

(ii) Ionic crystals always maintain electrical neutrality.

(iii) Vacancy and interstitial defects are shown as frankel and schottky defects.

Frenkel Defect :

(i) Shown by ionic compounds.

(ii) In this, smaller ion (usually cation) is dislocated from its normal site to an interstitial site.

(iii) It creats a vacancy defects at its original site and an interstitial defect at its new location.

(iv) Also called dislocation defect.

(v) Density of the solid remains same.

(vi) Shown by ionic compounds which have large difference in the size of ions.

(vi) e.g. `ZnS`, `AgCl`, `AgBr` and `AgI`

Schottky Defect :

(i) It is a vacancy defect.

(ii) Shown by ionic compounds.

(iii) In order to maintain electrical neutrality,

The no. of missing cations = no. of missing anion

(iv) Density of the solid decreases.

(v) No. of defects in solid is quite significant. e.g. there are `10^6` schottky pairs per `cm^3` at room temperature and in `1cm^3` there are `10^(22)` ions. So, there is one schottky defect per `10^(16)` ions.

(vi) Shown by ionic compounds in which cations and anions are of similar size.

(vii) e.g. `NaCl`, `KCl`, `CsCl` and `AgBr`

Note : `AgBr` shows both Frenkel and Schottky defects.

Impurity Defects :

If molten `NaCl` containing a little amount of `SrCl_2` is crystallised, some of the sites of `Na^(+)` ions are occupied by `Sr^(2+)` (Fig.1.27). Each `Sr^(2+)` replaces two `Na^(+)` ions. It occupies the site of one ion and the other site remains vacant. The cationic vacancies thus produced are equal in number to that of `Sr^(2+)` ions. Another similar example is the solid solution of `CdCl_2` and `AgCl`.

Non-Stoichiometric Defects :

These defects are of two types :

(i) Metal Excess Defect

(ii) Metal Deficiency Defect

(i) Metal Excess Defect :

(a) Metal Excess Defect due to Anionic Vacancies : Alkali 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`. Sodium atoms to lose electron to form `Na^(+)` ions. The released electrons diffuse into the crystal and occupy anionic sites (Fig. 1.28). As a result, the crystal 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 is due to the excitation of these electrons when they absorb energy from the visible light falling on the crystals. Similarly, excess of lithium makes `LiCl` crystals pink and excess of potassium makes `KCl` crystals violet (or lilac).

(b) Metal Excess Defect due to the presence of Extra Cations at Interstitial Sites : Zinc oxide is white in colour at room temperature. On heating it loses oxygen and turns yellow

`ZnO oversettext(heating)→ Zn^(2+) +1/2 O_2 +2 e^(-)`

Due to this, there is excess of zinc in the crystal and its formula becomes `Zn_(1+x)O`. The excess `Zn^(2+)` ions move to interstitial sites and the electrons to neighbouring interstitial sites.

(ii) Metal Deficiency Defect : Many solids are difficult to prepare in the stoichiometric composition and contain less amount of the metal as compared to the stoichiometric proportion. Example is `FeO` which is mostly found with a composition of `Fe_(0.95) O`. It may range from `Fe_(0.93) O` to `Fe_(0.96)O`. In crystals of `FeO`, loss of positive charge (`Fe^(2+)`) is made up by the presence of required number of `Fe^(3+)` ions.