The Solid State

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Imperfections in Solids or Crystal defects

Irregularity in the arrangement of constituent particles in solids is called crystal defect or imperfection in solids.

There are two types of crystal defects - Point Defects and Line Defects.

Point Defects: Irregularities or deviation from the ideal arrangement of constituent particles around the point or atom in a crystalline solid is known as point defects.

Line Defects: Irregularities or deviation from the ideal arrangement of constituent particles in an entire row of the lattice is known as line defects.

This defect occurs in ionic compounds. It arises when foreign atoms are present in lattice site.

If molten NaCl containing a little amount of SrCl₂ is crystallised, some of the sites of Na+ ions are occupied by Sr²⁺.Each Sr²⁺ 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 Sr²⁺ ions. Another similar example is the solid solution of CdCl2 and AgCl.

The defects in which stoichiometry of the compound is disturbed.

There are of two types:

1. Metal excess defect
2. Metal deficiency defect.

I) Metal excess defects due to anionic vacancies:

1. Alkali halides like NaCl and KCl show this type of defect.
2. The anionic vacancies are occupied by unpaired electrons called F-centres (from the German word Farbenzenter for colour centre/point).
3. Example: They impart a yellow colour to the crystal of NaCl, Excess of lithium makes LiCl crystals pink, Excess of potassium makes KCl crystals violet

II) Metal excess defects due to the presence of interstitial cations:

1. Zinc oxide is white in colour at room temperature. On heating, it loses oxygen and turns yellow.
   
   
   
   
2. The excess Zn²⁺ ions move to interstitial sites and the electrons to neighbouring interstitial sites.

Metal deficiency due to the absence of cations: Positive ions missing from their lattice sites. Extra negative charge is balanced by some neary metal ions with higher positive charge. e.g. FeO, FeS.

A typical example of this type is FeO which is mostly found with a composition of Fe_0.95O. It may actually range from Fe_0.93O to Fe_0.96O. In crystals of FeO some Fe²⁺ cations are missing and the loss of positive charge is made up by the presence of the required number of Fe³⁺ ions.

Point Defects: Point Defects are divided into three types:

1. Stoichiometric Defects
2. Impurities Defects
3. Non-stoichiometric Defects

Stoichiometric Defects: – It is a type of point defects which does not disturb the stoichiometry of solid. This is also known as Intrinsic or Thermodynamic Defects.

Types of stoichiometric defects: Vacancy Defects, Interstitial defects, Frenkel Defects, Schottky Defects.

Vacancy Defects	Interstitial Defects
When some lattice sites left vacant while the formation of the crystal, the defect is called Vacancy Defects.	Sometime in the formation of the lattice structure, some of the atoms occupy the interstitial site, the defect arising because of this is called Interstitial Defects.
In vacancy defects, an atom is missing from its regular atomic site. Because of missing of the atom the density of substance decreases, i.e. because of vacancy defects.	In interstitial defect, some atoms occupy sites at which; generally there is no atom in the crystal structure. Because of the interstitial defects, the number of atoms becomes larger than the number of lattice sites.
The vacancy defect develops on heating of a substance.	Increase in a number of atoms increases the density of the substance, i.e. interstitial defects increase the density of the substance.

The vacancy defects and interstitial defects are found only in non-ionic compounds. Such defects found in ionic compounds are known as Frenkel Defects and Schottky Defects.