Chemistry: Solid State: Structure of Ionic Compounds and Defects in Ionic Crystals (For CBSE, ICSE, IAS, NET, NRA 2022)

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Structure of Ionic Compounds of Mx2 Type

Calcium Fluoride

  • In this structure the Ca2 + ions form a fcc arrangement.
  • The fluoride ions are located in the tetrahedral voids.
  • Some of the ionic compounds like Na2O have antifluorite structure.
  • In this structure the positions of cations and the anions in fluorite structures are interchanged. So, it is called as antifluorite structure.
  • In Na2O the oxide ions form the ccp and the sodium ions occupy the tetrahedral voids.
Calcium Fluoride

Defects in Ionic Crystals

  • The crystalline solid are constituent particles that are arranged in an ordered three-dimensional network.
  • Every crystal has some deviations from the perfect order. These deviations are called imperfections or defects.
  • These defects can be broadly grouped into two types.
  • These are stoichiometric and non-stoichiometric defects depending on these disturb the stoichiometry of the crystalline material.
  • In such compounds the numbers of positive and negative ions are in stoichiometric proportions.

There are two kinds of stoichiometric defects, they are:

  • Schottky defects
  • Frenkel defects

Schottky Defects

  • This type of defect are due to the absence of some positive and negative ions from their positions.
  • These unoccupied lattice sites are called as holes.
  • Such defects are found in ionic compounds in which the positive and negative ions are of similar size e. g. , NaCl and CsCl.
  • The number of missing positive and negative ions are equal.
  • The presence of Schottky defects decreases the density of the crystal.
Schottky Defects

Frenkel Defects

  • This type of defect occurs when some ions move from their lattice positions and occupy interstitial sites.
  • The interstitial sites refer to the positions in between the ions.
  • When the ion leaves its lattice site a hole is created there.
  • ZnS and AgBr are examples of Frenkel defects.
  • In these ionic compounds the positive and negative ions are of quite different sizes.
  • Generally, the positive ions leave their lattice positions, as these are smaller and can accommodate themselves in the interstitial sites.
  • The Frenkel defects do not change the density of the solids
Frenkel Defects
  • These defects cause the crystal to conduct electricity.
  • The conduction is due to the movement of ions into the holes.
  • When an ion moves into a hole it creates a new hole, which is occupied by another ion, and the process continues.

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