Chemistry notes for class 12-Solid State-Types of Voids in Solids

Voids are also known as holes in the unit cell.Voids in solid states mean the vacant space between the constituent particles in a closed packed structure.

Close packing in solids can be generally done in three ways: 1D close packing, 2D close packing, and 3D close packing. In 2 dimensional structures when the atoms are arranged in square close packing and hexagonal close packing, we see empty spaces left over between the atoms. In the case of hexagonal packing, these voids are in triangular shapes and are known as the triangular voids/ trigonal voids.

The void enclosed by three spheres/ particles in contact is called triangular voids/ trigonal voids. These voids and the spheres surrounding it are in the same plane. In this type of void, only a small sphere of radius 0.155 times of the bigger sphere can fit. Thus, in ionic crystals, the cation will occupy the trigonal voids enclosed by anions.

In three dimensional closed packing of constituent particle shows following two types of voids:

Tetrahedral Voids:

A tetrahedral void is formed when one sphere or particle is placed in the depression formed by three particles. The vacant space or void among the four constituent particles having a tetrahedral arrangement in the crystal lattice makes a tetrahedral void.

Characteristics of tetrahedral voids

The vacant space or void is surrounded by four atomic spheres. Hence co-ordination number of the tetrahedral void is 4.
The atom in the tetrahedral void is in contact with four atoms placed at four corners of a tetrahedron.
This void is formed when a triangular void made coplanar atoms is in contact with the fourth atom above or below it.
The volume of the void is much smaller than that of the spherical particle.
If R is the radius of the constituent spherical particle, then the radius of the tetrahedral void is 0.225 R.
If the number of close packed spheres is N, then the number of tetrahedral voids is 2N.

Octahedral Void:

An octahedral void is formed when three close-packed spheres forming an equilateral triangle, are placed over another set of three spheres, in opposite directions. The vacant space or void at the centre of six spheres or atoms forms an octahedral void.

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