Question

What are voids ?
What are tetrahedral and octahedral voids ?

Answer 1

Answer:

The crystalline solids are characterized by their space lattices and close packing of atoms. However, these structures have some gaps in their arrangement we call voids. Many scientists believe that this spacing between atoms is of crucial importance to the properties of the substance. So let us study tetrahedral voids and octahedral voids.

Tetrahedral Voids

Tetrahedral voids and octahedral voids(Source: Chemistrystackexchange)

Now there are two types of three-dimensional close packing in crystals. One such type is Cubic Close packing. Here the two-dimensional structures are stacked in a specific alignment. The layers alternate with each other. So the second layer will be located in the depression of the first layer.

You will notice that a triangle-shaped void is seen in this type of alignment. The sphere which is in the depression will leave a void between itself and the sphere in the layer above. This void is the Tetrahedral Void.

There is a simple way to calculate the number of Tetrahedral Voids in a lattice. Here if the number of spheres (i.e. unit cells) is said to be “n”, then the number of voids will be twice as many. So the number of tetrahedral voids will be “2n”.

The void is much smaller than the sphere, i.e. it has a smaller volume. And the coordination number of a tetrahedral void is four because of the void forms at the center of four spheres.

Octahedral Voids

If you observe a three-dimensional structure of a crystal lattice you will observe the gaps in between the spheres. These are the voids. As you see that tetrahedral voids are triangular in shape. When two such voids combine, from two different layers they form an octahedral void.

So when the tetrahedral void of the first layer and the tetrahedral void of the second layer align together, they form an octahedral void. Here the void forms at the center of six spheres. So we say the coordination number of an octahedral void is six.

To calculate octahedral void, if the number of spheres in a structure is “n”, then the number of octahedral voids will also be the same. i.e. “n”.

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Explanation:

Answer 2

                    Voids

The vacant space in a closed packed arrangement are called voids.There are two types of Voids (i)  Tetrahedral voids (ii)  octahedral voids.

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       Tetrahedral void

A void Surrounded by four spheres in tetrahedral positions are called tetrahedral voids.

(i)  It is a triangular void

(ii) It is surrounded by 4 sphere

(iii) Can be observed in the edges of the unit cell.

(iv) Coordination number is four.

(v) Number of tetrahedral void is 2n or  twice the number of octahedral voids.

       Octahedral voids

A void surrounded by 6 spheres in octahedral positions are called octahedral voids.

(i)  It is surrounded by 6 spheres

(ii) Can be observed in the centre of the unit cells

(iii) Coordination number is six.

(iv) Number of octahedral void is n or half the number of tetrahedral voids.