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Voids literally mean gaps between the constituent particles. 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. These empty spaces are called voids and in the case of hexagonal packing, these voids are in triangular shapes and are known as theĀ triangular voids.

Packing of Spheres in 2 dimension

These triangular voids in hexagonal packing are shown in two orientations. We can see that in one row, the apex of the triangle points in an upward direction and in the other row it points in the downward direction.

In the three-dimensional structure, both in CCP and HCP close packing in solids, about 26% of total space is vacant and is not covered by spheres. These empty spaces are known as the interstitial voids, interstices or holes. The above voids in solids depend on the number of spheres.

There are two types of interstitial voids in a 3D structure:

  1. Tetrahedral voids: In cubic close packed structure, spheres of the second layer lie above the triangular voids of the first layer. Each sphere touches the three spheres of the first layer. By joining the centre of these four spheres, it forms a tetrahedron and the empty space left over by joining the centre of these spheres forms a tetrahedral void.

In closed packed structure the number of tetrahedral voids is two times the number of spheres.

Let the number of spheres be n.

Then the number of tetrahedral voids will be 2n.

  1. Octahedral voids: Adjacent to tetrahedral voids you can find octahedral voids. So now, what are Octahedral Voids? When the triangular voids of the first layer coincides with the triangular voids of the layer above or below it, we get a void that is formed by enclosing six spheres. This vacant space formed by combining the triangular voids of the first layer and that of the second layer are called Octahedral Voids.

If the number of spheres in a close-packed structure is n, then the number of octahedral voids will be n.