What are interstitial sites ? Discuss tetrahedral and octrahedral interstitial sites in a close packed arrangment.

Explanation

In close packing of identical spheres, spheres touch each other only at one point, there must be some empty space between them. This empty space between the spheres is known as void or interstital site.
The interstitial site between four identical spheres (three in one layer and one in the next layer) is called tetrahedral void or tetrahedral site. Radius ratio for tetrahedral site is
`r_("void")/(r_("sphere"))=0.225`
In c.c.p. for f.c.c. arrangement, centres of the eight small cubes represent the tetrahedral sites. For n atoms in c.c.p. arrangement, there are 2n tetrahedral sites.
The interstitial site between six identical spheres three in one layer and three in the next layer is known as octrahedral void. Radius ratio for octrahedral site is
`r_("void")/(r_("sphere"))=0.414`
In c.c.p. for f.c.c. arrangement, centres of the edges and centre of the cube represent the octahedral sites. For n atoms in c.c.p. arrangement, there are n octahedral sites.