# Interstices in Crystals

Interstices in Crystals

It has been shown that the particles are closely packed in the crystals even then there is some empty space left in between the spheres. This is known as interstices (or interstitial site of holes or empty space or voids).

The interstices are of two types:

(i) Tetrahedral-interstices: We have seen that in hexagonal close packing (hcp) and cubic close packing (ccp) each sphere of second layer touches three spheres of first layer. Thus they, leave a small space in between which is known as tetrahedral site or interstices. Since a sphere touches three spheres in the lower layer and three spheres in the upper layer hence there are two tetrahedral sites associated with one sphere. It may be noted that a tetrahedral site does not mean that the site is tetrahedral in geometry.

But it means that this site is surrounded by four spheres and the centres of these four spheres lie at the apices of a regular tetrahedron.

(ii) Octahedral-interstices: Hexagonal close packing (hcp) and cubic close packing (ccp) also form another type of interstices (or site) which is called octahedral site (or interstices). In the following figure two layers of close packed spheres are shown.

The spheres of first layer are shown by full circles while those of second layer by dotted circles. Two triangles are drawn by joining the centers of three touching spheres of both the layers. The apices of these triangles point in opposite directions.

On super-imposing these triangles on one another an octahedral site is created. It may be noted that an octahedral site does not mean that the hole is octahedral in shape but it means that this site is surrounded by six nearest neighbour lattice points arranged octahedrally.

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