Tetrahedral Void and Octahedral Void : Generally we can define the void as the unoccupied empty space in unit cell is called void or empty space in unit cell. Voids are also known as holes in unit cell. In three dimensional closed packing of constituent particle shows following two types of voids:
A tetrahedral void is formed when one sphere or particle is placed in the depression formed by three particles.
Definition of Tetrahedral Void: The vacant space or void among the four constituent particles having tetrahedral arrangement in the crystal lattice is called tetrahedral void.
The shape of this void is not tetrahedral but the arrangement of four spheres around the void is tetrahedral. The lines joining the centres of four atoms forms a tetrahedron. A tetrahedral void is formed when a triangular void made by three coplanar spheres is in contact with fourth sphere above or below it.
Characteristics of Tetrahedral Void:
- The volume of the void is much smaller than that of the atom or sphere.
- Larger the size of the sphere, more is the size of the void.
- If R is the radius of the constituent atom, then the radius of the tetrahedral void is 0.225 R.
- Coordination number of tetrahedral void is four.
- There are two tetrahedral voids per sphere, in the crystal lattice. If the number of closed packed sphere s in N then the number of tetrahedral voids is 2N.
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.
Definition of Octahedral Void: The vacant space or void at the centre of six spheres or atoms which are placed octahedrally is called octahedral void.
Characteristics of Octahedral Void:
- The volume of the void is small.
- There is one octahedral void at the body centre and twelve octahedral void positions at twelve edge centres.
- If R is the radius of constituent atom, then the radius of the octahedral void is 0.414 R.
- The coordination number of octahedral void is six.
- There is one octahedral void per sphere in the crystal lattice. If the number of closed packed spheres is N then the number of octahedral voids is N.