## Unit – VI C |

## Characteristics of Voids & Packings

### Characteristics of Tetrahedral Voids:

- The vacant space or void is surrounded by four atomic spheres. Hence co-ordination number of tetrahedral void is 4.
- The atom in tetrahedral void is in contact with four atoms placed at four corners of tetrahedron.
- This void is formed when a triangular void made coplanar atoms is in contact with the fourth atom above or below it.
- The volume of the void is much smaller than that of the spherical particle.
- If R is the radius of the constituent spherical particle, then the radius of tetrahedral void is 0.225 R.
- If the number of closed packed spheres is N, then the number of tetrahedral voids is 2N.

### Characteristics of Octahedral Voids:

- The vacant space or void is surrounded by six atomic spheres. Hence, co-ordination number of tetrahedral void is 6.
- The atom in octahedral void is in contact with six atoms placed at six corners of octahedron.
- This void is formed when two sets of equilateral triangles pointing in opposite direction with six spheres..
- The volume of the void is small.
- If R is the radius of the constituent spherical particle, then the radius of octahedral void is 0.414 R.
- If the number of closed packed spheres is N, then the number of octahedral voids is N.

### Characteristics of Hexagonal Close Packing:

- In this three dimensional arrangement of unit cell, spheres of third layer are placed on triangular shaped tetrahedral voids of the second layer.
- In this arrangement the spheres of third layer lie exactly above the spheres of the first layer.
- The first layer and third layer of this arrangement are identical.
- The arrangement of hexagonal close packing is represented as ABAB type.

### Characteristics of Cubic Close Packing:

- In this three dimensional arrangement of unit cell, spheres of third layer are placed in the positions of tetrahedral voids having apices upward..
- In this arrangement the spheres of third layer do not lie exactly above the spheres of the first layer. Actually the sphere of fourth layer lie exactly above the spheres of the first layer.
- The first layer and third layer of this arrangement are different.
- The arrangement of cubic close packing is represented as ABCABC type.

### Characteristics of Crystal Lattice:

- Crystal lattice is a regular arrangement of constituent particles of a crystalline solids in three dimensional space.
- It consists of large number of unit cells.
- Crystal lattice is defined in terms of properties of unit cell.
- On break up it forms numerous unit cells.
- Crystal lattice can be obtained , handled and studied in laboratory during experiments.
- It is macroscopic property.

### Characteristics of Unit Cell:

- A unit cell is the smallest structural repeating unit of crystalline solid.
- It is fundamental unit of crystal lattice which possesses all the properties of the crystal.
- Unit cell defines fundamental properties crystal lattice.
- Unit cells are fundamental unit, hence can not be divided further.
- Unit cell is hypothetical concept Hence it can not be obtained during experiments.
- It is microscopic property.