Types of Unit Cells

There are three types of non-primitive unit cells.

Taking the example of a cubic unit cell we can see that there are three types. They are:

fig 2.2 - Types of unit cells

Face centred cubic

The particles are present not only at the corners but also at the centre of each face of the unit cell.

Body centred cubic

In addition to the particles at the corner of the cube, there is one present at the body centre position of the unit cell.

Another type of unit cell, called end centered unit cell, is possible for orthorhombic and monoclinic crystal types. In an end centered unit cell, there are lattice points in the face centers of only one set of opposite faces, in addition to the lattice points at the corners of the unit cell.

It may be noted that all the four types of unit cells are not possible for each crystal class i.e., simply face centered, end centered and body centered.

Although each crystal system is expected to have four different unit cells, but actually all of them cannot exist in each case.

Bravais

has established fourteen different types of lattices called Bravais lattices according to the arrangement of the points in the different unit cells involved.

These are listed below:

Thus, there are seven crystal types for which fourteen different types of lattices (7 primitive or simple, 3 body centred, 2 face centred and 2 end centred are possible).

The following fourteen different types of lattices are known as Bravais lattices.

fig 2.3 - Fourteen Bravais Lattices