Unit Cell

UNIT CELL

Consider a two-dimensional space lattice as shown in fig. 1.6.

It is found that when a parallelogram ABCD is repeated by an integral multiple of vectors a and b corresponding to AB and AD, the whole pattern (or array) is obtained.

The region ABCD is known as unit cell. a and b are basis vectors.

The choice of the unit cell is not unique. It can be constructed in a number of ways like A' B' C' D' or A" B" C" D" without affecting the symmetry of the crystal (fig. 1.6).

Definition

The unit cell is defined as the smallest geometric figure which is repeated in three dimension to derive the actual crystal structure.

The unit cell fully represents the characteristics of the entire crystal.

The same principle is extended for a three-dimensional case. A unit cell in three-dimension is shown fig. 1.7.

It is also defined as the smallest volume of a solid from which the entire crystal structure is constructed by translational repetition in three-dimensions.