4. Different Classes of Crystals

DIFFERENT CLASSES OF CRYSTALS

Based on different permutations of a, b, c and a, b, g we define different crystal classes.

Seven Crystal System

Table : 4

Hint for memorise : CaTORacHMT

Note : In 3-D 14 different types of unit cell are found and these are also known as 14 Bravais lattice.

Types of unit cell :

In every crystal class, the positioning of the lattice points may be different. Based upon these different positions occupied by the lattice points, we have different types of unit cells.

1. Simple / primitive type of unit cell : If lattice points or the particles of the solid are present only at the corners of the unit cell.

2. Body centred unit cell : lattice point are at the corners as well as at the body centre.

3. Face centred unit cell : lattice points are at corners as well as at each of the face centres.

4. End centred unit cell : lattice points are at the corners as well as at centre of any of two opposite faces.

Each of these arrangements corresponds to a unique and different type of arrangement.These 14 different arrangements are called the 14  Bravais lattices.

In any lattice, the surrounding of each and every lattice point is exactly identical