Types of Unit Cell

What is Unit Cell?

The simplest repeating unit in a crystal is called a unit cell. Each unit cell is defined in terms of lattice points the points in space about which the particles are free to vibrate in a crystal.

PRIMITIVE UNIT CELL

Unit cells in which the constituent particles are present only at the corners are called primitive unit cells.

NON-PRIMITIVE UNIT CELL

Unit cells in which the constituent particles are present not only at the corners of the cell but also at some other position.

TYPE OF NON-PRIMITIVE UNIT CELL

• Face centered
• End-centered
• Body centered

Face centered:

1. The face centered cubic structure has atoms located at each of the corners and the centers of all the cubic faces
2. Each of the corner atoms is the corner of another cube so the corner atoms are shared among eight unit cells.
3. Additionally, each of its six face centered atoms is shared with an adjacent atom. Since 12 of its atoms are shared, it is said to have a coordination number of 12.
4. The fcc unit cell consists of a net total of four atoms.

End-centered

1. If one constituent particle lies at the centre of any two opposite faces besides the particles lying at the corners, it is known as End-Centred Unit Cell

Body centered:

1. The body-centered cubic unit cell has atoms at each of the eight corners of a cube (like the cubic unit cell) plus one atom in the center of the cube.
2. Each of the corner atoms is the corner of another cube so the corner atoms are shared among eight unit cells.
3. It is said to have a coordination number of 8.
4. The bcc unit cell consists of a net total of two atoms; one in the center and eight eighths from corners atoms.

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