Different types of unit cells

Different Types of Unit Cells

In crystallography, a unit cell is the smallest repeating structural unit that defines the entire structure of a crystal. The repetition of unit cells in three dimensions forms the space lattice of a crystalline solid. Depending on the arrangement of particles within the unit cell, there are various types of unit cells that differ in geometry and the number of particles they contain.

Classification of Unit Cells

Unit cells are primarily categorized into two types:

• Primitive Unit Cell: The particles are located only at the corners of the cell.
• Centered Unit Cells: Apart from the corner positions, particles are also located at the center of the body or the faces.

Types of Centered Unit Cells

Based on the additional locations of lattice points, centered unit cells are further divided into:

• Body-Centered Unit Cell (BCC): Contains particles at all eight corners and one at the center of the cell.
• Face-Centered Unit Cell (FCC): Contains particles at each corner and the centers of all six faces.
• End-Centered Unit Cell: Contains particles at all corners and at the centers of two opposite faces (usually the top and bottom or side faces).

Bravais Lattices

When various types of unit cells are combined with the seven crystal lattice systems, they result in 14 unique three-dimensional arrangements, referred to as Bravais lattices. These include combinations across seven crystal systems such as cubic, tetragonal, orthorhombic, and more.

Summary of Common Unit Cells

Understanding the different types of unit cells is essential for analyzing the internal structure of solids. Each unit cell type impacts the physical properties of the material, including density, packing efficiency, and mechanical strength.