What is the Difference Between Unit Cell and Primitive Cell?

The difference between a unit cell and a primitive cell lies in the number of lattice points they contain and their geometry. Here are the key differences:

• Unit Cell: A unit cell is the smallest repetitive unit of a crystal system that represents the repeating pattern of the lattice structure. It is a three-dimensional structure and can be defined as the smallest group of atoms that has the overall symmetry of a crystal, from which the entire lattice can be built up by repetition in three dimensions.
• Primitive Cell: A primitive cell is a type of unit cell that contains exactly one lattice point. It is the smallest possible unit cell of a lattice. In a primitive cell, lattice points are located at each of its eight vertices. Primitive cells can be either two-dimensional or three-dimensional structures.

In summary, a unit cell is a more general concept representing the smallest repetitive unit of a crystal system, while a primitive cell is a specific type of unit cell that contains only one lattice point.

Comparative Table: Unit Cell vs Primitive Cell

The main difference between a unit cell and a primitive cell is that a unit cell is the smallest group of atoms that has the overall symmetry of a crystal, while a primitive cell is the smallest possible unit cell of a lattice. Here is a comparison table highlighting the differences between unit cells and primitive cells:

A unit cell is a repeating part of the crystal structure, and its geometry can be parallelepiped or parallelogram depending on the space dimension (3D or 2D). On the other hand, a primitive cell is the smallest minimum volume cell corresponding to a single lattice point of a structure, and its geometry can also be parallelogram or parallelepiped depending on the space dimension (2D or 3D). The primitive cell is often used as a simpler representation of the unit cell in crystallographic investigations and computational materials science.