A unit cell is the smallest repeating unit of a crystal lattice that shows the complete structure and arrangement of particles (atoms, ions, or molecules) in the solid.
When this unit cell is repeated in all directions, it generates the entire crystal lattice.
Key features of a unit cell:
- Basic building block – It represents the simplest portion of the lattice that can build the whole crystal.
- Lattice points – Each corner or position in the unit cell represents atoms, ions, or molecules.
- Parameters – A unit cell is described by:
- Edge lengths: a, b, c (lengths of sides)
- Angles: α, β, γ (angles between edges)
- Types –
- Primitive (simple) unit cell: particles only at the corners.
- Non-primitive unit cells: particles at corners and also at faces, body, or edges (e.g., face-centered cubic, body-centered cubic).
Examples:
- In sodium chloride (NaCl), the unit cell is face-centered cubic (FCC).
- In cesium chloride (CsCl), the unit cell is body-centered cubic (BCC).