A unit cell is the smallest repeating unit in a crystal lattice that, when stacked in three-dimensional space without gaps, reproduces the entire crystal structure. It serves as the fundamental building block of the crystal

. Key points about a unit cell:

• It is defined by lattice points, which are points in space representing the periodic arrangement of atoms, ions, or molecules in the crystal

• The shape of a unit cell is typically a parallelogram in two dimensions or a parallelepiped in three dimensions, with lattice points at the vertices

• There are two main types of unit cells:
  • Primitive unit cell : Contains exactly one lattice point (considering fractional contributions from atoms at corners, edges, or faces). It is the smallest possible unit cell

* **Non-primitive (conventional) unit cell** : May contain more than one lattice point, including additional lattice points on faces, edges, or inside the cell, chosen to reflect the full symmetry of the crystal

• Examples of non-primitive unit cells include:
  • Body-centered cubic (BCC) unit cell: has lattice points at the eight corners plus one in the center, totaling two lattice points per cell

* Face-centered cubic (FCC) unit cell: has lattice points at the corners and at the centers of each face, totaling four lattice points per cell

In summary, the unit cell is a fundamental concept in crystallography that captures the repeating pattern of a crystal's structure in the smallest possible volume, allowing the entire crystal to be described by its repetition