What defines a primitive unit cell?

A primitive unit cell is defined as the smallest repeating unit in a crystal lattice that, when repeated in three-dimensional space, can generate the entire lattice. It contains only one lattice point, which means that it encompasses the smallest volume that reflects the symmetry and structure of the crystal.

This definition is significant because in crystallography, the primitive unit cell is the most fundamental building block of a crystal structure. The lattice point within a primitive unit cell is positioned such that the entire lattice can be reconstructed purely by the translation of this unit in three dimensions.

In contrast, other unit cells that contain more than one lattice point, such as those with two, three, or four lattice points, are categorized as non-primitive and typically represent a more complex arrangement of atoms or molecules within the crystal structure. These are often referred to as conventional unit cells, which may include additional symmetry and can combine multiple primitive cells into a larger unit cell.