Enhetcell

Enhetcell, the Swedish term for unit cell, is the fundamental repeating unit of a crystal lattice. It is the smallest portion of a crystal that, when translated through the lattice vectors, reproduces the entire lattice. A unit cell is described by three edge lengths a, b, c and three interaxial angles α, β, γ, which define its geometry. It may contain a motif or basis of atoms attached to each lattice point; repeating the unit cell builds the full crystal.

Unit cells are categorized as primitive (P), containing one lattice point per cell, or non-primitive such as body-centered (I) and face-centered (F), or base-centered, depending on the arrangement of lattice points. In three dimensions there are 14 Bravais lattices, grouped into seven crystal systems: triclinic, monoclinic, orthorhombic, tetragonal, trigonal (rhombohedral), hexagonal, and cubic. The conventional cell is often chosen to highlight symmetry, while the primitive cell is the smallest repeating unit.

The volume of a unit cell is V = |a · (b × c)|, which can also be expressed in terms of a, b, c and the angles α, β, γ. The density of a material can be determined from the number of formula units per cell, the cell volume, and the molar mass. The unit cell concept underpins crystallography and materials science, underpinning the interpretation of diffraction patterns, calculation of structure factors, and the connection between atomic arrangements and macroscopic properties.