egységsejt

The unit cell, or egységsejt (Hungarian term), is the smallest repeating unit in a crystal lattice that, by translational repetition, builds the entire crystal. It provides a compact description of the lattice geometry and symmetry.

A unit cell is defined by three lattice vectors a, b, c and the angles between them, α, β, γ. Its geometry is described by the lattice parameters (a, b, c, α, β, γ) and it can take different crystal system shapes such as cubic, tetragonal, orthorhombic, monoclinic, triclinic, or rhombohedral. The lattice itself is an abstract array of points; a motif or basis attached to each lattice point specifies the actual atoms or groups that occupy those positions.

Unit cells can be primitive (P) or centered. A primitive cell contains one lattice point per cell; centered cells include additional lattice points (I for body-centered, F for face-centered, C for base-centered). In three dimensions there are 14 Bravais lattices, which enumerate all distinct lattice symmetries.

Conventional cells are often chosen to reflect symmetry and to simplify description, even if they contain more than one lattice point. For example, a face-centered cubic lattice has a conventional cell that contains four lattice points, whereas a primitive cell would contain only one; the primitive cell is still definable but is less convenient for describing symmetry.

Unit cells are central to crystallography and diffraction methods. By analyzing diffraction patterns, researchers determine the unit cell parameters and the arrangement of atoms inside the basis. The cell volume V can be computed from the parameters as V = abc sqrt(1 − cos^2 α − cos^2 β − cos^2 γ + 2 cos α cos β cos γ).