Basis of Crystal structure

BASIS

Definition

The crystal structure is obtained by adding a unit assembly of atoms to each lattice point. This unit assembly is called basis.

Explanation

A basis may be a single atom or assembly of atoms which is identical in composition, arrangement and orientation.

When the basis is repeated in a space lattice with correct periodicity in all three directions, then it gives the actual crystal structure.

Therefore, a space lattice combined with a basis gives a crystal structure.

ie., Space lattice + Basis → Crystal structure

The basis combined with lattice points is shown in fig 1.5 in which two atoms (represented by circles of smaller and large radii) are added to one lattice point (represented by a black dot).

For many metals, the number of atoms in basis is one (Aluminium and Barium crystals), two or three or more.

For example in NaCl and KCl, each basis has two atoms and in CaF₂, basis has three atoms. But, for many complicated structures, the basis exceeds more than 1000 atoms.

Note

A space lattice refers to the geometry of a set of points in space whereas a crystal structure refers to the actual arrangement of atoms in space.