Pearson symbol

The Pearson symbol, or Pearson notation, is used in crystallography as a means of describing a crystal structure, and was originated by W.B. Pearson. The symbol is made up of two letters followed by a number. For example:

The two (italicised) letters specify the Bravais lattice. The lower case letter specifies the crystal system, and the upper case letter the lattice type. The figure gives the number of the atoms in the unit cell. IUPAC (2005)

The letters A, B and C were formerly used instead of S. When the centred face cuts the X-axis, the Bravais lattice is called A-centred. In analogy, when the centred face cuts the Y- or Z-axis, we have B- or C-centring, respectively.

The fourteen possible Bravais lattices are identified by the first two letters:

Pearson symbol and space group

The Pearson symbol does not uniquely identify the space group of a crystal structure, for example both the NaCl structure, (space group Fm3m) and diamond (space group Fd3m) have the same Pearson symbol cF8.