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 [1] The symbol is made up of two letters followed by a number for example:
- Diamond structure, cF8
- Rutile structure, tP6
The first two letters specify the Bravais lattice, the first lower case letter specifying the crystal class, and the second upper case letter the lattice type. The number suffix is the number of the atoms in the unit cell. IUPAC (2005)[2] recommends that this symbol can be used to specify the structure of inorganic solids where there is possible ambiguity, and should be italicised.
| a | triclinic |
| m | monoclinic |
| o | orthorhombic |
| t | tetragonal |
| h | hexagonal and rhombohedral |
| c | cubic |
| C | Side face centred |
| F | All face centred |
| I | Body centred |
| R | Rhombohedral |
| P | Primitive |
The fourteen possible Bravais lattices are identified by the first two letters:
| Crystal class | Lattice symbol | Pearson symbol letters |
|---|---|---|
| Triclinic | P | aP |
| Monoclinic | P | mP |
| C | mC | |
| Orthorhombic | P | oP |
| C | oC | |
| F | oF | |
| I | oI | |
| Tetragonal | P | tP |
| I | tI | |
| Hexagonal (and trigonal P) | P | hP |
| Rhombohedral | R | hR |
| Cubic | P | cP |
| F | cF | |
| I | cI |
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.
References
- ^ W.B. Pearson, A Handbook of Lattice Spacings and Structures of Metals and Alloys,Vol. 2, Pergamon Press, Oxford, 1967,
- ^ NOMENCLATURE OF INORGANIC CHEMISTRY IUPAC Recommendations 2005 ed. N. G. Connelly et al. RSC Publishing http://www.chem.qmul.ac.uk/iupac/bioinorg/
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