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The Pearson symbol, or Pearson notation, is used in crystallography as a means of describing a
crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric pattern ...
, and was originated by W. B. Pearson. The symbol is made up of two letters followed by a number. For example: * Diamond structure, ''cF''8 * Rutile structure, ''tP''6 The two (italicised) letters specify the
Bravais lattice In geometry and crystallography, a Bravais lattice, named after , is an infinite array of discrete points generated by a set of discrete translation operations described in three dimensional space by : \mathbf = n_1 \mathbf_1 + n_2 \mathbf_2 + n ...
. The lower-case letter specifies the crystal family, and the upper-case letter the centering type. The number at the end of the Pearson symbol gives the number of the atoms in the conventional unit cell.Nomenclature of Inorganic Chemistry IUPAC Recommendations 2005
IR-3.4.4, pp. 49–51; IR-11.5, pp. 241–242.
IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
.
})Page 124 in chapter 3. "Crystallography: Internal order and symmetry" in Cornelius Klein & Cornelius S. Hurlbut, Jr.: "Manual of Mineralogy", 21st edition, 1993, John Wiley & Sons, Inc., ., , 2 , - , R , , Rhombohedral centring (see below) , , 3 , - , F , , All faces centred , , 4 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 Fmm) and diamond (space group Fdm) have the same Pearson symbol ''cF''8. Confusion also arises in the rhombohedral lattice, which is alternatively described in a centred hexagonal (''a'' = ''b'', ''c'', ''α'' = ''β'' = 90°, ''γ'' = 120°) or primitive rhombohedral (''a'' = ''b'' = ''c'', ''α'' = ''β'' = ''γ'') setting. The more commonly used hexagonal setting has 3 translationally equivalent points per unit cell. The Pearson symbol refers to the hexagonal setting in its letter code (''hR''), but the following figure gives the number of translationally equivalent points in the primitive rhombohedral setting. Examples: ''hR''1 and ''hR''2 are used to designate the Hg and Bi structures respectively. Because there are many possible structures that can correspond to one Pearson symbol, a prototypical compound may be useful to specify. Examples of how to write this would be ''hP''12-MgZn_2 or ''cF''8-C. Prototypical compounds for particular structures can be found on the Inorganic Crystal Structure Database (ICSD) or on the AFLOW Library of Crystallographic Prototypes.


Caution

The Pearson symbol should only be used to designate simple structures (elements, some binary compound) where the number of atoms per unit cell equals, ideally, the number of translationally equivalent points.


References


External Links

* *{{cite web, url=http://www.aflow.org/prototype-encyclopedia/, title=AFLOW Library of Crystallographic Prototypes, accessdate=13 December 2022


Further reading


United States Naval Research Laboratory - Pearson symbol (Examples and pictures)
Crystallography