In crystallography, a crystallographic point group is a set of symmetry operations, like rotations or reflections, that leave a point fixed while moving each atom of the crystal to the position of an atom of the same kind. That is, an infinite crystal would look exactly the same before and after any of the operations in its point group. In the classification of crystals, each point group corresponds to a crystal class.

There are infinitely many 3D point groups; in crystallography, however, they are restricted to be compatible with the discrete translation symmetries of a crystal lattice. This crystallographic restriction of the infinite families of general point groups results in 32 crystallographic point groups.

The point group of a crystal, among other things, determines some of the crystal's optical properties, such as whether it is birefringent, or whether it shows the Pockels effect.

Contents 1 Notation 1.1 Schönflies notation 1.2 Hermann-Mauguin notation 2 See also 3 External links

[edit] Notation

The point groups are denoted by their component symmetries. There are a few standard notations used by crystallographers, mineralogists, and physicists.

For the correspondence of the two systems below, see crystal system.

[edit] Schönflies notation

In Schönflies notation, point groups are denoted by a letter symbol with a subscript. The symbols used in crystallography mean the following:

• The letter O (for octahedron) indicates that the group has the symmetry of an octahedron (or cube), with (O_h) or without (O) improper operations (those that change handedness).
• The letter T (for tetrahedron) indicates that the group has the symmetry of a tetrahedron. T_d includes improper operations, T excludes improper operations, and T_h is T with the addition of an inversion.
• C_n (for cyclic) indicates that the group has an n-fold rotation axis. C_nh is C_n with the addition of a mirror (reflection) plane perpendicular to the axis of rotation. C_nv is C_n with the addition of a mirror plane parallel to the axis of rotation.
• S_n (for Spiegel, German for mirror) denotes a group that contains only an n-fold rotation-reflection axis.
• D_n (for dihedral, or two-sided) indicates that the group has an n-fold rotation axis plus a twofold axis perpendicular to that axis. D_nh has, in addition, a mirror plane perpendicular to the n-fold axis. D_nv has, in addition to the elements of D_n, mirror planes parallel to the n-fold axis.

Due to the crystallographic restriction theorem, n = 1, 2, 3, 4, or 6 in 2 or 3 dimension space.

n	1	2	3	4	6
Cn	C1	C2	C3	C4	C6
Cnv	C1v=C1h	C2v	C3v	C4v	C6v
Cnh	C1h	C2h	C3h	C4h	C6h
Dn	D1=C2	D2	D3	D4	D6
Dnh	D1h=C2v	D2h	D3h	D4h	D6h
Dnd	D1d=C2h	D2d	D3d	D4d	D6d
Sn	S1=C1h	S2	S3=C3h	S4	S6

D_4d and D_6d are actually forbidden because they contain improper rotations with n=8 and 12 respectively. The 27 point groups in the table plus T, T_d, T_h, O and O_h constitute 32 crystallographic point groups

[edit] Hermann-Mauguin notation

An abbreviated form of the Hermann-Mauguin notation commonly used for space groups also serves to describe crystallographic point groups. Group names are

• 1, 1
• 2, m, ²⁄_m
• 222, mm2, mmm
• 4,4, ⁴⁄_m, 422, 4mm, 42m, ⁴⁄_mmm
• 3, 3, 32, 3m, 3m
• 6, 6, ⁶⁄_m, 622, 6mm, 62m, ⁶⁄_mmm
• 23, m3, 432, 43m, m3m

The correspondence between the three notations is:

Hermann-Mauguin	Schoenflies	Orbifold
1	C1	11		2	C2	22		222	D2	222		4	C4	44		3	C3	33		6	C6	66		23	T	332
1	S2	1x		m	C1h	1*		mm2	C2v	*22		4	S4	2x		3	S6	3x		6	C3h	3*		m3	Th	3*2
				2/m	C2h	2*		mmm	D2h	*222		4/m	C4h	4*		32	D3	223		6/m	C6h	6*		432	O	432
												422	D4	224		3m	C3v	*33		622	D6	226		43m	Td	*332
												4mm	C4v	*44		3m	D3d	2*3		6mm	C6v	*66		m3m	Oh	*432
												42m	D2d	2*2						62m	D3h	*223
												4/m mm	D4h	*224						6/m mm	D6h	*226

[edit] See also

• Molecular symmetry
• Point group
• Space group
• Point groups in three dimensions
• Crystal system

[edit] External links

• Pictorial overview of the 32 groups