Crystal classes

In crystallography, a crystallographic point group is a set of symmetry operations, like rotations or reflections, that leave a central point fixed while moving other directions and faces of the crystal to the positions of features of the same kind. For a periodic crystal (as opposed to a quasicrystal), the group must also be consistent with maintenance of the three-dimensional translational symmetry that defines crystallinity. The macroscopic properties of a 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 is also known as a crystal class.

There are infinitely many three-dimensional point groups. However, the crystallographic restriction of the infinite families of general point groups results in there being only 32 crystallographic point groups. These 32 point groups are one-and-the same as the 32 types of morphological (external) crystalline symmetries derived in 1830 by Johann Friedrich Christian Hessel from a consideration of observed crystal forms.

The point group of a crystal, among other things, determines directional variation of the physical properties that arise from its structure, including optical properties such as whether it is birefringent, or whether it shows the Pockels effect.

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.

Schoenflies notation

Main article: Schoenflies notation

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

  • Cn (for cyclic) indicates that the group has an n-fold rotation axis. Cnh is Cn with the addition of a mirror (reflection) plane perpendicular to the axis of rotation. Cnv is Cn with the addition of a mirror plane parallel to the axis of rotation.
  • S2n (for Spiegel, German for mirror) denotes a group that contains only a 2n-fold rotation-reflection axis.
  • Dn (for dihedral, or two-sided) indicates that the group has an n-fold rotation axis plus n twofold axes perpendicular to that axis. Dnh has, in addition, a mirror plane perpendicular to the n-fold axis. Dnd has, in addition to the elements of Dn, mirror planes parallel to the n-fold axis.
  • The letter T (for tetrahedron) indicates that the group has the symmetry of a tetrahedron. Td includes improper rotation operations, T excludes improper rotation operations, and Th is T with the addition of an inversion.
  • The letter O (for octahedron) indicates that the group has the symmetry of an octahedron (or cube), with (Oh) or without (O) improper operations (those that change handedness).

Due to the crystallographic restriction theorem, n = 1, 2, 3, 4, or 6 in 2- or 3-dimensional 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
S2n S2 S4 S6 S8 S12

D4d and D6d are actually forbidden because they contain improper rotations with n=8 and 12 respectively. The 27 point groups in the table plus T, Td, Th, O and Oh constitute 32 crystallographic point groups.

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 2m 222 m mm2 mmm
3 3 32 3m 3m
4 4 4m 422 4mm 42m 4mmm
6 6 6m 622 6mm 62m 6mmm
23 m3 432 43m m3m

The correspondence between different notations

Crystal family Crystal system Hermann-Mauguin
(full symbol)
Hermann-Mauguin
(short symbol)
Shubnikov[1] Schoenflies Orbifold Coxeter Order
Triclinic
1 1 1\ C1 11 [ ]+ 1
1 1 \tilde{2} Ci = S2 x [2+,2+] 2
Monoclinic
2 2 2\ C2 22 [2]+ 2
m m m\ Cs = C1h * [ ] 2
\color{Black}\tfrac{2}{m} 2/m 2:m\ C2h 2* [2,2+] 4
Orthorhombic
222 222 2:2\ D2 = V 222 [2,2]+ 4
mm2 mm2 2 \cdot m\ C2v *22 [2] 4
\color{Black}\tfrac{2}{m}\tfrac{2}{m}\tfrac{2}{m} mmm m \cdot 2:m\ D2h = Vh *222 [2,2] 8
Tetragonal
4 4 4\ C4 44 [4]+ 4
4 4 \tilde{4} S4 2x [2+,4+] 4
\color{Black}\tfrac{4}{m} 4/m 4:m\ C4h 4* [2,4+] 8
422 422 4:2\ D4 422 [4,2]+ 8
4mm 4mm 4 \cdot m\ C4v *44 [4] 8
42m 42m \tilde{4}\cdot m D2d = Vd 2*2 [2+,4] 8
\color{Black}\tfrac{4}{m}\tfrac{2}{m}\tfrac{2}{m} 4/mmm m \cdot 4:m\ D4h *422 [4,2] 16
Hexagonal
Trigonal
3 3 3\ C3 33 [3]+ 3
3 3 \tilde{6} S6 = C3i 3x [2+,6+] 6
32 32 3:2\ D3 322 [3,2]+ 6
3m 3m 3 \cdot m\ C3v *33 [3] 6
3\color{Black}\tfrac{2}{m} 3m \tilde{6}\cdot m D3d 2*3 [2+,6] 12
Hexagonal
6 6 6\ C6 66 [6]+ 6
6 6 3:m\ C3h 3* [2,3+] 6
\color{Black}\tfrac{6}{m} 6/m 6:m\ C6h 6* [2,6+] 12
622 622 6:2\ D6 622 [6,2]+ 12
6mm 6mm 6 \cdot m\ C6v *66 [6] 12
6m2 6m2 m \cdot 3:m\ D3h *322 [3,2] 12
\color{Black}\tfrac{6}{m}\tfrac{2}{m}\tfrac{2}{m} 6/mmm m \cdot 6:m\ D6h *622 [6,2] 24
Cubic
23 23 3/2\ T 332 [3,3]+ 12
\color{Black}\tfrac{2}{m}3 m3 \tilde{6}/2 Th 3*2 [3+,4] 24
432 432 3/4\ O 432 [4,3]+ 24
43m 43m 3/\tilde{4} Td *332 [3,3] 24
\color{Black}\tfrac{4}{m}3\color{Black}\tfrac{2}{m} m3m \tilde{6}/4 Oh *432 [4,3] 48

See also

References

External links

  • Point-group symbols in International Tables for Crystallography (2006). Vol. A, ch. 12.1, pp. 818-820
  • Names and symbols of the 32 crystal classes in International Tables for Crystallography (2006). Vol. A, ch. 10.1, p. 794
  • Pictorial overview of the 32 groups
  • Point Groups - Flow Chart
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.