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Crystallographic point group

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In crystallography, a crystallographic point group is a set of symmetry operations, like rotations or reflections, that leave a central point fixed while moving the edges and faces of the crystal to the positions of features of the same size and shape. For a periodic crystal (as opposed to a quasicrystal), the group must also maintain the three-dimensional translational symmetry that defines crystallinity. The geometric properties of a crystal must look exactly the same before and after applying any of the operations in its point group. In the classification of crystals, each point group defines a so-called (geometric) crystal class.

There are infinitely many three-dimensional point groups. However, the crystallographic restriction on the 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 determines, among other things, the directional variation of physical properties that arise from its structure, including optical properties such as birefringency, or electro-optical features such as the Pockels effect.

Contents

NotationEdit

The point groups are named according to their component symmetries. There are several standard notations used by crystallographers, mineralogists, and physicists.

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

Schoenflies notationEdit

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 n mirror planes parallel to the axis of rotation.
  • S2n (for Spiegel, German for mirror) denotes a group with 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 notationEdit

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

Class Group names  
Cubic 23 m3 432 43m m3m
Hexagonal 6 6 6m 622 6mm 6m2 6mmm
Trigonal 3 3 32 3m 3m
Tetragonal 4 4 4m 422 4mm 42m 4mmm
Orthorhombic 222 mm2 mmm
Monoclinic 2 2m m
Triclinic 1 1 Subgroup relations of the 32 crystallographic point groups
(rows represent group orders from bottom to top as: 1,2,3,4,6,8,12,16,24, and 48.)

The correspondence between different notationsEdit

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

See alsoEdit

ReferencesEdit

  1. ^ "Archived copy". Archived from the original on 2013-07-04. Retrieved 2011-11-25. Cite uses deprecated parameter |deadurl= (help)CS1 maint: archived copy as title (link)

External linksEdit