Crystallographic point group explained

In crystallography, a crystallographic point group is a three dimensional point group whose symmetry operations are compatible with a three dimensional crystallographic lattice. According to the crystallographic restriction it may only contain one-, two-, three-, four- and sixfold rotations or rotoinversions. This reduces the number of crystallographic point groups to 32 (from an infinity of general point groups). These 32 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.

In the classification of crystals, to each space group is associated a crystallographic point group by "forgetting" the translational components of the symmetry operations. That is, by turning screw rotations into rotations, glide reflections into reflections and moving all symmetry elements into the origin. Each crystallographic point group defines the (geometric) crystal class of the crystal.

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.

Notation

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 notation

See 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:

• 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 n mirror planes parallel to the axis of rotation.
• S_2n (for Spiegel, German for mirror) denotes a group with only a 2n-fold rotation-reflection axis.
• D_n (for dihedral, or two-sided) indicates that the group has an n-fold rotation axis plus n twofold axes perpendicular to that axis. D_nh has, in addition, a mirror plane perpendicular to the n-fold axis. D_nd has, in addition to the elements of D_n, mirror planes parallel to the n-fold axis.
• The letter T (for tetrahedron) indicates that the group has the symmetry of a tetrahedron. T_d includes improper rotation operations, T excludes improper rotation operations, and T_h is T with the addition of an inversion.
• The letter O (for octahedron) indicates that the group has the symmetry of an octahedron, with (O_h) 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

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.

Hermann–Mauguin notation

See main article: 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

Crystal family	Crystal system	Group names
Cubic		23	m		432	3m	mm
Hexagonal	Hexagonal	6		^6⁄m	622	6mm	m2	6/mmm
	Trigonal	3			32	3m	m
Tetragonal		4		^4⁄m	422	4mm	2m	4/mmm
Orthorhombic					222		mm2	mmm
Monoclinic		2		^2⁄m		m
Triclinic		1

The correspondence between different notations

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

Isomorphisms

Many of the crystallographic point groups share the same internal structure. For example, the point groups, 2, and m contain different geometric symmetry operations, (inversion, rotation, and reflection, respectively) but all share the structure of the cyclic group C₂. All isomorphic groups are of the same order, but not all groups of the same order are isomorphic. The point groups which are isomorphic are shown in the following table:^[2]

Hermann-Mauguin	Schoenflies	Order	Abstract group
1	C1	1		G ^{1} _{1}
	Ci = S2	2		G ^{1} _{2}
2	C2	2
m	Cs = C1h	2
3	C3	3		G ^{1} _{3}
4	C4	4		G ^{1} _{4}
	S4	4
2/m	C2h	4	D2 = C2 × C2	G ^{2} _{4}
222	D2 = V	4
mm2	C2v	4
	C3i = S6	6	C6	G ^{1} _{6}
6	C6	6
	C3h	6
32	D3	6	D3	G ^{2} _{6}
3m	C3v	6
mmm	D2h = Vh	8	D2 × C2	G ^{3} _{8}
4/m	C4h	8	C4 × C2	G ^{2} _{8}
422	D4	8	D4	G ^{4} _{8}
4mm	C4v	8
2m	D2d = Vd	8
6/m	C6h	12	C6 × C2	G ^{2} _{12}
23	T	12	A4	G ^{5} _{12}
m	D3d	12	D6	G ^{3} _{12}
622	D6	12
6mm	C6v	12
m2	D3h	12
4/mmm	D4h	16	D4 × C2	G ^{9} _{16}
6/mmm	D6h	24	D6 × C2	G ^{5} _{24}
m	Th	24	A4 × C2	G ^{10} _{24}
432	O	24	S4	G ^{7} _{24}
3m	Td	24
mm	Oh	48	S4 × C2	G ^{7} _{48}

This table makes use of cyclic groups (C₁, C₂, C₃, C₄, C₆), dihedral groups (D₂, D₃, D₄, D₆), one of the alternating groups (A₄), and one of the symmetric groups (S₄). Here the symbol " × " indicates a direct product.

Deriving the crystallographic point group (crystal class) from the space group

1. Leave out the Bravais lattice type.
2. Convert all symmetry elements with translational components into their respective symmetry elements without translation symmetry. (Glide planes are converted into simple mirror planes; screw axes are converted into simple axes of rotation.)
3. Axes of rotation, rotoinversion axes, and mirror planes remain unchanged.

See also

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

References

1. Web site: (International Tables) Abstract . 2011-11-25 . dead . https://archive.today/20130704042455/http://it.iucr.org/Ab/ch12o1v0001/sec12o1o3/ . 2013-07-04 .
2. Novak . I . Molecular isomorphism . European Journal of Physics . IOP Publishing . 16 . 4 . 1995-07-18 . 0143-0807 . 10.1088/0143-0807/16/4/001 . 151–153. 1995EJPh...16..151N . 250887121 .

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