Crystal system

Crystals can be classified in other ways, including crystal families and lattice systems. These make use of space groups, lattices, and point groups. The various classifications are often confused: in particular the trigonal crystal system is often confused with the rhombohedral lattice system, and the term "crystal system" is sometimes used to mean one of the others.

Spaces with fewer than three dimensions have the same number of crystal systems, crystal families, and lattice systems. In one-dimensional space, there is one crystal system. In 2D space, there are four crystal systems: oblique, rectangular, square, and hexagonal.

Crystal family

A crystal family is determined by lattices and point groups. It is formed by combining crystal systems that have space groups assigned to a common lattice system. In three dimensions, the crystal families and systems are identical, except the hexagonal and trigonal crystal systems, which are combined into one hexagonal crystal family. The six crystal families are triclinic, monoclinic, orthorhombic, tetragonal, hexagonal, and cubic.

Hexagonal hanksite crystal, with threefold c-axis symmetry

The 7 crystal systems consist of 32 crystal classes (corresponding to the 32 crystallographic point groups) as shown in the following table below:

Crystal family	Crystal system	Point group / Crystal class	Schönflies	Hermann–Mauguin	Orbifold	Coxeter	Point symmetry	Order	Abstract group
triclinic		pedial	C1	1	11	[ ]+	enantiomorphic polar	1	trivial ${\displaystyle \mathbb {Z} _{1}}$
		pinacoidal	Ci (S2)	1	1x	[2,1+]	centrosymmetric	2	cyclic ${\displaystyle \mathbb {Z} _{2}}$
monoclinic		sphenoidal	C2	2	22	[2,2]+	enantiomorphic polar	2	cyclic ${\displaystyle \mathbb {Z} _{2}}$
		domatic	Cs (C1h)	m	*11	[ ]	polar	2	cyclic ${\displaystyle \mathbb {Z} _{2}}$
		prismatic	C2h	2/m	2*	[2,2+]	centrosymmetric	4	Klein four ${\displaystyle \mathbb {V} =\mathbb {Z} _{2}\times \mathbb {Z} _{2}}$
orthorhombic		rhombic-disphenoidal	D2 (V)	222	222	[2,2]+	enantiomorphic	4	Klein four ${\displaystyle \mathbb {V} =\mathbb {Z} _{2}\times \mathbb {Z} _{2}}$
		rhombic-pyramidal	C2v	mm2	*22	[2]	polar	4	Klein four ${\displaystyle \mathbb {V} =\mathbb {Z} _{2}\times \mathbb {Z} _{2}}$
		rhombic-dipyramidal	D2h (Vh)	mmm	*222	[2,2]	centrosymmetric	8	${\displaystyle \mathbb {V} \times \mathbb {Z} _{2}}$
tetragonal		tetragonal-pyramidal	C4	4	44	[4]+	enantiomorphic polar	4	cyclic ${\displaystyle \mathbb {Z} _{4}}$
		tetragonal-disphenoidal	S4	4	2x	[2+,2]	non-centrosymmetric	4	cyclic ${\displaystyle \mathbb {Z} _{4}}$
		tetragonal-dipyramidal	C4h	4/m	4*	[2,4+]	centrosymmetric	8	${\displaystyle \mathbb {Z} _{4}\times \mathbb {Z} _{2}}$
		tetragonal-trapezohedral	D4	422	422	[2,4]+	enantiomorphic	8	dihedral ${\displaystyle \mathbb {D} _{8}=\mathbb {Z} _{4}\rtimes \mathbb {Z} _{2}}$
		ditetragonal-pyramidal	C4v	4mm	*44	[4]	polar	8	dihedral ${\displaystyle \mathbb {D} _{8}=\mathbb {Z} _{4}\rtimes \mathbb {Z} _{2}}$
		tetragonal-scalenohedral	D2d (Vd)	42m or 4m2	2*2	[2+,4]	non-centrosymmetric	8	dihedral ${\displaystyle \mathbb {D} _{8}=\mathbb {Z} _{4}\rtimes \mathbb {Z} _{2}}$
		ditetragonal-dipyramidal	D4h	4/mmm	*422	[2,4]	centrosymmetric	16	${\displaystyle \mathbb {D} _{8}\times \mathbb {Z} _{2}}$
hexagonal	trigonal	trigonal-pyramidal	C3	3	33	[3]+	enantiomorphic polar	3	cyclic ${\displaystyle \mathbb {Z} _{3}}$
		rhombohedral	C3i (S6)	3	3x	[2+,3+]	centrosymmetric	6	cyclic ${\displaystyle \mathbb {Z} _{6}=\mathbb {Z} _{3}\times \mathbb {Z} _{2}}$
		trigonal-trapezohedral	D3	32 or 321 or 312	322	[3,2]+	enantiomorphic	6	dihedral ${\displaystyle \mathbb {D} _{6}=\mathbb {Z} _{3}\rtimes \mathbb {Z} _{2}}$
		ditrigonal-pyramidal	C3v	3m or 3m1 or 31m	*33	[3]	polar	6	dihedral ${\displaystyle \mathbb {D} _{6}=\mathbb {Z} _{3}\rtimes \mathbb {Z} _{2}}$
		ditrigonal-scalenohedral	D3d	3m or 3m1 or 31m	2*3	[2+,6]	centrosymmetric	12	dihedral ${\displaystyle \mathbb {D} _{12}=\mathbb {Z} _{6}\rtimes \mathbb {Z} _{2}}$
	hexagonal	hexagonal-pyramidal	C6	6	66	[6]+	enantiomorphic polar	6	cyclic ${\displaystyle \mathbb {Z} _{6}=\mathbb {Z} _{3}\times \mathbb {Z} _{2}}$
		trigonal-dipyramidal	C3h	6	3*	[2,3+]	non-centrosymmetric	6	cyclic ${\displaystyle \mathbb {Z} _{6}=\mathbb {Z} _{3}\times \mathbb {Z} _{2}}$
		hexagonal-dipyramidal	C6h	6/m	6*	[2,6+]	centrosymmetric	12	${\displaystyle \mathbb {Z} _{6}\times \mathbb {Z} _{2}}$
		hexagonal-trapezohedral	D6	622	622	[2,6]+	enantiomorphic	12	dihedral ${\displaystyle \mathbb {D} _{12}=\mathbb {Z} _{6}\rtimes \mathbb {Z} _{2}}$
		dihexagonal-pyramidal	C6v	6mm	*66	[6]	polar	12	dihedral ${\displaystyle \mathbb {D} _{12}=\mathbb {Z} _{6}\rtimes \mathbb {Z} _{2}}$
		ditrigonal-dipyramidal	D3h	6m2 or 62m	*322	[2,3]	non-centrosymmetric	12	dihedral ${\displaystyle \mathbb {D} _{12}=\mathbb {Z} _{6}\rtimes \mathbb {Z} _{2}}$
		dihexagonal-dipyramidal	D6h	6/mmm	*622	[2,6]	centrosymmetric	24	${\displaystyle \mathbb {D} _{12}\times \mathbb {Z} _{2}}$
cubic		tetartoidal	T	23	332	[3,3]+	enantiomorphic	12	alternating ${\displaystyle \mathbb {A} _{4}}$
		diploidal	Th	m3	3*2	[3+,4]	centrosymmetric	24	${\displaystyle \mathbb {A} _{4}\times \mathbb {Z} _{2}}$
		gyroidal	O	432	432	[4,3]+	enantiomorphic	24	symmetric ${\displaystyle \mathbb {S} _{4}}$
		hextetrahedral	Td	43m	*332	[3,3]	non-centrosymmetric	24	symmetric ${\displaystyle \mathbb {S} _{4}}$
		hexoctahedral	Oh	m3m	*432	[4,3]	centrosymmetric	48	${\displaystyle \mathbb {S} _{4}\times \mathbb {Z} _{2}}$

The point symmetry of a structure can be further described as follows. Consider the points that make up the structure, and reflect them all through a single point, so that (x,y,z) becomes (−x,−y,−z). This is the 'inverted structure'. If the original structure and inverted structure are identical, then the structure is centrosymmetric. Otherwise it is non-centrosymmetric. Still, even in the non-centrosymmetric case, the inverted structure can in some cases be rotated to align with the original structure. This is a non-centrosymmetric achiral structure. If the inverted structure cannot be rotated to align with the original structure, then the structure is chiral or enantiomorphic and its symmetry group is enantiomorphic.[1]

A direction (meaning a line without an arrow) is called polar if its two-directional senses are geometrically or physically different. A symmetry direction of a crystal that is polar is called a polar axis.[2] Groups containing a polar axis are called polar. A polar crystal possesses a unique polar axis (more precisely, all polar axes are parallel). Some geometrical or physical property is different at the two ends of this axis: for example, there might develop a dielectric polarization as in pyroelectric crystals. A polar axis can occur only in non-centrosymmetric structures. There cannot be a mirror plane or twofold axis perpendicular to the polar axis, because they would make the two directions of the axis equivalent.

The crystal structures of chiral biological molecules (such as protein structures) can only occur in the 65 enantiomorphic space groups (biological molecules are usually chiral).

There are seven different kinds of crystal systems, and each kind of crystal system has four different kinds of centerings (primitive, base-centered, body-centered, face-centered). However, not all of the combinations are unique; some of the combinations are equivalent while other combinations are not possible due to symmetry reasons. This reduces the number of unique lattices to the 14 Bravais lattices.

The distribution of the 14 Bravais lattices into lattice systems and crystal families is given in the following table.

Crystal family	Lattice system	Point group (Schönflies notation)	14 Bravais lattices
			Primitive (P)	Base-centered (S)	Body-centered (I)	Face-centered (F)
Triclinic (a)		Ci	aP
Monoclinic (m)		C2h	mP	mS
Orthorhombic (o)		D2h	oP	oS	oI	oF
Tetragonal (t)		D4h	tP		tI
Hexagonal (h)	Rhombohedral	D3d	hR
	Hexagonal	D6h	hP
Cubic (c)		Oh	cP		cI	cF

In geometry and crystallography, a Bravais lattice is a category of translative symmetry groups (also known as lattices) in three directions.

Such symmetry groups consist of translations by vectors of the form

R = n₁a₁ + n₂a₂ + n₃a₃,

where n₁, n₂, and n₃ are integers and a₁, a₂, and a₃ are three non-coplanar vectors, called primitive vectors.

These lattices are classified by the space group of the lattice itself, viewed as a collection of points; there are 14 Bravais lattices in three dimensions; each belongs to one lattice system only. They represent the maximum symmetry a structure with the given translational symmetry can have.

All crystalline materials (not including quasicrystals) must, by definition, fit into one of these arrangements.

For convenience a Bravais lattice is depicted by a unit cell which is a factor 1, 2, 3, or 4 larger than the primitive cell. Depending on the symmetry of a crystal or other pattern, the fundamental domain is again smaller, up to a factor 48.

The Bravais lattices were studied by Moritz Ludwig Frankenheim in 1842, who found that there were 15 Bravais lattices. This was corrected to 14 by A. Bravais in 1848.

‌The four-dimensional unit cell is defined by four edge lengths (a, b, c, d) and six interaxial angles (α, β, γ, δ, ε, ζ). The following conditions for the lattice parameters define 23 crystal families

The names here are given according to Whittaker.[3] They are almost the same as in Brown et al,[4] with exception for names of the crystal families 9, 13, and 22. The names for these three families according to Brown et al are given in parenthesis.

The relation between four-dimensional crystal families, crystal systems, and lattice systems is shown in the following table.[3][4] Enantiomorphic systems are marked with an asterisk. The number of enantiomorphic pairs is given in parentheses. Here the term "enantiomorphic" has a different meaning than in the table for three-dimensional crystal classes. The latter means, that enantiomorphic point groups describe chiral (enantiomorphic) structures. In the current table, "enantiomorphic" means that a group itself (considered as a geometric object) is enantiomorphic, like enantiomorphic pairs of three-dimensional space groups P3₁ and P3₂, P4₁22 and P4₃22. Starting from four-dimensional space, point groups also can be enantiomorphic in this sense.

• Crystal cluster – Group of crystals formed in an open space with form determined by their internal crystal structure
• Crystal structure – Ordered arrangement of atoms, ions, or molecules in a crystalline material
• List of space groups
• Polar point group

• Hahn, Theo, ed. (2002). International Tables for Crystallography, Volume A: Space Group Symmetry. International Tables for Crystallography. Vol. A (5th ed.). Berlin, New York: Springer-Verlag. doi:10.1107/97809553602060000100. ISBN 978-0-7923-6590-7.

Wikimedia Commons has media related to Crystal systems.

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• Crystal system at the Online Dictionary of Crystallography
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• Learning Crystallography