Crystal system

The diamond crystal structure belongs to the face-centered cubic lattice, with a repeated two-atom pattern.

In crystallography, the terms crystal system, crystal family, and lattice system each refer to one of several classes of space groups, lattices, point groups, or crystals. Informally, two crystals are in the same crystal system if they have similar symmetries, although there are many exceptions to this.

Crystal systems, crystal families and lattice systems are similar but slightly different, and there is widespread confusion between them: in particular the trigonal crystal system is often confused with the rhombohedral lattice system, and the term "crystal system" is sometimes used to mean "lattice system" or "crystal family".

Space groups and crystals are divided into seven crystal systems according to their point groups, and into seven lattice systems according to their Bravais lattices. Five of the crystal systems are essentially the same as five of the lattice systems, but the hexagonal and trigonal crystal systems differ from the hexagonal and rhombohedral lattice systems. The six crystal families are formed by combining the hexagonal and trigonal crystal systems into one hexagonal family, in order to eliminate this confusion.

Overview[edit]

Hexagonal hanksite crystal, with threefold c-axis symmetry

A lattice system is a class of lattices with the same set of lattice point groups, which are subgroups of the arithmetic crystal classes. The 14 Bravais lattices are grouped into seven lattice systems: triclinic, monoclinic, orthorhombic, tetragonal, rhombohedral, hexagonal, and cubic.

In a crystal system, a set of point groups and their corresponding space groups are assigned to a lattice system. Of the 32 point groups that exist in three dimensions, most are assigned to only one lattice system, in which case both the crystal and lattice systems have the same name. However, five point groups are assigned to two lattice systems, rhombohedral and hexagonal, because both exhibit threefold rotational symmetry. These point groups are assigned to the trigonal crystal system. In total there are seven crystal systems: triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal, and cubic.

A crystal family is determined by lattices and point groups. It is formed by combining crystal systems which 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. In total there are six crystal families: triclinic, monoclinic, orthorhombic, tetragonal, hexagonal, and cubic.

Spaces with less 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.

The relation between three-dimensional crystal families, crystal systems and lattice systems is shown in the following table:

Note: there is no "trigonal" lattice system. To avoid confusion of terminology, the term "trigonal lattice" is not used.

Crystal classes[edit]

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

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	${\displaystyle \mathbb {S} _{4}\times \mathbb {Z} _{2}}$

Point symmetry can be thought of in the following fashion: consider the coordinates which make up the structure, and project 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 for non-centrosymmetric case, inverted structure in some cases can be rotated to align with the original structure. This is the case of non-centrosymmetric achiral structure. If the inverted structure cannot be rotated to align with the original structure, then the structure is chiral (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 polar symmetry^{[clarification needed]} direction of a crystal is called a polar axis.^[2] Groups containing a polar axis are called polar. A polar crystal possess a "unique" axis (found in no other directions) such that some geometrical or physical property is different at the two ends of this axis. It may develop a dielectric polarization, e.g. in pyroelectric crystals. A polar axis can occur only in non-centrosymmetric structures. There should also not be a mirror plane or twofold axis perpendicular to the polar axis, because they will make both 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).

Bravais lattices[edit]

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

Crystal family	Lattice system	Schönflies	14 Bravais Lattices
			Primitive	Base-centered	Body-centered	Face-centered
triclinic		Ci
monoclinic		C2h
orthorhombic		D2h
tetragonal		D4h
hexagonal	rhombohedral	D3d
	hexagonal	D6h
cubic		Oh

In geometry and crystallography, a Bravais lattice is a category of symmetry groups for translational symmetry in three directions, or correspondingly, a category of translation lattices.

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 space group of the translation lattice itself; there are 14 Bravais lattices in three dimensions; each can apply in one lattice system only. They represent the maximum symmetry a structure with the translational symmetry concerned can have.

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

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.

In four-dimensional space[edit]

‌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 are 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.

See also[edit]

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

References[edit]

This article lacks ISBNs for the books listed in it. Please make it easier to conduct research by listing ISBNs. If the {{Cite book}} or {{citation}} templates are in use, you may add ISBNs automatically, or discuss this issue on the talk page. (August 2017)

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

External links[edit]

• Overview of the 32 groups
• Mineral galleries – Symmetry
• all cubic crystal classes, forms, and stereographic projections (interactive java applet)
• Crystal system at the Online Dictionary of Crystallography
• Crystal family at the Online Dictionary of Crystallography
• Lattice system at the Online Dictionary of Crystallography
• Conversion Primitive to Standard Conventional for VASP input files
• Learning Crystallography