# Crystal system

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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.

## Contents

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, to eliminate this confusion.

## Overview

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:

Crystal familyCrystal systemRequired symmetries of the point group Point groups Space groups Bravais lattices Lattice system
Triclinic TriclinicNone221Triclinic
Monoclinic Monoclinic1 twofold axis of rotation or 1 mirror plane 3132Monoclinic
Orthorhombic Orthorhombic3 twofold axes of rotation or 1 twofold axis of rotation and 2 mirror planes3594Orthorhombic
Tetragonal Tetragonal1 fourfold axis of rotation7682Tetragonal
Hexagonal Trigonal1 threefold axis of rotation571Rhombohedral
181Hexagonal
Hexagonal1 sixfold axis of rotation727
Cubic Cubic4 threefold axes of rotation5363Cubic
67Total32230147
Note: there is no "trigonal" lattice system. To avoid confusion of terminology, the term "trigonal lattice" is not used.

## Crystal classes

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

Crystal familyCrystal system Point group / Crystal class Schönflies Hermann–Mauguin Orbifold Coxeter Point symmetry Order Abstract group
triclinic pedialC1111[ ]+ enantiomorphic polar 1trivial ${\displaystyle \mathbb {Z} _{1}}$
pinacoidalCi (S2)11x[2,1+] centrosymmetric 2 cyclic ${\displaystyle \mathbb {Z} _{2}}$
monoclinic sphenoidalC2222[2,2]+ enantiomorphic polar 2 cyclic ${\displaystyle \mathbb {Z} _{2}}$
domaticCs (C1h)m*11[ ] polar 2 cyclic ${\displaystyle \mathbb {Z} _{2}}$
prismatic C2h2/m2*[2,2+] centrosymmetric 4 Klein four ${\displaystyle \mathbb {V} =\mathbb {Z} _{2}\times \mathbb {Z} _{2}}$
orthorhombic rhombic-disphenoidalD2 (V)222222[2,2]+ enantiomorphic 4 Klein four ${\displaystyle \mathbb {V} =\mathbb {Z} _{2}\times \mathbb {Z} _{2}}$
rhombic-pyramidal C2vmm2*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-pyramidalC4444[4]+ enantiomorphic polar 4 cyclic ${\displaystyle \mathbb {Z} _{4}}$
tetragonal-disphenoidalS442x[2+,2] non-centrosymmetric 4 cyclic ${\displaystyle \mathbb {Z} _{4}}$
tetragonal-dipyramidalC4h4/m4*[2,4+] centrosymmetric 8${\displaystyle \mathbb {Z} _{4}\times \mathbb {Z} _{2}}$
tetragonal-trapezohedralD4422422[2,4]+ enantiomorphic 8 dihedral ${\displaystyle \mathbb {D} _{8}=\mathbb {Z} _{4}\rtimes \mathbb {Z} _{2}}$
ditetragonal-pyramidalC4v4mm*44[4] polar 8 dihedral ${\displaystyle \mathbb {D} _{8}=\mathbb {Z} _{4}\rtimes \mathbb {Z} _{2}}$
tetragonal-scalenohedralD2d (Vd)42m or 4m22*2[2+,4] non-centrosymmetric 8 dihedral ${\displaystyle \mathbb {D} _{8}=\mathbb {Z} _{4}\rtimes \mathbb {Z} _{2}}$
ditetragonal-dipyramidalD4h4/mmm*422[2,4] centrosymmetric 16${\displaystyle \mathbb {D} _{8}\times \mathbb {Z} _{2}}$
hexagonal trigonaltrigonal-pyramidalC3333[3]+ enantiomorphic polar 3 cyclic ${\displaystyle \mathbb {Z} _{3}}$
rhombohedralC3i (S6)33x[2+,3+] centrosymmetric 6 cyclic ${\displaystyle \mathbb {Z} _{6}=\mathbb {Z} _{3}\times \mathbb {Z} _{2}}$
trigonal-trapezohedralD332 or 321 or 312322[3,2]+ enantiomorphic 6 dihedral ${\displaystyle \mathbb {D} _{6}=\mathbb {Z} _{3}\rtimes \mathbb {Z} _{2}}$
ditrigonal-pyramidalC3v3m or 3m1 or 31m*33[3] polar 6 dihedral ${\displaystyle \mathbb {D} _{6}=\mathbb {Z} _{3}\rtimes \mathbb {Z} _{2}}$
ditrigonal-scalenohedralD3d3m or 3m1 or 31m2*3[2+,6] centrosymmetric 12 dihedral ${\displaystyle \mathbb {D} _{12}=\mathbb {Z} _{6}\rtimes \mathbb {Z} _{2}}$
hexagonalhexagonal-pyramidalC6666[6]+ enantiomorphic polar 6 cyclic ${\displaystyle \mathbb {Z} _{6}=\mathbb {Z} _{3}\times \mathbb {Z} _{2}}$
trigonal-dipyramidalC3h63*[2,3+] non-centrosymmetric 6 cyclic ${\displaystyle \mathbb {Z} _{6}=\mathbb {Z} _{3}\times \mathbb {Z} _{2}}$
hexagonal-dipyramidalC6h6/m6*[2,6+] centrosymmetric 12${\displaystyle \mathbb {Z} _{6}\times \mathbb {Z} _{2}}$
hexagonal-trapezohedralD6622622[2,6]+ enantiomorphic 12 dihedral ${\displaystyle \mathbb {D} _{12}=\mathbb {Z} _{6}\rtimes \mathbb {Z} _{2}}$
dihexagonal-pyramidalC6v6mm*66[6] polar 12 dihedral ${\displaystyle \mathbb {D} _{12}=\mathbb {Z} _{6}\rtimes \mathbb {Z} _{2}}$
ditrigonal-dipyramidalD3h6m2 or 62m*322[2,3] non-centrosymmetric 12 dihedral ${\displaystyle \mathbb {D} _{12}=\mathbb {Z} _{6}\rtimes \mathbb {Z} _{2}}$
dihexagonal-dipyramidalD6h6/mmm*622[2,6] centrosymmetric 24${\displaystyle \mathbb {D} _{12}\times \mathbb {Z} _{2}}$
cubic tetartoidalT23332[3,3]+ enantiomorphic 12 alternating ${\displaystyle \mathbb {A} _{4}}$
diploidalThm33*2[3+,4] centrosymmetric 24${\displaystyle \mathbb {A} _{4}\times \mathbb {Z} _{2}}$
gyroidalO432432[4,3]+ enantiomorphic 24 symmetric ${\displaystyle \mathbb {S} _{4}}$
hextetrahedral Td43m*332[3,3] non-centrosymmetric 24 symmetric ${\displaystyle \mathbb {S} _{4}}$
hexoctahedral Ohm3m*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).

## Bravais lattices

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 familyLattice systemPoint 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)RhombohedralD3d

hR

HexagonalD6h

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[ clarification needed ] 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.

## In four-dimensional space

‌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

Crystal families in 4D space
No.FamilyEdge lengthsInteraxial angles
1Hexaclinicabcdαβγδεζ ≠ 90°
2Triclinicabcdαβγ ≠ 90°
δ = ε = ζ = 90°
3Diclinicabcdα ≠ 90°
β = γ = δ = ε = 90°
ζ ≠ 90°
4Monoclinicabcdα ≠ 90°
β = γ = δ = ε = ζ = 90°
5Orthogonalabcdα = β = γ = δ = ε = ζ = 90°
6Tetragonal monoclinicab = cdα ≠ 90°
β = γ = δ = ε = ζ = 90°
7Hexagonal monoclinicab = cdα ≠ 90°
β = γ = δ = ε = 90°
ζ = 120°
8Ditetragonal diclinica = db = cα = ζ = 90°
β = ε ≠ 90°
γ ≠ 90°
δ = 180° − γ
9Ditrigonal (dihexagonal) diclinica = db = cα = ζ = 120°
β = ε ≠ 90°
γδ ≠ 90°
cos δ = cos β − cos γ
10Tetragonal orthogonalab = cdα = β = γ = δ = ε = ζ = 90°
11Hexagonal orthogonalab = cdα = β = γ = δ = ε = 90°, ζ = 120°
12Ditetragonal monoclinica = db = cα = γ = δ = ζ = 90°
β = ε ≠ 90°
13Ditrigonal (dihexagonal) monoclinica = db = cα = ζ = 120°
β = ε ≠ 90°
γ = δ ≠ 90°
cos γ = −1/2cos β
14Ditetragonal orthogonala = db = cα = β = γ = δ = ε = ζ = 90°
15Hexagonal tetragonala = db = cα = β = γ = δ = ε = 90°
ζ = 120°
16Dihexagonal orthogonala = db = cα = ζ = 120°
β = γ = δ = ε = 90°
17Cubic orthogonala = b = cdα = β = γ = δ = ε = ζ = 90°
18Octagonala = b = c = dα = γ = ζ ≠ 90°
β = ε = 90°
δ = 180° − α
19Decagonala = b = c = dα = γ = ζβ = δ = ε
cos β = −1/2 − cos α
20Dodecagonala = b = c = dα = ζ = 90°
β = ε = 120°
γ = δ ≠ 90°
21Diisohexagonal orthogonala = b = c = dα = ζ = 120°
β = γ = δ = ε = 90°
22Icosagonal (icosahedral)a = b = c = dα = β = γ = δ = ε = ζ
cos α = −1/4
23Hypercubica = b = c = dα = β = γ = δ = ε = ζ = 90°

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 systems in 4D space
No. of
crystal family
Crystal familyCrystal systemNo. of
crystal system
Point groupsSpace groupsBravais latticesLattice system
IHexaclinic1221Hexaclinic P
IITriclinic23132Triclinic P, S
IIIDiclinic32123Diclinic P, S, D
IVMonoclinic442076Monoclinic P, S, S, I, D, F
VOrthogonalNon-axial orthogonal5221Orthogonal KU
1128Orthogonal P, S, I, Z, D, F, G, U
Axial orthogonal63887
VITetragonal monoclinic77882Tetragonal monoclinic P, I
VIIHexagonal monoclinicTrigonal monoclinic8591Hexagonal monoclinic R
151Hexagonal monoclinic P
Hexagonal monoclinic9725
VIIIDitetragonal diclinic*101 (+1)1 (+1)1 (+1)Ditetragonal diclinic P*
IXDitrigonal diclinic*112 (+2)2 (+2)1 (+1)Ditrigonal diclinic P*
XTetragonal orthogonalInverse tetragonal orthogonal12571Tetragonal orthogonal KG
3515Tetragonal orthogonal P, S, I, Z, G
Proper tetragonal orthogonal13101312
XIHexagonal orthogonalTrigonal orthogonal1410812Hexagonal orthogonal R, RS
1502Hexagonal orthogonal P, S
Hexagonal orthogonal1512240
XIIDitetragonal monoclinic*161 (+1)6 (+6)3 (+3)Ditetragonal monoclinic P*, S*, D*
XIIIDitrigonal monoclinic*172 (+2)5 (+5)2 (+2)Ditrigonal monoclinic P*, RR*
XIVDitetragonal orthogonalCrypto-ditetragonal orthogonal185101Ditetragonal orthogonal D
165 (+2)2Ditetragonal orthogonal P, Z
Ditetragonal orthogonal196127
XVHexagonal tetragonal20221081Hexagonal tetragonal P
XVIDihexagonal orthogonalCrypto-ditrigonal orthogonal*214 (+4)5 (+5)1 (+1)Dihexagonal orthogonal G*
5 (+5)1Dihexagonal orthogonal P
Dihexagonal orthogonal231120
Ditrigonal orthogonal221141
161Dihexagonal orthogonal RR
XVIICubic orthogonalSimple cubic orthogonal24591Cubic orthogonal KU
965Cubic orthogonal P, I, Z, F, U
Complex cubic orthogonal2511366
XVIIIOctagonal*262 (+2)3 (+3)1 (+1)Octagonal P*
XIXDecagonal27451Decagonal P
XXDodecagonal*282 (+2)2 (+2)1 (+1)Dodecagonal P*
XXIDiisohexagonal orthogonalSimple diisohexagonal orthogonal299 (+2)19 (+5)1Diisohexagonal orthogonal RR
19 (+3)1Diisohexagonal orthogonal P
Complex diisohexagonal orthogonal3013 (+8)15 (+9)
XXIIIcosagonal317202Icosagonal P, SN
XXIIIHypercubicOctagonal hypercubic3221 (+8)73 (+15)1Hypercubic P
107 (+28)1Hypercubic Z
Dodecagonal hypercubic3316 (+12)25 (+20)
Total23 (+6)33 (+7)227 (+44)4783 (+111)64 (+10)33 (+7)

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## References

1. Flack, Howard D. (2003). "Chiral and Achiral Crystal Structures". Helvetica Chimica Acta. 86 (4): 905–921. CiteSeerX  . doi:10.1002/hlca.200390109 via Wiley Online Library.
2. Hahn 2002, p. 804.
3. Whittaker, E. J. W. (1985). An Atlas of Hyperstereograms of the Four-Dimensional Crystal Classes. Oxford: Clarendon Press. ISBN   978-0-19-854432-6. OCLC   638900498.
4. Brown, H.; Bülow, R.; Neubüser, J.; Wondratschek, H.; Zassenhaus, H. (1978). Crystallographic Groups of Four-Dimensional Space. New York: Wiley. ISBN   978-0-471-03095-9. OCLC   939898594.