Crystal system

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

In crystallography, a crystal system is a set of point groups (a group of geometric symmetries with at least one fixed point). A lattice system is a set of Bravais lattices (an infinite array of discrete points). Space groups (symmetry groups of a configuration in space) are classified into crystal systems according to their point groups, and into lattice systems according to their Bravais lattices. Crystal systems that have space groups assigned to a common lattice system are combined into a crystal family.

Contents

The seven crystal systems are triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal, and cubic. Informally, two crystals are in the same crystal system if they have similar symmetries (though there are many exceptions).

Classifications

Crystals can be classified in three ways: lattice systems, crystal systems and crystal families. 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 "lattice system" or "crystal family".

Lattice system

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

Crystal system

A crystal system is a set of point groups in which the point groups themselves and their corresponding space groups are assigned to a lattice system. Of the 32 crystallographic 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.

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 hexagonal and trigonal crystal systems are combined into one hexagonal crystal family.

Hexagonal hanksite crystal, with threefold c-axis symmetry Hanksite.JPG
Hexagonal hanksite crystal, with threefold c-axis symmetry

Comparison

Five of the crystal systems are essentially the same as five of the lattice systems. The hexagonal and trigonal crystal systems differ from the hexagonal and rhombohedral lattice systems. These are combined into the hexagonal crystal family.

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

Crystal familyCrystal system Lattice system Required symmetries of the point group Point groups Space groups Bravais lattices
Triclinic TriclinicTriclinicNone221
Monoclinic MonoclinicMonoclinic1 twofold axis of rotation or 1 mirror plane 3132
Orthorhombic OrthorhombicOrthorhombic3 twofold axes of rotation or 1 twofold axis of rotation and 2 mirror planes3594
Tetragonal TetragonalTetragonal1 fourfold axis of rotation7682
Hexagonal Trigonal Rhombohedral 1 threefold axis of rotation571
TrigonalHexagonal1 threefold axis of rotation5181
HexagonalHexagonal1 sixfold axis of rotation7271
Cubic CubicCubic4 threefold axes of rotation5363
677Total3223014
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
pinacoidalCi (S2)11x[2,1+] centrosymmetric 2 cyclic
monoclinic sphenoidalC2222[2,2]+ enantiomorphic polar 2 cyclic
domaticCs (C1h)m*11[ ] polar 2 cyclic
prismatic C2h2/m2*[2,2+] centrosymmetric 4 Klein four
orthorhombic rhombic-disphenoidalD2 (V)222222[2,2]+ enantiomorphic 4 Klein four
rhombic-pyramidal C2vmm2*22[2] polar 4 Klein four
rhombic-dipyramidal D2h (Vh)mmm*222[2,2] centrosymmetric 8
tetragonal tetragonal-pyramidalC4444[4]+ enantiomorphic polar 4 cyclic
tetragonal-disphenoidalS442x[2+,2] non-centrosymmetric 4 cyclic
tetragonal-dipyramidalC4h4/m4*[2,4+] centrosymmetric 8
tetragonal-trapezohedralD4422422[2,4]+ enantiomorphic 8 dihedral
ditetragonal-pyramidalC4v4mm*44[4] polar 8 dihedral
tetragonal-scalenohedralD2d (Vd)42m or 4m22*2[2+,4] non-centrosymmetric 8 dihedral
ditetragonal-dipyramidalD4h4/mmm*422[2,4] centrosymmetric 16
hexagonal trigonaltrigonal-pyramidalC3333[3]+ enantiomorphic polar 3 cyclic
rhombohedralC3i (S6)33x[2+,3+] centrosymmetric 6 cyclic
trigonal-trapezohedralD332 or 321 or 312322[3,2]+ enantiomorphic 6 dihedral
ditrigonal-pyramidalC3v3m or 3m1 or 31m*33[3] polar 6 dihedral
ditrigonal-scalenohedralD3d3m or 3m1 or 31m2*3[2+,6] centrosymmetric 12 dihedral
hexagonalhexagonal-pyramidalC6666[6]+ enantiomorphic polar 6 cyclic
trigonal-dipyramidalC3h63*[2,3+] non-centrosymmetric 6 cyclic
hexagonal-dipyramidalC6h6/m6*[2,6+] centrosymmetric 12
hexagonal-trapezohedralD6622622[2,6]+ enantiomorphic 12 dihedral
dihexagonal-pyramidalC6v6mm*66[6] polar 12 dihedral
ditrigonal-dipyramidalD3h6m2 or 62m*322[2,3] non-centrosymmetric 12 dihedral
dihexagonal-dipyramidalD6h6/mmm*622[2,6] centrosymmetric 24
cubic tetartoidalT23332[3,3]+ enantiomorphic 12 alternating
diploidalThm33*2[3+,4] centrosymmetric 24
gyroidalO432432[4,3]+ enantiomorphic 24 symmetric
hextetrahedral Td43m*332[3,3] non-centrosymmetric 24 symmetric
hexoctahedral Ohm3m*432[4,3] centrosymmetric 48

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 lattice systems, and each kind of lattice 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 7 lattice systems 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 Triclinic.svg

aP

Monoclinic (m)C2h Monoclinic.svg

mP

Base-centered monoclinic.svg

mS

Orthorhombic (o)D2h Orthorhombic.svg

oP

Orthorhombic-base-centered.svg

oS

Orthorhombic-body-centered.svg

oI

Orthorhombic-face-centered.svg

oF

Tetragonal (t)D4h Tetragonal.svg

tP

Tetragonal-body-centered.svg

tI

Hexagonal (h)RhombohedralD3d Rhombohedral.svg

hR

HexagonalD6h Hexagonal latticeFRONT.svg

hP

Cubic (c)Oh Cubic.svg

cP

Cubic-body-centered.svg

cI

Cubic-face-centered.svg

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 other dimensions

Two-dimensional space

In two-dimensional space, there are four crystal systems (oblique, rectangular, square, hexagonal), four crystal families (oblique, rectanguar, square, hexagonal), and four lattice systems (oblique, rectangular, square, and hexagonal). [3] [4]

Crystal familyCrystal systemCrystallographic point groupsNo. of plane groupsBravais lattices
Oblique (monoclinic)Oblique1, 22mp
Rectangular (orthorhombic)Rectangularm, 2mm7op, oc
Square (tetragonal)Square4, 4mm3tp
HexagonalHexagonal3, 6, 3m, 6mm5hp
Total410175

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. [5] They are almost the same as in Brown et al., [6] 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 parentheses.

The relation between four-dimensional crystal families, crystal systems, and lattice systems is shown in the following table. [5] [6] 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 systemLattice systemNo. of
crystal system
Point groupsSpace groupsBravais lattices
IHexaclinicHexaclinic P1221
IITriclinicTriclinic P, S23132
IIIDiclinicDiclinic P, S, D32123
IVMonoclinicMonoclinic P, S, S, I, D, F442076
VOrthogonalNon-axial orthogonalOrthogonal KU5221
Orthogonal P, S, I, Z, D, F, G, U1128
Axial orthogonal63887
VITetragonal monoclinicTetragonal monoclinic P, I77882
VIIHexagonal monoclinicTrigonal monoclinicHexagonal monoclinic R8591
Hexagonal monoclinic P151
Hexagonal monoclinic9725
VIIIDitetragonal diclinic*Ditetragonal diclinic P*101 (+1)1 (+1)1 (+1)
IXDitrigonal diclinic*Ditrigonal diclinic P*112 (+2)2 (+2)1 (+1)
XTetragonal orthogonalInverse tetragonal orthogonalTetragonal orthogonal KG12571
Tetragonal orthogonal P, S, I, Z, G3515
Proper tetragonal orthogonal13101312
XIHexagonal orthogonalTrigonal orthogonalHexagonal orthogonal R, RS1410812
Hexagonal orthogonal P, S1502
Hexagonal orthogonal1512240
XIIDitetragonal monoclinic*Ditetragonal monoclinic P*, S*, D*161 (+1)6 (+6)3 (+3)
XIIIDitrigonal monoclinic*Ditrigonal monoclinic P*, RR*172 (+2)5 (+5)2 (+2)
XIVDitetragonal orthogonalCrypto-ditetragonal orthogonalDitetragonal orthogonal D185101
Ditetragonal orthogonal P, Z165 (+2)2
Ditetragonal orthogonal196127
XVHexagonal tetragonalHexagonal tetragonal P20221081
XVIDihexagonal orthogonalCrypto-ditrigonal orthogonal*Dihexagonal orthogonal G*214 (+4)5 (+5)1 (+1)
Dihexagonal orthogonal P5 (+5)1
Dihexagonal orthogonal231120
Ditrigonal orthogonal221141
Dihexagonal orthogonal RR161
XVIICubic orthogonalSimple cubic orthogonalCubic orthogonal KU24591
Cubic orthogonal P, I, Z, F, U965
Complex cubic orthogonal2511366
XVIIIOctagonal*Octagonal P*262 (+2)3 (+3)1 (+1)
XIXDecagonalDecagonal P27451
XXDodecagonal*Dodecagonal P*282 (+2)2 (+2)1 (+1)
XXIDiisohexagonal orthogonalSimple diisohexagonal orthogonalDiisohexagonal orthogonal RR299 (+2)19 (+5)1
Diisohexagonal orthogonal P19 (+3)1
Complex diisohexagonal orthogonal3013 (+8)15 (+9)
XXIIIcosagonalIcosagonal P, SN317202
XXIIIHypercubicOctagonal hypercubicHypercubic P3221 (+8)73 (+15)1
Hypercubic Z107 (+28)1
Dodecagonal hypercubic3316 (+12)25 (+20)
Total23 (+6)33 (+7)33 (+7)227 (+44)4783 (+111)64 (+10)

See also

References

  1. Flack, Howard D. (2003). "Chiral and Achiral Crystal Structures". Helvetica Chimica Acta. 86 (4): 905–921. CiteSeerX   10.1.1.537.266 . doi:10.1002/hlca.200390109.
  2. Hahn 2002, p. 804.
  3. Giacovazzo, Carmelo (10 February 2011). Fundamentals of Crystallography (3rd ed.). Oxford University Press. ISBN   978-0-19-957366-0.
  4. Hahn, Theo (2005). International Tables for Crystallography Volume A: Space-Group Symmetry (5th ed.). Table 2.1.2.1: Springer.{{cite book}}: CS1 maint: location (link)
  5. 1 2 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.
  6. 1 2 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.

Works cited