# Space group

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In mathematics, physics and chemistry, a space group is the symmetry group of a configuration in space, usually in three dimensions. [1] In three dimensions, there are 219 distinct types, or 230 if chiral copies are considered distinct. Space groups are also studied in dimensions other than 3 where they are sometimes called Bieberbach groups, and are discrete cocompact groups of isometries of an oriented Euclidean space.

## Contents

In crystallography, space groups are also called the crystallographic or Fedorov groups, and represent a description of the symmetry of the crystal. A definitive source regarding 3-dimensional space groups is the International Tables for Crystallography (Hahn (2002)).

## History

Space groups in 2 dimensions are the 17 wallpaper groups which have been known for several centuries, though the proof that the list was complete was only given in 1891, after the much more difficult classification of space groups had largely been completed. [2] In 1879 the German mathematician Leonhard Sohncke listed the 65 space groups (called Sohncke groups) whose elements preserve the chirality. [3] More accurately, he listed 66 groups, but both the Russian mathematician and crystallographer Evgraf Fedorov and the German mathematician Arthur Moritz Schoenflies noticed that two of them were really the same. The space groups in three dimensions were first enumerated in 1891 by Fedorov [4] (whose list had two omissions (I43d and Fdd2) and one duplication (Fmm2)), and shortly afterwards in 1891 were independently enumerated by Schönflies [5] (whose list had four omissions (I43d, Pc, Cc, ?) and one duplication (P421m)). The correct list of 230 space groups was found by 1892 during correspondence between Fedorov and Schönflies. [6] Barlow  ( 1894 ) later enumerated the groups with a different method, but omitted four groups (Fdd2, I42d, P421d, and P421c) even though he already had the correct list of 230 groups from Fedorov and Schönflies; the common claim that Barlow was unaware of their work is incorrect.[ citation needed ] Burckhardt (1967) describes the history of the discovery of the space groups in detail.

## Elements

The space groups in three dimensions are made from combinations of the 32 crystallographic point groups with the 14 Bravais lattices, each of the latter belonging to one of 7 lattice systems. What this means is that the action of any element of a given space group can be expressed as the action of an element of the appropriate point group followed optionally by a translation. A space group is thus some combination of the translational symmetry of a unit cell (including lattice centering[ clarification needed ]), the point group symmetry operations of reflection, rotation and improper rotation (also called rotoinversion), and the screw axis and glide plane symmetry operations. The combination of all these symmetry operations results in a total of 230 different space groups describing all possible crystal symmetries.

### Elements fixing a point

The elements of the space group fixing a point of space are the identity element, reflections, rotations and improper rotations.

### Translations

The translations form a normal abelian subgroup of rank 3, called the Bravais lattice. There are 14 possible types of Bravais lattice. The quotient of the space group by the Bravais lattice is a finite group which is one of the 32 possible point groups.

### Glide planes

A glide plane is a reflection in a plane, followed by a translation parallel with that plane. This is noted by ${\displaystyle a}$, ${\displaystyle b}$, or ${\displaystyle c}$, depending on which axis the glide is along. There is also the ${\displaystyle n}$ glide, which is a glide along the half of a diagonal of a face, and the ${\displaystyle d}$ glide, which is a fourth of the way along either a face or space diagonal of the unit cell. The latter is called the diamond glide plane as it features in the diamond structure. In 17 space groups, due to the centering of the cell, the glides occur in two perpendicular directions simultaneously, i.e. the same glide plane can be called b or c, a or b, a or c. For example, group Abm2 could be also called Acm2, group Ccca could be called Cccb. In 1992, it was suggested to use symbol e for such planes. The symbols for five space groups have been modified:

Space group no.3941646768
New symbolAem2Aea2CmceCmmeCcce
Old SymbolAbm2Aba2CmcaCmmaCcca

### Screw axes

A screw axis is a rotation about an axis, followed by a translation along the direction of the axis. These are noted by a number, n, to describe the degree of rotation, where the number is how many operations must be applied to complete a full rotation (e.g., 3 would mean a rotation one third of the way around the axis each time). The degree of translation is then added as a subscript showing how far along the axis the translation is, as a portion of the parallel lattice vector. So, 21 is a twofold rotation followed by a translation of 1/2 of the lattice vector.

### General formula

The general formula for the action of an element of a space group is

y = M.x + D

where M is its matrix, D is its vector, and where the element transforms point x into point y. In general, D = D(lattice) + D(M), where D(M) is a unique function of M that is zero for M being the identity. The matrices M form a point group that is a basis of the space group; the lattice must be symmetric under that point group, but the crystal structure itself may not be symmetric under that point group as applied to any particular point (that is, without a translation). For example, the diamond cubic structure does not have any point where the cubic point group applies.

The lattice dimension can be less than the overall dimension, resulting in a "subperiodic" space group. For (overall dimension, lattice dimension):

## Notation

There are at least ten methods of naming space groups. Some of these methods can assign several different names to the same space group, so altogether there are many thousands of different names.

Number
The International Union of Crystallography publishes tables of all space group types, and assigns each a unique number from 1 to 230. The numbering is arbitrary, except that groups with the same crystal system or point group are given consecutive numbers.
International symbol notation
Hermann–Mauguin notation
The Hermann–Mauguin (or international) notation describes the lattice and some generators for the group. It has a shortened form called the international short symbol, which is the one most commonly used in crystallography, and usually consists of a set of four symbols. The first describes the centering of the Bravais lattice (P, A, C, I, R or F). The next three describe the most prominent symmetry operation visible when projected along one of the high symmetry directions of the crystal. These symbols are the same as used in point groups, with the addition of glide planes and screw axis, described above. By way of example, the space group of quartz is P3121, showing that it exhibits primitive centering of the motif (i.e., once per unit cell), with a threefold screw axis and a twofold rotation axis. Note that it does not explicitly contain the crystal system, although this is unique to each space group (in the case of P3121, it is trigonal).
In the international short symbol the first symbol (31 in this example) denotes the symmetry along the major axis (c-axis in trigonal cases), the second (2 in this case) along axes of secondary importance (a and b) and the third symbol the symmetry in another direction. In the trigonal case there also exists a space group P3112. In this space group the twofold axes are not along the a and b-axes but in a direction rotated by 30°.
The international symbols and international short symbols for some of the space groups were changed slightly between 1935 and 2002, so several space groups have 4 different international symbols in use.

The viewing directions of the 7 crystal systems are shown as follows.

Position in the symbolTriclinicMonoclinicOrthorhombicTetragonalTrigonalHexagonalCubic
1baccca
2baaa[111]
3c[110][210][210][110]
Hall notation [7]
Space group notation with an explicit origin. Rotation, translation and axis-direction symbols are clearly separated and inversion centers are explicitly defined. The construction and format of the notation make it particularly suited to computer generation of symmetry information. For example, group number 3 has three Hall symbols: P 2y (P 1 2 1), P 2 (P 1 1 2), P 2x (P 2 1 1).
Schönflies notation
The space groups with given point group are numbered by 1, 2, 3, … (in the same order as their international number) and this number is added as a superscript to the Schönflies symbol for the point group. For example, groups numbers 3 to 5 whose point group is C2 have Schönflies symbols C1
2
, C2
2
, C3
2
.
Fedorov notation
Shubnikov symbol
Strukturbericht designation
A related notation for crystal structures given a letter and index: A Elements (monatomic), B for AB compounds, C for AB2 compounds, D for Am Bn compounds, (E, F, ..., K More complex compounds), L Alloys, O Organic compounds, S Silicates. Some structure designation share the same space groups. For example, space group 225 is A1, B1, and C1. Space group 221 is Ah, and B2. [8] However, crystallographers would not use Strukturbericht notation to describe the space group, rather it would be used to describe a specific crystal structure (e.g. space group + atomic arrangement (motif)).
Orbifold notation (2D)
Fibrifold notation (3D)
As the name suggests, the orbifold notation describes the orbifold, given by the quotient of Euclidean space by the space group, rather than generators of the space group. It was introduced by Conway and Thurston, and is not used much outside mathematics. Some of the space groups have several different fibrifolds associated to them, so have several different fibrifold symbols.
Coxeter notation
Spatial and point symmetry groups, represented as modications of the pure reflectional Coxeter groups.
Geometric notation [9]
A geometric algebra notation.

## Classification systems

There are (at least) 10 different ways to classify space groups into classes. The relations between some of these are described in the following table. Each classification system is a refinement of the ones below it.

(Crystallographic) space group types (230 in three dimensions)
Two space groups, considered as subgroups of the group of affine transformations of space, have the same space group type if they are conjugate by a chirality-preserving affine transformation. In three dimensions, for 11 of the affine space groups, there is no chirality-preserving map from the group to its mirror image, so if one distinguishes groups from their mirror images these each split into two cases (Such as P41 and P43). So there are 54 + 11 = 65 space group types that preserve chirality (the Sohncke groups).
Affine space group types (219 in three dimensions)
Two space groups, considered as subgroups of the group of affine transformations of space, have the same affine space group type if they are conjugate under an affine transformation. The affine space group type is determined by the underlying abstract group of the space group. In three dimensions there are 54 affine space group types that preserve chirality.
Arithmetic crystal classes (73 in three dimensions)
Sometimes called Z-classes. These are determined by the point group together with the action of the point group on the subgroup of translations. In other words, the arithmetic crystal classes correspond to conjugacy classes of finite subgroup of the general linear group GLn(Z) over the integers. A space group is called symmorphic (or split) if there is a point such that all symmetries are the product of a symmetry fixing this point and a translation. Equivalently, a space group is symmorphic if it is a semidirect product of its point group with its translation subgroup. There are 73 symmorphic space groups, with exactly one in each arithmetic crystal class. There are also 157 nonsymmorphic space group types with varying numbers in the arithmetic crystal classes.

Arithmetic crystal classes may be interpreted as different orientations of the point groups in the lattice, with the group elements' matrix components being constrained to have integer coefficients in lattice space. This is rather easy to picture in the two-dimensional, wallpaper group case. Some of the point groups have reflections, and the reflection lines can be along the lattice directions, halfway in between them, or both.

• None: C1: p1; C2: p2; C3: p3; C4: p4; C6: p6
• Along: D1: pm, pg; D2: pmm, pmg, pgg; D3: p31m
• Between: D1: cm; D2: cmm; D3: p3m1
• Both: D4: p4m, p4g; D6: p6m
(geometric) Crystal classes (32 in three dimensions)Bravais flocks (14 in three dimensions)
Sometimes called Q-classes. The crystal class of a space group is determined by its point group: the quotient by the subgroup of translations, acting on the lattice. Two space groups are in the same crystal class if and only if their point groups, which are subgroups of GLn(Z), are conjugate in the larger group GLn(Q).These are determined by the underlying Bravais lattice type.

These correspond to conjugacy classes of lattice point groups in GLn(Z), where the lattice point group is the group of symmetries of the underlying lattice that fix a point of the lattice, and contains the point group.

Crystal systems (7 in three dimensions) Lattice systems (7 in three dimensions)
Crystal systems are an ad hoc modification of the lattice systems to make them compatible with the classification according to point groups. They differ from crystal families in that the hexagonal crystal family is split into two subsets, called the trigonal and hexagonal crystal systems. The trigonal crystal system is larger than the rhombohedral lattice system, the hexagonal crystal system is smaller than the hexagonal lattice system, and the remaining crystal systems and lattice systems are the same.The lattice system of a space group is determined by the conjugacy class of the lattice point group (a subgroup of GLn(Z)) in the larger group GLn(Q). In three dimensions the lattice point group can have one of the 7 different orders 2, 4, 8, 12, 16, 24, or 48. The hexagonal crystal family is split into two subsets, called the rhombohedral and hexagonal lattice systems.
Crystal families (6 in three dimensions)
The point group of a space group does not quite determine its lattice system, because occasionally two space groups with the same point group may be in different lattice systems. Crystal families are formed from lattice systems by merging the two lattice systems whenever this happens, so that the crystal family of a space group is determined by either its lattice system or its point group. In 3 dimensions the only two lattice families that get merged in this way are the hexagonal and rhombohedral lattice systems, which are combined into the hexagonal crystal family. The 6 crystal families in 3 dimensions are called triclinic, monoclinic, orthorhombic, tetragonal, hexagonal, and cubic. Crystal families are commonly used in popular books on crystals, where they are sometimes called crystal systems.

Conway ,Delgado Friedrichs,andHusonet al. ( 2001 ) gave another classification of the space groups, called a fibrifold notation, according to the fibrifold structures on the corresponding orbifold. They divided the 219 affine space groups into reducible and irreducible groups. The reducible groups fall into 17 classes corresponding to the 17 wallpaper groups, and the remaining 35 irreducible groups are the same as the cubic groups and are classified separately.

## In other dimensions

### Bieberbach's theorems

In n dimensions, an affine space group, or Bieberbach group, is a discrete subgroup of isometries of n-dimensional Euclidean space with a compact fundamental domain. Bieberbach ( 1911 , 1912 ) proved that the subgroup of translations of any such group contains n linearly independent translations, and is a free abelian subgroup of finite index, and is also the unique maximal normal abelian subgroup. He also showed that in any dimension n there are only a finite number of possibilities for the isomorphism class of the underlying group of a space group, and moreover the action of the group on Euclidean space is unique up to conjugation by affine transformations. This answers part of Hilbert's eighteenth problem. Zassenhaus (1948) showed that conversely any group that is the extension[ when defined as? ] of Zn by a finite group acting faithfully is an affine space group. Combining these results shows that classifying space groups in n dimensions up to conjugation by affine transformations is essentially the same as classifying isomorphism classes for groups that are extensions of Zn by a finite group acting faithfully.

It is essential in Bieberbach's theorems to assume that the group acts as isometries; the theorems do not generalize to discrete cocompact groups of affine transformations of Euclidean space. A counter-example is given by the 3-dimensional Heisenberg group of the integers acting by translations on the Heisenberg group of the reals, identified with 3-dimensional Euclidean space. This is a discrete cocompact group of affine transformations of space, but does not contain a subgroup Z3.

### Classification in small dimensions

This table gives the number of space group types in small dimensions, including the numbers of various classes of space group. The numbers of enantiomorphic pairs are given in parentheses.

DimensionsCrystal families (sequence in the OEIS )Crystal systems (sequence in the OEIS )Bravais lattices (sequence in the OEIS )Abstract crystallographic point groups (sequence in the OEIS )Geometric crystal classes, Q-classes, crystallographic point groups (sequence in the OEIS )Arithmetic crystal classes, Z-classes (sequence in the OEIS )Affine space group types (sequence in the OEIS )Crystallographic space group types (sequence in the OEIS )
0 [lower-alpha 1] 11111111
1 [lower-alpha 2] 11122222
2 [lower-alpha 3] 445910131717
3 [lower-alpha 4] 6714183273219 (+11)230
4 [lower-alpha 5] 23 (+6)33 (+7)64 (+10)118227 (+44)710 (+70)4783 (+111)4894
5 [lower-alpha 6] 32591892399556079222018 (+79)222097
6 [lower-alpha 7] 912518411594710385308 (+?)28927915 (+?)?
1. Trivial group
2. One is the group of integers and the other is the infinite dihedral group; see symmetry groups in one dimension.
3. These 2D space groups are also called wallpaper groups or plane groups.
4. In 3D, there are 230 crystallographic space group types, which reduces to 219 affine space group types because of some types being different from their mirror image; these are said to differ by enantiomorphous character (e.g. P3112 and P3212). Usually space group refers to 3D. They were enumerated independently by Barlow (1894), Fedorov (1891) and Schönflies (1891).
5. The 4895 4-dimensional groups were enumerated by HaroldBrown,Rolf Bülow,andJoachim Neubüseret al. (1978) Neubüser, Souvignier & Wondratschek (2002) corrected the number of enantiomorphic groups from 112 to 111, so total number of groups is 4783 + 111 = 4894. There are 44 enantiomorphic point groups in 4-dimensional space. If we consider enantiomorphic groups as different, the total number of point groups is 227 + 44 = 271.
6. Plesken & Schulz (2000) enumerated the ones of dimension 5. Souvignier (2003) counted the enantiomorphs.
7. Plesken & Schulz (2000) enumerated the ones of dimension 6, later the corrected figures were found. [10] Initially published number of 826 Lattice types in Plesken & Hanrath (1984) was corrected to 841 in Opgenorth, Plesken & Schulz (1998). See also Janssen et al. (2002) . Souvignier (2003) counted the enantiomorphs, but that paper relied on old erroneous CARAT data for dimension 6.

### Magnetic groups and time reversal

In addition to crystallographic space groups there are also magnetic space groups (also called two-color (black and white) crystallographic groups or Shubnikov groups). These symmetries contain an element known as time reversal. They treat time as an additional dimension, and the group elements can include time reversal as reflection in it. They are of importance in magnetic structures that contain ordered unpaired spins, i.e. ferro-, ferri- or antiferromagnetic structures as studied by neutron diffraction. The time reversal element flips a magnetic spin while leaving all other structure the same and it can be combined with a number of other symmetry elements. Including time reversal there are 1651 magnetic space groups in 3D ( Kim 1999 , p.428). It has also been possible to construct magnetic versions for other overall and lattice dimensions (Daniel Litvin's papers, ( Litvin 2008 ), ( Litvin 2005 )). Frieze groups are magnetic 1D line groups and layer groups are magnetic wallpaper groups, and the axial 3D point groups are magnetic 2D point groups. Number of original and magnetic groups by (overall, lattice) dimension:( Palistrant 2012 )( Souvignier 2006 )

Overall
dimension
Lattice
dimension
Ordinary groupsMagnetic groups
NameSymbolCountSymbolCount
00Zero-dimensional symmetry group${\displaystyle G_{0}}$1${\displaystyle G_{0}^{1}}$2
10 One-dimensional point groups ${\displaystyle G_{10}}$2${\displaystyle G_{10}^{1}}$5
1 One-dimensional discrete symmetry groups ${\displaystyle G_{1}}$2${\displaystyle G_{1}^{1}}$7
20 Two-dimensional point groups ${\displaystyle G_{20}}$10${\displaystyle G_{20}^{1}}$31
1 Frieze groups ${\displaystyle G_{21}}$7${\displaystyle G_{21}^{1}}$31
2 Wallpaper groups ${\displaystyle G_{2}}$17${\displaystyle G_{2}^{1}}$80
30 Three-dimensional point groups ${\displaystyle G_{30}}$32${\displaystyle G_{30}^{1}}$122
1 Rod groups ${\displaystyle G_{31}}$75${\displaystyle G_{31}^{1}}$394
2 Layer groups ${\displaystyle G_{32}}$80${\displaystyle G_{32}^{1}}$528
3 Three-dimensional space groups ${\displaystyle G_{3}}$230${\displaystyle G_{3}^{1}}$1651
40Four-dimensional point groups${\displaystyle G_{40}}$271${\displaystyle G_{40}^{1}}$1202
1${\displaystyle G_{41}}$343
2${\displaystyle G_{42}}$1091
3${\displaystyle G_{43}}$1594
4Four-dimensional discrete symmetry groups${\displaystyle G_{4}}$4894${\displaystyle G_{4}^{1}}$62227

## Table of space groups in 2 dimensions (wallpaper groups)

Table of the wallpaper groups using the classification of the 3-dimensional space groups:

Crystal system
(Bravais lattice)
Geometric class
Point group
Arithmetic
class
Wallpaper groups
(cell diagram)
Schön. Orbifold Cox. Ord.
Oblique
C1(1)[ ]+1Nonep1
(1)

C2(22)[2]+2Nonep2
(2222)

Rectangular
(Centered rhombic)
D1(*)[ ]2Alongpm
(**)
pg
(××)
D2(*22)[2]4Alongpmm
(*2222)
pmg
(22*)
Rhombic
(Centered rectangular)
D1(*)[ ]2Betweencm
(*×)

D2(*22)[2]4Betweencmm
(2*22)
pgg
(22×)
Square
C4(44)[4]+4Nonep4
(442)

D4(*44)[4]8Bothp4m
(*442)
p4g
(4*2)
Hexagonal
C3(33)[3]+3Nonep3
(333)

D3(*33)[3]6Betweenp3m1
(*333)
p31m
(3*3)
C6(66)[6]+6Nonep6
(632)

D6(*66)[6]12Bothp6m
(*632)

For each geometric class, the possible arithmetic classes are

• None: no reflection lines
• Along: reflection lines along lattice directions
• Between: reflection lines halfway in between lattice directions
• Both: reflection lines both along and between lattice directions

## Table of space groups in 3 dimensions

# Crystal system
(count)
Bravais lattice
Point group Space groups (international short symbol)
Int'l Schön. Orbifold Cox. Ord.
1 Triclinic
(2)
1C111[ ]+1P1
21Ci[2+,2+]2P1
3–5 Monoclinic
(13)
2C222[2]+2P2, P21
C2
6–9mCs*11[ ]2Pm, Pc
Cm, Cc
10–152/mC2h2*[2,2+]4P2/m, P21/m
C2/m, P2/c, P21/c
C2/c
16–24 Orthorhombic
(59)

222D2222[2,2]+4P222, P2221, P21212, P212121, C2221, C222, F222, I222, I212121
25–46mm2C2v*22[2]4Pmm2, Pmc21, Pcc2, Pma2, Pca21, Pnc2, Pmn21, Pba2, Pna21, Pnn2
Cmm2, Cmc21, Ccc2, Amm2, Aem2, Ama2, Aea2
Fmm2, Fdd2
Imm2, Iba2, Ima2
47–74mmmD2h*222[2,2]8Pmmm, Pnnn, Pccm, Pban, Pmma, Pnna, Pmna, Pcca, Pbam, Pccn, Pbcm, Pnnm, Pmmn, Pbcn, Pbca, Pnma
Cmcm, Cmce, Cmmm, Cccm, Cmme, Ccce
Fmmm, Fddd
Immm, Ibam, Ibca, Imma
75–80 Tetragonal
(68)

4C444[4]+4P4, P41, P42, P43, I4, I41
81–824S4[2+,4+]4P4, I4
83–884/mC4h4*[2,4+]8P4/m, P42/m, P4/n, P42/n
I4/m, I41/a
89–98422D4224[2,4]+8P422, P4212, P4122, P41212, P4222, P42212, P4322, P43212
I422, I4122
99–1104mmC4v*44[4]8P4mm, P4bm, P42cm, P42nm, P4cc, P4nc, P42mc, P42bc
I4mm, I4cm, I41md, I41cd
111–12242mD2d2*2[2+,4]8P42m, P42c, P421m, P421c, P4m2, P4c2, P4b2, P4n2
I4m2, I4c2, I42m, I42d
123–1424/mmmD4h*224[2,4]16P4/mmm, P4/mcc, P4/nbm, P4/nnc, P4/mbm, P4/mnc, P4/nmm, P4/ncc, P42/mmc, P42/mcm, P42/nbc, P42/nnm, P42/mbc, P42/mnm, P42/nmc, P42/ncm
I4/mmm, I4/mcm, I41/amd, I41/acd
143–146 Trigonal
(25)
3C333[3]+3P3, P31, P32
R3
147–1483S6[2+,6+]6P3, R3
149–15532D3223[2,3]+6P312, P321, P3112, P3121, P3212, P3221
R32
156–1613mC3v*33[3]6P3m1, P31m, P3c1, P31c
R3m, R3c
162–1673mD3d2*3[2+,6]12P31m, P31c, P3m1, P3c1
R3m, R3c
168–173 Hexagonal
(27)
6C666[6]+6P6, P61, P65, P62, P64, P63
1746C3h3*[2,3+]6P6
175–1766/mC6h6*[2,6+]12P6/m, P63/m
177–182622D6226[2,6]+12P622, P6122, P6522, P6222, P6422, P6322
183–1866mmC6v*66[6]12P6mm, P6cc, P63cm, P63mc
187–1906m2D3h*223[2,3]12P6m2, P6c2, P62m, P62c
191–1946/mmmD6h*226[2,6]24P6/mmm, P6/mcc, P63/mcm, P63/mmc
195–199 Cubic
(36)

23T332[3,3]+12P23, F23, I23
P213, I213
200–206m3Th3*2[3+,4]24Pm3, Pn3, Fm3, Fd3, Im3, Pa3, Ia3
207–214432O432[3,4]+24P432, P4232
F432, F4132
I432
P4332, P4132, I4132
215–22043mTd*332[3,3]24P43m, F43m, I43m
P43n, F43c, I43d
221–230m3mOh*432[3,4]48Pm3m, Pn3n, Pm3n, Pn3m
Fm3m, Fm3c, Fd3m, Fd3c
Im3m, Ia3d

Note: An e plane is a double glide plane, one having glides in two different directions. They are found in seven orthorhombic, five tetragonal and five cubic space groups, all with centered lattice. The use of the symbol e became official with Hahn (2002).

The lattice system can be found as follows. If the crystal system is not trigonal then the lattice system is of the same type. If the crystal system is trigonal, then the lattice system is hexagonal unless the space group is one of the seven in the rhombohedral lattice system consisting of the 7 trigonal space groups in the table above whose name begins with R. (The term rhombohedral system is also sometimes used as an alternative name for the whole trigonal system.) The hexagonal lattice system is larger than the hexagonal crystal system, and consists of the hexagonal crystal system together with the 18 groups of the trigonal crystal system other than the seven whose names begin with R.

The Bravais lattice of the space group is determined by the lattice system together with the initial letter of its name, which for the non-rhombohedral groups is P, I, F, A or C, standing for the principal, body centered, face centered, A-face centered or C-face centered lattices.

## Derivation of the crystal class from the space group

1. Leave out the Bravais 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.

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In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic system. They form a rectangular prism with a parallelogram as its base. Hence two pairs of vectors are perpendicular, while the third pair makes an angle other than 90°.

In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a rectangular base and height (c), such that a, b, and c are distinct. All three bases intersect at 90° angles, so the three lattice vectors remain mutually orthogonal.

In geometry and crystallography, a Bravais lattice, named after Auguste Bravais (1850), is an infinite array of discrete points generated by a set of discrete translation operations described in three dimensional space by:

In crystallography, a crystallographic point group is a set of symmetry operations, corresponding to one of the point groups in three dimensions, such that each operation would leave the structure of a crystal unchanged i.e. the same kinds of atoms would be placed in similar positions as before the transformation. For example, in a primitive cubic crystal system, a rotation of the unit cell by 90 degree around an axis that is perpendicular to two parallel faces of the cube, intersecting at its center, is a symmetry operation that projects each atom to the location of one of its neighbor leaving the overall structure of the crystal unaffected.

In crystallography, the tricliniccrystal system is one of the 7 crystal systems. A crystal system is described by three basis vectors. In the triclinic system, the crystal is described by vectors of unequal length, as in the orthorhombic system. In addition, the angles between these vectors must all be different and may not include 90°.

The Schoenfliesnotation, named after the German mathematician Arthur Moritz Schoenflies, is a notation primarily used to specify point groups in three dimensions. Because a point group alone is completely adequate to describe the symmetry of a molecule, the notation is often sufficient and commonly used for spectroscopy. However, in crystallography, there is additional translational symmetry, and point groups are not enough to describe the full symmetry of crystals, so the full space group is usually used instead. The naming of full space groups usually follows another common convention, the Hermann–Mauguin notation, also known as the international notation.

In geometry, a point group is a group of geometric symmetries (isometries) that keep at least one point fixed. Point groups can exist in a Euclidean space with any dimension, and every point group in dimension d is a subgroup of the orthogonal group O(d). Point groups can be realized as sets of orthogonal matrices M that transform point x into point y:

In geometry, a point group in three dimensions is an isometry group in three dimensions that leaves the origin fixed, or correspondingly, an isometry group of a sphere. It is a subgroup of the orthogonal group O(3), the group of all isometries that leave the origin fixed, or correspondingly, the group of orthogonal matrices. O(3) itself is a subgroup of the Euclidean group E(3) of all isometries.

In geometry, orbifold notation is a system, invented by the mathematician John Conway, for representing types of symmetry groups in two-dimensional spaces of constant curvature. The advantage of the notation is that it describes these groups in a way which indicates many of the groups' properties: in particular, it follows William Thurston in describing the orbifold obtained by taking the quotient of Euclidean space by the group under consideration.

In geometry, Hermann–Mauguin notation is used to represent the symmetry elements in point groups, plane groups and space groups. It is named after the German crystallographer Carl Hermann and the French mineralogist Charles-Victor Mauguin. This notation is sometimes called international notation, because it was adopted as standard by the International Tables For Crystallography since their first edition in 1935.

In crystallography, the hexagonal crystal family is one of the six crystal families, which includes two crystal systems and two lattice systems.

In geometry a parallelohedron is a polyhedron that can be translated without rotations in 3-dimensional Euclidean space to fill space with a honeycomb in which all copies of the polyhedron meet face-to-face. There are five types of parallelohedron, first identified by Evgraf Fedorov in 1885 in his studies of crystallographic systems: the cube, hexagonal prism, rhombic dodecahedron, elongated dodecahedron, and truncated octahedron.

A line group is a mathematical way of describing symmetries associated with moving along a line. These symmetries include repeating along that line, making that line a one-dimensional lattice. However, line groups may have more than one dimension, and they may involve those dimensions in its isometries or symmetry transformations.

In solid state physics, the magnetic space groups, or Shubnikov groups, are the symmetry groups which classify the symmetries of a crystal both in space, and in a two-valued property such as electron spin. To represent such a property, each lattice point is colored black or white, and in addition to the usual three-dimensional symmetry operations, there is a so-called "antisymmetry" operation which turns all black lattice points white and all white lattice points black. Thus, the magnetic space groups serve as an extension to the crystallographic space groups which describe spatial symmetry alone.

## References

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