Hermann–Mauguin notation
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 (who introduced it in 1928) and the French mineralogist CharlesVictor Mauguin (who modified it in 1931). 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.
The Hermann–Mauguin notation, compared with the Schoenflies notation, is preferred in crystallography because it can easily be used to include translational symmetry elements, and it specifies the directions of the symmetry axes.^{1}
Contents
Point groups
Rotation axes are denoted by a number n — 1, 2, 3, 4, 5, 6, 7, 8 ... (angle of rotation φ = 360° / n) For improper rotations HermanMauguin symbols show rotoinversion axes, unlike Schoenflies and Shubnikov notations, where preference is given to rotationreflection axes. The rotoinversion axes are represented by the corresponding number with a macron, n — 1, 3, 4, 5, 6, 7, 8 ... The symbol for a mirror plane (rotoinversion axis 2) is m. The direction of a mirror plane is defined as direction perpendicular to it (the direction of 2 axis).
HermanMauguin symbols show symmetrically nonequivalent axes and planes. The direction of a symmetry element is represented by its position in the HermanMauguin symbol. If a rotation axis n and a mirror plane m have the same direction (i.e. the plane is perpendicular to axis n), then they are denoted as fraction or n/m.
If two or more axes have the same direction, the axis with higher symmetry is shown. Higher symmetry here means that the axis generates a pattern with more points. For example, rotation axes 3, 4, 5, 6, 7, 8 generate 3, 4, 5, 6, 7, 8point patterns, respectively. Improper rotation axes 3, 4, 5, 6, 7, 8 generate 6, 4, 10, 6, 14, 8point patterns, respectively. If both the rotation and rotoinversion axes satisfy the previous rule, the rotation axis should be chosen. For example, combination is equivalent to 6. Because 6 generates 6 points, and 3 generates only 3, 6 should be written instead of (not , because 6 already contains mirror plane m). The same situations is in the case when both 3 and 3 axes are present — 3 should be written. However we write , not , because both 4 and 4 generate four points. The analogous case is the combination, where 2, 3, 6, 3, and 6 axes are present; axes 3, 6, and 6 all generate 6point patterns, but the latter should be used because it is a rotation axis — the symbol will be .
Finally, the HermanMauguin symbol depends on the type of the group.
Groups without higherorder axes (axes of order three or more)
These groups may contain only twofold axes, mirror planes, and inversion center. These are the crystallographic point groups 1 and 1 (triclinic crystal system), 2, m, and (monoclinic), and 222, , and mm2 (orthorhombic). If the symbol contains three positions, then they denote symmetry elements in the X, Y, Z directions, respectively.
Groups with one higherorder axis
 First position — primary direction — Z direction, assigned to the higherorder axis.
 Second position — symmetrically equivalent secondary directions, which are perpendicular to Zaxis. These can be 2, m, or .
 Third position — symmetrically equivalent tertiary directions, passing between secondary directions. These can be 2, m, or .
These are the crystallographic groups 3, 32, 3m, 3, and 3 (trigonal crystal system), 4, 422, 4mm, 4, 42m, , and (tetragonal), and 6, 622, 6mm, 6, 6m2, , and (hexagonal). Analogously, symbols of noncrystallographic groups (with axis of order 5, 7, 8, 9 ...) can be constructed. These groups can be arranged in the following table
n  3  4  5  6  7  8  ...  

n  3  4  5  6  7  8 


n2 or n22  32  422  52  622  72  822 


nm or nmm  3m  4mm  5m  6mm  7m  8mm 


= 6  = 10  = 14 


m2 = 6m2  m2 = 10m2  m2 = 14m2 




or  3  42m  5  6m2  7  82m 

The symbols that shouldn't be used are shown in red text.
It can be noticed that in groups with oddorder axes n and n the third position in symbol is always absent, because all n directions, perpendicular to higherorder axis, are symmetrically equivalent. For example, in the picture of a triangle all three mirror planes (S_{0}, S_{1}, S_{2}) are equivalent — all of them pass through one vertex and the center of the opposite side. For evenorder axes n and n there are secondary directions and tertiary directions. For example, in the picture of a regular hexagon one can distinguish two sets of mirror planes — three planes go through two opposite vertexes, and three other planes go though the centers of opposite sides. In this case any of two sets can be chosen as secondary directions, the rest set will be tertiary directions. Hence groups 42m, 62m, 82m, ... can be written as 4m2, 6m2, 8m2, ... For symbols of point groups this order usually doesn't matter; however, it will be important for HermanMauguin symbols of corresponding space groups, where secondary directions are directions of symmetry elements along unit cell translations b and c, while the tertiary directions correspond to the direction between unit cell translations b and c. For example, symbols P6m2 and P62m denote two different space groups. This also applies to symbols of space groups with oddorder axes 3 and 3. The perpendicular symmetry elements can go along unit cell translations b and c or between them. Space groups P321 and P312 are examples of the former and the latter cases, respectively.
The symbol of point group 3 may be confusing; the corresponding Schoenflies symbol is D_{3d}, which means that the group consists of 3fold axis, three perpendicular 2fold axes, and 3 vertical diagonal planes passing between these 2fold axes, so it seems that the group can be denoted as 32m or 3m2. However, one should remember that, unlike Schoenflies notation, the direction of a plane in a HermanMauguin symbol is defined as the direction perpendicular to the plane, and in the D_{3d} group all mirror planes are perpendicular to 2fold axes, so they should be written in the same position as . Second, these complexes generate an inversion center, which combining with the 3fold rotation axis generates a 3 rotoinversion axis.
Groups with are called limit groups or Curie groups.
Groups with several higherorder axes
These are the crystallographic groups of a cubic crystal system: 23, 432, 3, 43m, and 3. All of them contain four diagonal 3fold axes. These axes are arranged as 3fold axes in a cube, directed along its four space diagonals (the cube has 3 symmetry). These symbols are constructed the following way:
 First position — symmetrically equivalent directions of coordinate axes X, Y, Z. They are equivalent due to the presence of diagonal 3fold axes.
 Second position — diagonal 3 or 3 axes.
 Third position — diagonal directions between any two of three coordinate axes X, Y, and Z. These can be 2, m, or .
All HermanMauguin symbols presented above are called full symbols. For many groups they can be simplified by omitting nfold rotation axes in positions. This can be done if the rotation axis can be unambiguously obtained from the combination of symmetry elements presented in the symbol. For example, the short symbol for is mmm, for is mm, and for 3 is m3m. In groups containing one higherorder axis, this higherorder axis cannot be omitted. For example, symbols and can be simplified to 4/mmm (or mm) and 6/mmm (or mm), but not to mmm; the short symbol for 3 is 3m. The full and short symbols for all 32 crystallographic point groups are given in crystallographic point groups page.
Besides five cubic groups, there are two more noncrystallographic icosahedral groups (I and I_{h} in Schoenflies notation) and two limit groups (K and K_{h} in Schoenflies notation). The HermanMauguin symbols were not designed for noncrystallographic groups, so their symbols are rather nominal and based on similarity to symbols of the crystallographic groups of a cubic crystal system.^{2}^{3}^{4}^{5}^{6} Group I can be denoted as 235, 25, 532, 53. The possible short symbols for I_{h} are m35, m5, m5m, 53m. The possible symbols for limit groups are or for K and or or for K_{h}.
Plane groups
Plane groups can be depicted using the HermannMauguin system. The first letter is either lowercase p or c to represent primitive or centered unit cells. The next number is the rotational symmetry, as given above. The presence of mirror planes are denoted m, while glide reflections are denoted g.
Space groups
The symbol of Space groups is defined by combining the uppercase letter describing the lattice type with symbols specifying the symmetry elements. The symmetry elements are ordered the same way as in the symbol of corresponding point group (group that is obtained if one removes all translational components from the space group). The symbols for symmetry elements are more diverse, because in addition to rotations axes and mirror planes, space group may contain more complex symmetry elements — screw axes (combination of rotation and translation) and glide planes (combination of mirror reflection and translation). As a result, many different space groups can correspond to the same point group. For example, choosing different lattice types and glide planes one can generate 28 different space groups from point group mmm, e.g. Pmmm, Pnnn, Pccm, Pban, Cmcm, Ibam, Fmmm, Fddd.
Lattice types
These are the Bravais lattices in three dimensions:
 P — Primitive
 I — Body centered (from the German "Innenzentriert")
 F — Face centered (from the German "Flächenzentriert")
 A — Base centered on A faces only
 B — Base centered on B faces only
 C — Base centered on C faces only
 R — Rhombohedral
Primitive, P  Base centered, C  Face centered, F  Body centered, I  Rhombohedral in hexagonal setting, R 
Screw axis
The screw axis is noted by a number, n, where the angle of rotation is . 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. For example, 2_{1} is a 180° (twofold) rotation followed by a translation of ½ of the lattice vector. 3_{1} is a 120° (threefold) rotation followed by a translation of ⅓ of the lattice vector.
The possible screw axis are: 2_{1}, 3_{1}, 3_{2}, 4_{1}, 4_{2}, 4_{3}, 6_{1}, 6_{2}, 6_{3}, 6_{4}, and 6_{5}. There are 4 enantiomorphic pairs of axes: (3_{1}  3_{2}), (4_{1}  4_{3}), (6_{1}  6_{5}), and (6_{2}  6_{4}). This enantiomorphism results in 11 pairs of enantiomorphic space groups, namely
Crystal system  Tetragonal  Trigonal  Hexagonal  Cubic  

First group Group Number 
P4_{1} 76 
P4_{1}22 91 
P4_{1}2_{1}2 92 
P3_{1} 144 
P3_{1}12 152 
P3_{1}21 151 
P6_{1} 169 
P6_{2} 171 
P6_{1}22 168 
P6_{2}22 181 
P4_{1}32 213 
Second group Group Number 
P4_{3} 78 
P4_{3}22 95 
P4_{3}2_{1}2 96 
P3_{2} 145 
P3_{2}12 154 
P3_{2}21 153 
P6_{5} 170 
P6_{4} 172 
P6_{5}22 169 
P6_{4}22 182 
P4_{3}32 212 
Glide planes
Glide planes are noted by , , or depending on which axis the glide is along. There is also the glide, which is a glide along the half of a diagonal of a face, and the glide, which is along a quarter of either a face or space diagonal of the unit cell. The glide is often called the diamond glide plane as it features in the diamond structure.
 , , or glide translation along half the lattice vector of this face.
 glide translation along with half a face diagonal.
 glide planes with translation along a quarter of a face diagonal.
 two glides with the same glide plane and translation along two (different) halflattice vectors.
References
 ^ Sands, Donald E. (1993). "Crystal Systems and Geometry". Introduction to Crystallography. Mineola, New York: Dover Publications, Inc. p. 165. ISBN 0486678393.
 ^ http://it.iucr.org/Ab/ch10o1v0001/#table10o1o4o3
 ^ Families of point groups
 ^ Vainshtein, Boris K., Modern Crystallography 1: Fundamentals of Crystals. Symmetry, and Methods of Structural Crystallography, Springer. 1994, page 93.
 ^ Point groups in three dimensions
 ^ Shubnikov, A.V., Belov, N.V. & others, Colored Symmetry, Oxford: Pergamon Press. 1964, page 70.
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