International
Tables for
Crystallography
Volume A1
Symmetry relations between space groups
Edited by Hans Wondratschek and Ulrich Müller

International Tables for Crystallography (2011). Vol. A1, ch. 2.1, pp. 76-79   | 1 | 2 |

## Section 2.1.3. I Maximal translationengleiche subgroups (t-subgroups)

Hans Wondratscheka* and Mois I. Aroyob

aInstitut für Kristallographie, Universität, D-76128 Karlsruhe, Germany, and bDepartamento de Física de la Materia Condensada, Facultad de Ciencias, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain
Correspondence e-mail:  wondra@physik.uni-karlsruhe.de

### 2.1.3. I Maximal translationengleiche subgroups (t-subgroups)

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#### 2.1.3.1. Introduction

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In this block, all maximal t-subgroups of the plane groups and the space groups are listed individually. Maximal t-sugroups are always non-isomorphic.

For the sequence of the subgroups, see Section 2.1.2.4. There are no lattice relations for t-subgroups because the lattice is retained. Therefore, the sequence is determined only by the rising value of the index and by the decreasing space-group number.

#### 2.1.3.2. A description in close analogy with IT A

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The listing is similar to that of IT A and presents on one line the following information for each subgroup : Conjugate subgroups are listed together and are connected by a left brace.

The symbols have the following meaning:

 [i] index of in ; HMS1 HM symbol of referred to the coordinate system and setting of . This symbol may be nonconventional; No. space-group No. of ; HMS2 conventional HM symbol of if HMS1 is not a conventional HM symbol; sequence sequence of numbers; the numbers refer to those coordinate triplets of the general position of that are retained in , cf. Remarks; for general position cf. Section 2.1.2.2.2; matrix matrix part of the transformation to the conventional setting corresponding to HMS2, cf. Section 2.1.3.3; shift column part of the transformation to the conventional setting corresponding to HMS2, cf. Section 2.1.3.3.

Remarks

• In the sequence column for space groups with centred lattices, the abbreviation (numbers)' means that the coordinate triplets specified by numbers' are to be taken plus those obtained by adding each of the centring translations, see the comments following Examples 2.1.3.2.2 and 2.1.3.2.3.

• The symbol HMS2 is omitted if HMS1 is a conventional HM symbol.

• The following deviations from the listing of IT A are introduced in these tables:

 No.: the space-group No. of is added. HMS2: In order to specify the setting clearly, the full HM symbol is listed for monoclinic subgroups, not the standard (short) HM symbol as in IT A. matrix, shift: These entries contain information on the trans­formation of from the setting of to the standard setting of . They are explained in Section 2.1.3.3.

In general, the numbers in the list Sequence' of follow the order of the numbers in the group , i.e. they rise monotonically. Sometimes this sequence is modified because those entries which have the same additional translations are joined together, see, e.g. the maximal k-subgroups of with Loss of centring translations'. In addition, in a class of conjugate subgroups, the mono­tonically rising order may be obeyed only for the first member of the conjugacy class. The order of the other members is then determined by the conjugation of the first member. (In IT A the monotonically rising order of the numbers is kept in all conjugate subgroups.)

#### Example 2.1.3.2.1

, No. 221, tetragonal t-subgroups

If is the order of the first sequence, then the second sequence follows the order , , . Here the C means a threefold rotation and is the conjugating element; for the second subgroup of the general position of ; for the third subgroup . In this example the columns w of the symmetry operations (and thus of the conjugating elements) are the zero columns o and could be omitted.

The description of the subgroups can be explained by the following four examples.

#### Example 2.1.3.2.2

, No. 8, UNIQUE AXIS b

I Maximal translationengleiche subgroups

 HMS1: is not a conventional HM symbol. Therefore, the conventional symbol is added as HMS2 after the space-group number 1 of . sequence: 1' means ; .

#### Example 2.1.3.2.3

, No. 43

I Maximal translationengleiche subgroups

 HMS1: is not a conventional HM symbol; therefore, the conventional symbol is added to the space-group No. 5 as HMS2. The setting unique axis c is inherited from . sequence: (1, 2) means:

#### Example 2.1.3.2.4

, No. 137, ORIGIN CHOICE 2

I Maximal translationengleiche subgroups

(59, ) 1; 2; 5; 6; 9; 10; 13; 14

 HMS1: The sequence of the directions in the HM symbol for a tetragonal space group is c, a, . From the parts , and of the full HM symbol of , only , and 1 remain in . Therefore, HMS1 is , and the con­ven­tional symbol is added as HMS2. No.: The space-group number of is 59. The setting origin choice 2 of is inherited from . sequence: The coordinate triplets of retained in are: (1) ; (2) ; (5) ; (6) ; (9) ; etc.

#### Example 2.1.3.2.5

, No. 151

 brace: The brace on the left-hand side connects the three conjugate monoclinic subgroups. HMS1: is not the conventional HM symbol for unique axis c but the constituent 2' of the nonconventional HM symbol refers to the directions , and , in the hexagonal basis. According to the rules of Section 2.1.2.5, the standard setting is unique axis b, as expressed by the HM symbol . HMS2: Note that the conventional monoclinic cell is centred. matrix, shift: The entries in the columns matrix' and shift' are explained in the following Section 2.1.3.3 and evaluated in Example 2.1.3.3.2.

#### 2.1.3.3. Basis transformation and origin shift

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Each t-subgroup is defined by its representatives, listed under sequence' by numbers each of which designates an element of . These elements form the general position of . They are taken from the general position of and, therefore, are referred to the coordinate system of . In the general position of , however, its elements are referred to the coordinate system of . In order to allow the transfer of the data from the coordinate system of to that of , the tools for this transformation are provided in the columns matrix' and shift' of the subgroup tables. The designation of the quantities is that of IT A Part 5 and is repeated here for convenience. The transformation described in this section is not restricted to translationengleiche subgroups but is applied to klassengleiche subgroups as well.

In the following, columns and rows are designated by boldface italic lower-case letters. Point coordinates , translation parts of the symmetry operations and shifts are represented by columns. The sets of basis vectors and are represented by rows [indicated by , which means transposed']. The quantities with unprimed symbols are referred to the coordinate system of , those with primes are referred to the coordinate system of .

The following columns will be used ( is analogous to w):

The matrices W and of the symmetry operations, as well as the matrix P for a change of basis and its inverse , are designated by boldface italic upper-case letters ( is analo­gous to W): Let be the row of basis vectors of and the basis of , then the basis is expressed in the basis by the system of equationsor

In matrix notation, this is

The column p of coordinates of the origin of is referred to the coordinate system of and is called the origin shift. The matrix–column pair (P, p) describes the transformation from the coordinate system of to that of , for details, cf. IT A, Part 5 . Therefore, P and p are listed in the subgroup tables in the columns matrix' and shift', cf. Section 2.1.3.2. The column matrix' is empty if there is no change of basis, i.e. if P is the unit matrix I. The column shift' is empty if there is no origin shift, i.e. if p is the column o consisting of zeroes only.

A change of the coordinate system, described by the matrix–column pair , changes the point coordinates from the column x to the column . The formulae for this change do not contain the pair itself, but the related pair :

Not only the point coordinates but also the matrix–column pairs for the symmetry operations are changed by a change of the coordinate system. A symmetry operation is described in the coordinate system of by the system of equations2ori.e. by the matrix–column pair (W, w). The symmetry operation will be described in the coordinate system of the subgroup by the equation and thus by the pair . This pair can be calculated from the pair by the equations and

These equations are rather complicated and unpleasant. They become simple when using augmented matrices and columns. In this case the formulae are reduced formally to normal matrix multiplication [the formalism is simpler but the necessary calculations are not, because the inversion of a (4 × 4) matrix is tedious if done by hand].

The matrices P, Q, W and W′ may be combined with the corresponding columns p, q, w and w′ to form (4 × 4) matrices (callled augmented matrices):3The coefficients of these augmented matrices are integer, rational or real numbers.

The (3 × 1) rows (a)T and (a′)T must be augmented to (4 × 1) rows by appending some vectors and , respectively, as fourth entries in order to enable matrix multiplication with the aug­mented matrices:with . As the vector one can take the zero vector , which results in , i.e.The relation between and is given by equation (2.1.3.10), which replaces equation (2.1.3.3),

Analogously, the (3 × 1) columns x and x′ must be augmented to (4 × 1) columns by a 1' in the fourth row in order to enable matrix multiplication with the augmented matrices:The three equations (2.1.3.4), (2.1.3.8) and (2.1.3.9) are replaced by the two equationsand

#### Example 2.1.3.3.1

Consider the data listed for the t-subgroups of , No. 31:This means that the transformation matrices and origin shifts are

The first subgroup is monoclinic, the symmetry direction is the b axis, which is standard. However, the glide direction is nonconventional. Therefore, the basis of is transformed to a basis of the subgroup such that the b axis is retained, the glide direction becomes the axis and the axis is chosen such that the basis is a right-handed one, the angle and the transformation matrix P is simple. This is done by the chosen matrix . The origin shift is the o column.

With equations (2.1.3.8) and (2.1.3.9), one obtains for the glide reflection , which is after standardization by .

For the second monoclinic subgroup, the symmetry direction is the (nonconventional) a axis. The rules of Section 2.1.2.5 require a change to the setting unique axis b'. A cyclic per­mutation of the basis vectors is the simplest way to achieve this. The reflection is now described by . Again there is no origin shift.

The third monoclinic subgroup is in the conventional setting unique axis c', but the origin must be shifted onto the screw axis. This is achieved by applying equation (2.1.3.9) with , which changes of to of .

#### Example 2.1.3.3.2

Evaluation of the t-subgroup data of the space group , No. 151, started in Example 2.1.3.2.5. The evaluation is now continued with the columns sequence', matrix' and shift'. They are used for the transformation of the elements of to their conventional form. Only the monoclinic t-subgroups are of interest here because the trigonal subgroup is already in the standard setting.

One takes from the tables of subgroups in Chapter 2.3 Designating the three matrices by , , , one obtainswith the corresponding inverse matricesand the origin shiftsFor the three new bases this means All these bases span ortho-hexagonal cells with twice the volume of the original hexagonal cell because for the matrices holds.

In the general position of , No.151, one findsThese entries represent the matrix–column pairs : Application of equations (2.1.3.8) on the matrices and (2.1.3.9) on the columns of the matrix–column pairs results in All translation vectors of are retained in the subgroups but the volume of the cells is doubled. Therefore, there must be centring-translation vectors in the new cells. For example, the application of equation (2.1.3.9) with to the translation of with the vector , i.e. , results in the column , i.e. the centring translation of the subgroup. Either by calculation or, more easily, from a small sketch one sees that the vectors for , for (and for ) correspond to the cell-centring translation vectors of the subgroup cells.