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(i) Extended symbols of centred groups A, B, C, I
By convention, the second line of the extended space-group symbol is the result of the multiplication of the first line by the centring translation (cf. Table 4.1.2.3
). As a consequence, the product of any two terms in one line is equal to the product of the corresponding two terms in the other line.
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(a) Class 222
The extended symbol of I222 (23) is the twofold axes intersect and one obtains .
Maximal k subgroups are P222 and (plus permutations) but not .
The extended symbol of is , where one obtains ; the twofold axes do not intersect. Thus, maximal non-isomorphic k subgroups are and (plus permutations), but not P222.
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(b) Class mm2
The extended symbol of Aea2 (41) is the following relations hold: and .
Maximal k subgroups are Pba2; Pcn2 (Pnc2); (); .
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(c) Class mmm
By convention, the first line of the extended symbol contains those symmetry elements for which the coordinate triplets are explicitly printed under Positions. From the two-line symbols, as defined in the example below, one reads not only the eight maximal k subgroups P of class mmm but also the location of their centres of symmetry, by applying the following rules:
If in the symbol of the P subgroup the number of symmetry planes, chosen from the first line of the extended symbol, is odd (three or one), the symmetry centre is at 0, 0, 0; if it is even (two or zero), the symmetry centre is at for the subgroups of C groups and at for the subgroups of I groups (Bertaut, 1976).
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Examples
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(1) According to these rules, the extended symbol of Cmce (64) is (see above). The four k subgroups with symmetry centres at 0, 0, 0 are Pmcb (Pbam); Pmna; Pbca; Pbnb (Pccn); those with symmetry centres at are Pbna (Pbcn); Pmca (Pbcm); Pmnb (Pnma); Pbcb (Pcca). These rules can easily be transposed to other settings.
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(2) The extended symbol of Ibam (72) is . The four subgroups with symmetry centre at 0, 0, 0 are Pbam; Pbcn; Pcan (Pbcn); Pccm;
those with symmetry centre at are Pccn; Pcam (Pbcm); Pbcm; Pban.
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(ii) Extended symbols of F-centred space groups
Maximal k subgroups of the groups F222, Fmm2 and Fmmm are C, A and B groups. The corresponding centring translations are and .
The (four-line) extended symbols of these groups can be obtained from the following scheme: The second, third, and fourth lines are the result of the multiplication of the first line by the centring translations w, u and v, respectively.
The following abbreviations are used: For the location of the symmetry elements in the above scheme, see Table 4.1.2.3
. In Table 4.3.2.1, the centring translations and the superscripts u, v, w have been omitted. The first two lines of the scheme represent the extended symbols of C222, Cmm2 and Cmmm. An interchange of the symmetry elements in the first two lines does not change the group. To obtain further maximal C subgroups, one has to replace symmetry elements of the first line by corresponding elements of the third or fourth line. Note that the symbol `e' is not used in the four-line symbols for Fmm2 and Fmmm in order to keep the above scheme transparent.
Maximal A- and B-centred subgroups can be obtained from the C subgroups by simple symmetry arguments.
In space groups Fdd2 (43) and Fddd (70), the nature of the d planes is not altered by the translations of the F lattice; for this reason, a two-line symbol for Fdd2 and a one-line symbol for Fddd are sufficient. There exist no maximal non-isomorphic k subgroups for these two groups.
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