C2/m (A2/m, 12) Space Group

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Table 3017a. C2/m (12) space group.

Name in the International Tables for Crystallography C2/m Lattice type C
Patterson space group 12 Point group 2/m
Crystal system Monoclinic Laue class 2/m
Asymm 0 ≤ x ≤ 1/2 and 0 ≤ y ≤ 1/4 and 0 ≤ z ≤ 1
Number of symmetry operators 8
Symmetry (atomic coordinates) X, Y, Z; X, -Y, Z; -X, Y, -Z; -X, -Y, -Z; 1/2+X, 1/2+Y, Z; 1/2+X, 1/2-Y, Z; 1/2-X, 1/2+Y, -Z; 1/2-X, 1/2-Y, -Z
Forbidden reflections The three space groups C2/m, C2 and Cm have the same systematic forbidden reflections which are caused by the C-centering (h+k = 2n+1). The other symmetry operations in the three space groups, e.g. 2-fold rotation axis (2) and mirror plane (m) in the C2/m space group, do not cause forbidden reflections.
Crystal examples

MoCl3 (molybdenum chloride): Mo at 0, 0.1417, 0, Cl1 at 0.502, 0.17, 0.223, and Cl2 at 0.0080, 0, 0.274. Na2Ti6O13.

Table 3017b. A2/m space group.

Name in the International Tables for Crystallography A2/m Lattice type A
Patterson space group Point group
Crystal system Monoclinic Laue class
Asymm
Number of symmetry operators
Symmetry (atomic coordinates) x, y, z; -x, -y, z; -x, -y, -z; x, y, -z
Crystal examples HICl4•4H2O, HAuCl4•4H2O PbMnO2.75: a = 3.2232 nm, b = 0.3831 nm, c = 3.5671 nm, β = 130°. Y7Ge2O12(F,OH)5: a = 0.5551 nm, b = 1.1287 nm, c = 1.2121 nm, γ = 108.84°. Prototype R3T29 structure: e.g. R3(Fe,Co)29, Nd3(Fe,Ti)29, Nd3Fe29-xTix TiO0.9 - TiO1.1
monoclinic sodium hexatitanate Na2Ti6O13 monoclinic sodium hexatitanate Na2Ti6O13 monoclinic sodium hexatitanate Na2Ti6O13
[1 0 -3] monoclinic sodium hexatitanate Na2Ti6O13 (a = 15.131 Å, b = 3.745 Å, c = 9.159Å, β = 99.3°) [1] [1 -1 -3] monoclinic sodium hexatitanate Na2Ti6O13 (a = 15.131 Å, b = 3.745 Å, c = 9.159Å, β = 99.3°) [1] [1 -1 -1] monoclinic sodium hexatitanate Na2Ti6O13 (a = 15.131 Å, b = 3.745 Å, c = 9.159Å, β = 99.3°) [1]
monoclinic sodium hexatitanate Na2Ti6O13
[0 -1 0] monoclinic sodium hexatitanate Na2Ti6O13 (a = 15.131 Å, b = 3.745 Å, c = 9.159Å, β = 99.3°) [1]

Figure 3017. Examples of indexed electron diffraction (or FFT) patterns of C2/m (12) crystals.

[1] Cheng-Yan Xu, Qi Zhang, Han Zhang, Liang Zhen, Jie Tang and Lu-Chang Qin, Synthesis and Characterization of Single-Crystalline Alkali Titanate Nanowires, J. AM. CHEM. SOC. 127, 11584-11585, 2005.

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