Combining symmetry operations

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Peter Hensley
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1 Combining symmetry operations An object can possess several symmetry elements Not all symmetry elements can be combined arbitrarily - for example, two perpendicular two-fold axes imply the existence of a third perpendicular two-fold Translational symmetry in 3D imposes limitations - only 2, 3, 4 and 6-fold rotation axes allow for space filling translational symmetry The allowed combinations of symmetry elements are called point groups - there are 32 point groups that give rise to periodicity in 3D

2 Space filling repeat patterns Only 2, 3, 4 and 6-fold rotations can produce space filling patterns Structure Determination by X-ray Crystallography, Ladd and Palmer, Plenum, 1994.

3 Point groups Show 3D repeat pattern Contain symmetry elements 32 point groups exist Structure Determination by X-ray Crystallography, Ladd and Palmer, Plenum, 1994.

4 Space groups When talking about crystal structures, people will usually report the space group of a crystal Space groups are made up from - lattice symmetry (translational) - point symmetry (not translational) - glide and/or screw axes (some translational component) There are 230 space groups 7 crystal systems 14 Bravais lattices 32 point groups 230 space groups

5 Screw axes A 2 1 screw axis translates an object by half a unit cell in the direction of the screw axis, followed by a 180 rotation Crystal Structure Analysis for Chemists and Biologists, Glusker, Lewis and Rossi, VCH, 1994.

6 Higher order screws Structure Determination by X-ray Crystallography Ladd and Palmer, Plenum, A C n screw axis translates an object by the unit cell dimension multiplied by n/c along the direction of the screw axis, followed by a C-fold rotation

7 Glide planes A glide plane corresponds to a reflection-translation operation - reflection through the glide plane - translation within the glide plane - exact translation depends on type of glide There are a, b, c, n and d glide planes - a, b and c glides correspond to translations of ½ a, ½ b and ½ c respectively - called axial glide planes - n glide corresponds to a translation of ½ a + ½ b, ½ a + ½ c, or ½ b + ½ c - called diagonal glide plane - d glide corresponds to a translation of ¼ a + ¼ b, ¼ a + ¼ c, or ¼ b + ¼ c - called diamond glide plane

8 Example of an a glide plane Crystal Structure Analysis for Chemists and Biologists, Glusker, Lewis and Rossi, VCH, 1994.

9 Graphical symbols used for symmetry operations International Tables for Crystallography, Vol. A, Kluwer, 1993.

10 Graphical symbols (2) International Tables for Crystallography, Vol. A, Kluwer, 1993.

11 Interpretation of space group symbols All space group symbols start with a letter corresponding to the lattice centering, followed by a collection of symbols for symmetry operations in the three lattice directions There are sometimes short notations for space group symbols - P is usually written as P 2 - primitive cell that has a two-fold rotation along the b axis - P (cannot be abbreviated) - primitive cell that has a 2 1 screw along each axis, orthorhombic - C m m a (full symbol: C 2/m 2/m 2/a) - C-centered cell with a mirror plane perpendicular to a and b and an a glide plane perpendicular to c - also has implied symmetry elements (e.g., the 2-fold rotations)

12 Limitations on combination of symmetry elements As for point groups, not all symmetry elements can be combined arbitrarily For three dimensional lattices - 14 Bravais lattices - 32 point groups - but only 230 space groups For two dimensional lattices - 5 lattices - 10 point groups - but only 17 plane groups

13 The 17 plane groups (1) Structure Determination by X-ray Crystallography, Ladd and Palmer, Plenum, 1994.

14 The 17 plane groups (2) Structure Determination by X-ray Crystallography, Ladd and Palmer, Plenum, 1994.

15 Symmetry elements parallel to the plane of projection International Tables for Crystallography, Vol. A, Kluwer, 1993.

16 Symmetry elements inclined to the plane of projection International Tables for Crystallography, Vol. A, Kluwer, 1993.

Figure.1. The conventional unit cells (thick black outline) of the 14 Bravais lattices. [crystallographic symmetry] 1

Figure.1. The conventional unit cells (thick black outline) of the 14 Bravais lattices. [crystallographic symmetry] 1 [crystallographic symmetry] The crystallographic space groups. Supplementary to { 9.6:324} In the 3-D space there are 7 crystal systems that satisfy the point (e.g., rotation, reflection and inversion)