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15.7: Symmetry

Last updated

Aug 24, 2023
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- 15.6: Close Packing and Packing Efficiency
- 15.8: Combining symmetry

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We have already met the concept symmetry in relation to crystal structures: the lattice generates the translational symmetry—the motif is repeated on every lattice point.

Other types of symmetry exist, including:

rotation axes
mirror planes
centre of symmetry
inversion axes (combination of rotation and centre of symmetry operations)

An n fold rotational symmetry operation rotates an object by 360°/n. Only n = 1, 2, 3, 4, and 6 are permitted in a periodic lattice

Examples of n-fold rotational symmetry

An object has mirror symmetry if reflection of the object in a plane brings it into coincidence with itself:

Examples of mirror symmetry

Some objects have special symmetry about an origin such that, for any point at position x, y, z, there is an exactly similar point at –x, –y, –z. The origin is called a centre of symmetry ( “inversion centre”). Such an object is said to be centrosymmetric:

An n-fold inversion axis is a combination of a rotation by 360/n followed by a centre of symmetry operation. An example of a 4-fold inversion axis is show in the following animation:

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