Lecture course on crystallography, 2015 Lecture 5: Symmetry in crystallography
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1 Dr Semën Gorfman Department of Physics, University of SIegen Lecture course on crystallography, 2015 Lecture 5: Symmetry in crystallography
2 What is symmetry? Symmetry is a property of an object to stay unchanged (invariant) after certain type of movement. 2
3 Symmetry operation Symmetry operation is the movement which keeps the object unchanged Rotation by 60 degrees Rotation by 90 degrees around the vertical axis Reflection in the plane through the middle Unchanged means it is not possible to distinguish two objects from any kind of physical experiments 3
4 Types of symmetry operation According to the type of movement there are different types of symmetry operations 1. Point symmetry: AT LEAST one point is fixed during the movement of the object Rotation Reflection Inversion Combination of any above 2. Space symmetry: no points are fixed during the movement of the object Translation (lattice translation) Combination of translation and any kind of point symmetry operations 4
5 Rotational symmetry Movement: rotation around selected direction by the angle a. The axis of rotation (symmetry element) is referred to n-fold SYMMETRY AXIS with n=360 / a. This symmetry is designated as n Vertically oriented symmetry axis 4 (a = 90 deg) 5 (a =72 deg) 6 (a = 60 deg) 5
6 Reflection symmetry Movement: reflection in the mirror. Each point is replaced by its mirror reflection. The mirror is referred to as MIRROR PLANE and is designated by m. 6
7 7
8 Inversion symmetry Movement: inversion relative to the origin (centre of inversion). Each point is replaced by the inverted one. X Y Z -X -Y -Z R 1 R 2 R 3 -R 3 -R 2 -R 1 8
9 n Designation of the symmetry axes and mirror Designation of an axis m 9
10 2-fold rotoinversion axis It is possible to combine some symmetry operations. As an example combination of rotation and inversion gives the symmetry associated with the rotoinversion. 2 1 Intermediate step (after rotation) Final position (after rotation plus inversion) 3 2-fold rotoinversion axis is equivalent to the mirror plane perpendicular to the axis 10
11 Designation of the rotoinversion axes n Designation of an axis 2 m
12 3-fold rotoinversion axis, fold rotoinversion axis is equivalent to independent existence of 3 fold axis and inversion 12
13 4-fold rotoinversion axis,
14 6-fold rotoinversion axis, fold rotoinversion axis is equivalent to independent existence of 3 fold axis and mirror plane 14
15 15
16 Further demonstration of symmetry 2 fold axis 3 fold axis 4 fold axis 6 fold axis 16
17 Further demonstration of symmetry Mirror plane Centre of inversion 17
18 Further demonstration of symmetry 3 fold rotoinversion axis 4 fold rotoinversion axis 6 fold rotoinversion axis 18
19 Crystal lattice as a symmetry operation The main feature of the 3D structure of a crystalline solid is lattice periodicity. A crystals DOES NOT HAVE TO have any of the symmetry described above, however it MUST sustain the periodic structure / long range order. The structure of a crystal must be described by the unit cell and crystal lattice. Consider a unit cell and crystal lattice 19
20 m m 20
21 21
22 22
23 CRYSTAL UNIT CELL + CRYSTAL LATTICE SYMMETRY of CRYSTAL SYMMETRY of UNIT CELL * SYMMETRY of CRYSTAL LATTICE The are no RESTRICTIONS on the SYMMETRY OF a UNIT CELL. There are however a certain restriction on the symmetry of the lattice. Therefore holding the long range (3D periodicity) order puts the natural restriction on the possible symmetry of a crystal 23
24 MAIN THEOREM of CRYSTALLOGRAPHY The only possible ROTATIONAL SYMMETRIES accepted by a crystal lattice are 1,2,3,4 and 6 A 2 -a a a a A 1 We consider the lattice vector, a and suppose there is a symmetry axis of the order n=360/a. The two lattice vectors A 1 and A 2 are obtained by rotation by the a of a and a in two opposite directions. If the rotation is the symmetry operation then A 1, A 2 and A 2 - A 1 are also the lattice vectors. However according to the construction A 2 -A 1 is parallel to a. That means the following condition must be hold: A 2 A 1 = N*a where N is integer number 24
25 a a a a According to the construction A 1 A 2 = 2 a sin (90 a) = 2 a cos a, a 90-a a Therefore cos a = N / 2. As cos a <=1 there are a few possibilities only A 2 A 1 N cos a a, deg Order of symmetry axis
26 IMPORTANT MESSAGE The 3D periodicity crystals puts the RESTRICTION on the POSSIBLE SYMMETRY OPERATION of a whole CRYSTAL STRUCTURE. In particular the maximum order of the rotation axis possible in crystals is 6. In addition 5-fold rotational symmetry is forbidden! 26
27 Depending on the geometry crystal lattice may sustain different symmetry elements. According to the symmetry of the LATTICES are all CRYSTAL are subdivided into a CRYSTAL SYSTEMS 27
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