1. PRESENTATION ON SYMMETRY IN CRYSTAL STRUCTURE REPRESENTED BY SATYAM CHAUHAN BT/ME/1601/011

2. WHAT IS CRYSTAL  "crystal" is based on the microscopic arrangement of atoms inside it, called the crystal structure  A crystal is a solid where the atoms form a periodic arrangement.  highly transparent glass with a high refractive index

3. What is meant by crystal structure?  In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material.

4. Symmetry

5. Symmetry Introduction 1.Symmetry defines the order resulting from how atoms are arranged and oriented in a crystal 2.Study the 2-D and 3-D order of minerals

6. 2-D Symmetry Operators  Mirror Planes (m) – reflection along a plane A line denotes mirror planes

7. 3-D Symmetry Operators Mirror Planes (m) – reflection along any plane in 3-D space

8. CRYSTAL SYMMETRY Crystals have inherent symmetry. The definite ordered arrangement of the faces and edges of a crystal is known as `crystal symmetry’. It is a powerful tool for the study of the internal structure of crystals.

9. For an unit cell of cubic lattice, the point at the body centre represents’ the `centre of symmetry’ and it is shown in the figure. What is a symmetry operation ? A Crystal may possess a number of planes or axes of symmetry but it can have only one centre of symmetry.

10. CENTRE OF SYMMETRY For an unit cell of cubic lattice, the point at the body centre represents’ the `centre of symmetry’ and it is shown in the figure.

11. PLANE OF SYMMETRY it is divided by an imaginary plane into two halv A crystal is said to have a plane of symmetry, when es, such that one is the mirror image of the other.

12. SYMMETRICAL AXES OF CUBE (a) Centre of symmetry 1 (b) Planes of symmetry 9 (Straight planes -3,Diagonal planes -6) (c) Diad axes 6 (d) Triad axes 4 (e) Tetrad axes 3 ---- Total number of symmetry elements = 23 ---- Thus the total number of symmetry elements of a cubic structure is 23.

13. ROTO INVERSION AXES X1 X1 X 1 2 3 4 Let us consider an axis xx, normal to the circle passing through the centre.

14. SCREW AXES The symmetry element that corresponds to such a motion is called a screw axis. A B C T θ

15. GLIDE PLANE mm1m1 a a / 2 Figure shows the operation of a glide plane

16. COMBINATION OF SYMMETRY ELEMENTS Apart from the different symmetry elements different combinations of the basic symmetry elements are also possible. They give rise to different symmetry points in the crystal. The combination of symmetry elements at a point is called a `point group