Crystallography Lec 6: 7 crystal system and 14 Bravais Lattices on the basis of symmetry

[@palaniappankarthik]

Defining crystals systems in terms of symmetry instead of lattice parameters is nicely explained. These are not explained in text books. Thanks to your efforts. I wish you to continue posting videos in this topic. Thank you professor.

[@rajeshprasadlectures]

Thank you

[@STUDYSSINGH]

Thank you sir it's just awesome. I could not understand why there are 7 crystal system but after watching your video everything is resolved.

[@anureji748]

Really appreciable.. Logically formulated and presented with clarity.. Thank you so much.

[@blzKrg]

How are there 7 different point groups? There are 3 point symmetries- Rotation, Reflection and Inversion and all lattices possess inversion symmetry. So shouldn't there be 4 point groups only?

[@rajeshprasadlectures]

First: there are different kinds of rotations: 2-fold, 3-fold, 4-fold, and 6-fold. Single axes of these give four different systems: monoclinic, trigonal, tetragonal, and hexagonal. Then there are combinations of these axes; three mutually orthogonal 2-fold axes and four 3-fold axes. These give orthorhombic and cubic systems respectively. Finally, the absence of all these gives triclinic.

[@moumitapaul2345]

sir , I have seen some videos where 32 crystallographic point groups are mentioned, but in this video you have explained 7 point group symmetries .what are the differences between these two?

[@rajeshprasadlectures]

Crystals (lattice+motif) can have 32 point groups. Lattice alone can have only 7 point groups.

[@smilecreation78]

Sir in what special case we can make bct unit cell as fcc ?

[@rajeshprasadlectures]

BCT with c/a = Sqrt [2] will actually be an FCC. This is another example of why crystal systems should not be defined simply by their lattice parameters. So, to avoid this case you will have to say that for tetragonal system is one with a=b.NE.c as well as for BCT c.NE. Sqrt [2]. This sort of conditions will soon make definitions based on lattice parameters very messy.

[@smilecreation78]

@@rajeshprasadlectures thank you sir

[@palaniappankarthik]

Thank you professor