Crystal structures, point groups (2016) - lecture 2

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  • Опубліковано 8 гру 2024

КОМЕНТАРІ • 11

  • @Imagon100
    @Imagon100 4 роки тому

    Professor, at 19:40 you say the water molecule has 2m symmetry, but the water molecule has 2 mirror planes (second one cutting all atoms in half). Is the second mirror plane implied in the 2m notation or is there a mistake in your presentation?
    Also the video at 23:35 is very confusing and it looks to me more like a mmm symmetry to me. Perhaps the video should contain a more 'skewed' shape that would indicate the lesser symmetry better (as in the real gypsum crystal).

    • @bhadeshia123
      @bhadeshia123  4 роки тому

      Regarding the water molecule, please see the caption to Figure 1.18, in Geometry of Crystals, Polycrystals and Phase Transformations, that you can download freely from www.phase-trans.msm.cam.ac.uk/2020/Crystallography_book.pdf

    • @bhadeshia123
      @bhadeshia123  4 роки тому

      For the Gypsum, one of the angles is not 90 degrees. See page 24 of www.phase-trans.msm.cam.ac.uk/2020/Crystallography_book.pdf

    • @TheLetterH111
      @TheLetterH111 4 роки тому

      Hi professor, I'm also a little unclear about the water molecule. I had a look at the caption in the book, but I can't see how this addresses the presence of another mirror plane that bisects all three atoms at once

    • @bhadeshia123
      @bhadeshia123  4 роки тому +1

      @@TheLetterH111 Without considering the electron clouds, the mirror simply reflects each atom on to itself because each atom then lies on the mirror. That is a trivial operation.

  • @MTim-vj7xg
    @MTim-vj7xg 8 років тому

    a crystal class corresponds to several point groups. Does the crystal class contain the point groups at same time? can it derive other point groups form one point group in a a crystal class?

    • @bhadeshia123
      @bhadeshia123  8 років тому

      The highest symmetry point group is reduced within a class by placing atoms.

  • @wanghou6219
    @wanghou6219 8 років тому

    Is point group th symmetry of crystal shape or lattice ?

    • @bhadeshia123
      @bhadeshia123  8 років тому

      It is a set of symmetry elements passing through a point. If a macroscopic crystal has its equilibrium shape, then the point group symmetry of that shape can be related to the crystal class.

  • @simmasrinivas8732
    @simmasrinivas8732 7 років тому

    what is motif of calcium fluorite?

    • @bhadeshia123
      @bhadeshia123  7 років тому

      Count the number of atoms in the cell, and divide by the number of lattice points, and then attribute that number to the chemical formula of calcium fluorite