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Crystal symmetry

  1. Sep 10, 2012 #1
    Hi all,
    Can anyone help explaining how during a phase transition, crystal will be more symmetric at high temperature
     
  2. jcsd
  3. Sep 10, 2012 #2

    DrDu

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    Think e.g. of the Ising model. You may think of the "spins" as describing a distortion of the unit cell, which may distort in either of two directions. The energy gained by a collective distortion = parallel alignment of N spins is of the order U=-N J where J is the spin spin interaction energy. On the other hand a completely disordered state has an entropy of S= k ln N while an ordered state has S=0.
    The relevant quantity is the free energy F=U-TS. At T=0 the ordered state will be more stable however at some critical temperature, the S term will outweigh the U term and the symmetric state with <S_z>=0 will be favoured.
     
  4. Sep 12, 2012 #3
    Can this b xplained on d basis of crystal structure?
     
  5. Sep 12, 2012 #4
    There are crystals that undergo changes of lattice symmetry when cooled, e.g. due to the Jahn-Teller effect.

    TbVO4 is one of the textbook examples, it is tetragonal (high symmetry) at high temperature and orthorhombic (lower symmetry) at low temperature.
     
  6. Sep 13, 2012 #5
    @ M Quack : Thank You
     
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