[Pw_forum] 回复： Re: [Pw_forum] Re:about the displacive instabilities

[li yan]

Dear Stefano,
      I'm sorry. I did not explain my question clearly.
      I found a softening transverse acaustic mode at X (TA(X)) point on the brillouin zone boundary in a NaCl structure.  Since  the first high-pressure phase that the experiment observed is CsCl structure,  could the soft mode be considered as  the driven force of the transition from NaCl to CsCl? 
      If it could not, may i give an intermediate state between the NaCl and CsCl phase if I can get a energy favored structure in terns of the softening mode in the NaCl structure? It is a monoclinic phase, obtained by moving the atoms along the displacements (order parameters) corresponding to the softening mode.
   
   Thank you very much for any advice. 
   
   Best regards.
   
  Stefano Baroni <baroni at sissa.it> дµÀ£º
  
    On Jun 15, 2007, at 3:20 AM, li yan wrote:

    Dear Stefano
       I'm so sorry and giving my delayed thanks to you, 
       I am not sure if i can continue the discussion of my previous question. I have read the papers you suggested. And the authors proposed that the softening mode coupled with the lattice strain can induced a second or a slightly first order pahse transition. But I stil have doubts in the displacive instabilitis. 
     The character of the  transition from NaCl to CsCl is thougt to be?first order in experiment. May I cosider this  transition to be induced by the lattice instability if I found the softening mode in the  NaCl phase at the pressure which is consitent with the experiment??lt;/DIV>
  

which normal-mode displacement would bring the NaCl structure to CsCl? what would be the order parameter of the transition?
  

  S.

    ---
  Stefano Baroni - SISSA?&?DEMOCRITOS National Simulation Center - Trieste
  [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)
  

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