[Pw_forum] Re:about the displacive instabilities

[li yan]

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? 
    Best regards. 
        
   
   
  Hi, Li Yan. This is a good question which, I am sure, has been =20
addressed many times in many places. There are for sure a number of =20
textbook references. The only one that occurs to me by heart is =20
unfortunately in French [N. Boccara, Symmetries brisees (Hermann, =20
Paris, 1976)]. Any textbook dealing with Landau's theory of phase =20
transitions would do. The literature in the field looks often more =20
complicated than it actually should. You may want to take a glance at =20=

a pair of old papers of mine where your issue is addressed using =20
minimum theoretical background.

NARDELLI MB, BARONI S, GIANNOZZI P
HIGH-PRESSURE LOW-SYMMETRY PHASES OF CESIUM-HALIDES
PHYSICAL REVIEW B 51 (13): 8060-8068 APR 1 1995

NARDELLI MB, BARONI S, GIANNOZZI P
PHONON SOFTENING AND HIGH-PRESSURE LOW-SYMMETRY PHASES OF CESIUM IODIDE
PHYSICAL REVIEW LETTERS 69 (7): 1069-1072 AUG 17 1992

Hope you will find something useful in them. If not, I will try to =20
write a short explanation, but for this you will have to wait for me =20
to find 1-2 hours of spare time, which is not going to be tomorrow ...

Cheers - Stefano


On Jan 9, 2007, at 8:54 AM, li yan wrote:

> Dear all,
>       Since the displacive phase transition is second order, why =20
> some first order transitions are considered to be induced by the =20
> displacive instabilities? would you please give me some advices or =20
> some reference papers about this?
>                                                                       =20=

>               best regards
> __________________________________________________

    

       
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