[Pw_forum] Pressure-induced phase transition

Eric Germaneau germaneau at gucas.ac.cn
Wed Jul 20 16:45:29 CEST 2011 


On 07/19/2011 10:21 PM, Hongsheng Zhao wrote:
> On 07/20/2011 09:24 PM, Eric Germaneau wrote:
>> Well, I don't know about "common tangent" concept.
> See the page 3 of this notes by Keith Refson:
>
> http://www.castep.org/CASTEP_talks_07/refson2.pdf
Thank you.
>
>
>> You can study structure stability looking at phonon dispersion for instance.
>> An instability could mean that the structure undergoes a phase
>> transition (or that it's not fully relaxed)
> What's the characteristic of the phonon dispersion when phase transition
> occurs?  More hints?
I noticed the reply of Pascal Boulet after I sent my post. He point it 
out very well.
>
>> You can also compute the free energy of your system where phonons will
>> give you the harmonic part (see PRL 64 045123 (2001)
>> <http://link.aps.org/doi/10.1103/PhysRevB.64.045123>).
>> So you can look at several thing when changing the pressure, among them
>> structure change, phonon, energy, free energy (not that easy).
>> Why not electronic density of sates?
> Again, do you have experience in using DOS for judging phase transition?
>    Any hints on the  the characteristic of it when the phase transition
> occurs?
I've no idea, that was just a suggestion.
I suppose some change may occur in the electronic structure ...
But the effect of the pressure on the electronic structure may be less 
significant than the temperature. I don't know.
An expert on /ab initio/ calculation would be able to give you an answer.
>
> Regards
>> Good luck,
>>
>> Éric.
>>
>> On 07/18/2011 09:46 PM, Amin Torabi wrote:
>>> Thanks Eric and Hongsheng for the discussion!
>>>
>>> Can I also have your comments on this "common tangent" concept in
>>> first order phase transitions? Is it applicable for my case?
>>>
>>>
>>> And how can I use phonon calculations to help me find the phase
>>> transition? I've heard about some "mode softening", but I am not sure
>>> how it can be used in this context? Could you refer me to some article
>>> explaining the theory behind this
>>>
>>> Thanks again
>>> Amin
>>>
>>>
>>>
>>>
>>>
>>> On Mon, Jul 18, 2011 at 9:28 PM, Hongsheng Zhao
>>> <zhaohscas at yahoo.com.cn<mailto:zhaohscas at yahoo.com.cn>>  wrote:
>>>
>>>      On 07/19/2011 06:49 PM, Eric Germaneau wrote:
>>>      >
>>>      >
>>>      >  On 07/18/2011 10:45 AM, Hongsheng Zhao wrote:
>>>      >>  On 07/19/2011 09:23 AM, Eric Germaneau wrote:
>>>      >>>  Hey Amin,
>>>      >>>
>>>      >>>  The approach you proposed is ok but you should relax the cell
>>>      as well.
>>>      >>>  By keeping the cell fix the system does not feel the pressure
>>>      you wish
>>>      >>>  to apply.
>>>      >>  Not so clear about the meaning of this sentence, could you
>>>      please give
>>>      >>  some more explanations? ;-(
>>>      >  Under pressure keeping the unit cell fix is useless since the system
>>>      >  won't feel it.
>>>      >  It just makes sense.
>>>      >  So, if you wish to see the effect of the pressure you have to
>>>      perform
>>>      >  vc-relax calculation.
>>>      >  Take it that way, in thermodynamic you have 2 important ensembles,
>>>      >  isochoric and isobaric.
>>>      >  So either the volume is fix and you get out the corresponding
>>>      pressure,
>>>      >  or the pressure is fix and you get out the corresponding volume.
>>>      >  Note that the volume of gas for instance decreases inversely
>>>      >  proportionally to pressure.
>>>      >  The pressure is the intensive variable related to the volume
>>>      (extensive).
>>>      >  You simply can not have both of them constant at the same time.
>>>
>>>      Yes, you're right. I'm so sorry for my previous misleading posts on
>>>      this thread. Thanks again.
>>>
>>>      >  Hope it helps.
>>>      >>
>>>      >>>  You can also change the volume, relax the positions only and
>>>      compute the
>>>      >>>  pressure.
>>>      >>>  The best is to compute the free energy as function of pressure
>>>      ans so
>>>      >>>  the QHA method is the best way to go.
>>>      >>  I think this method cann't do the job of
>>>      structure-searching-and-finding
>>>      >>  for unknown phases when we changing the external pressure.
>>>      >  Yes, it can but in the case of structure-searching it's very
>>>      expensive.
>>>      >  In that case you'd better generate random unit cell parameters,
>>>      random
>>>      >  positions and run vc-relax.
>>>      >  Even this can be expensive since you have to do it a lot of
>>>      times ....
>>>      >  Just take a look to this papers, PRL 97 1 (2006)
>>>      >  <http://link.aps.org/doi/10.1103/PhysRevLett.97.045504>  and JPCM 23
>>>      >  053201 (2011)
>>>      >
>>>      <http://stacks.iop.org/0953-8984/23/i=5/a=053201?key=crossref.44087776832bc9a0edd0dc492a67d376>.
>>>      >  It is the main work of Professor Pickard
>>>      >
>>>      <http://www.st-andrews.ac.uk/physics/condmat/pickard/Crystals%20ex%20nihilo.html>.
>>>      >  Best.
>>>
>>>      Thanks a lot for this information ;-)
>>>
>>>      Regards
>>>      >
>>>      >>
>>>      >>  Regards
>>>      >>
>>>      >>>  Note that at first you can ignore the thermal effect and
>>>      compute the
>>>      >>>  free energy from a regular phonon calculations.
>>>      >>>  I'm also very interested in such calculations but don't have
>>>      experiences
>>>      >>>  doing this by mean of first principal calculations.
>>>
>>>
>>>
>>>      --
>>>      Hongsheng Zhao<zhaohscas at yahoo.com.cn
>>>      <mailto:zhaohscas at yahoo.com.cn>>
>>>      School of Physics and Electrical Information Science,
>>>      Ningxia University, Yinchuan 750021, China
>>>      _______________________________________________
>>>      Pw_forum mailing list
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>>>
>>>
>>>
>>>
>>> --
>>> Amin Torabi
>>> Ph.D. Student
>>> Chemistry Department
>>> /The/ University /of/ Western Ontario
>>> London, On Canada, N6A 5B7
>>> Phone: 519-661-2111 Ext: 87871
>>> ****************************************
>>>
>>>
>>>
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>> --
>> /Be the change you wish to see in the world
>> / — Mahatma Gandhi —
>>
>> Dr. Éric Germaneau<mailto:germaneau at gucas.ac.cn>
>>
>> College of Physical Sciences
>> Graduate University of Chinese Academy of Sciences
>> Yuquan Road 19A
>> Beijing 100049
>> China
>>
>> /Please, if possible, don't send me MS Word or PowerPoint attachments
>> Why? See: http://www.gnu.org/philosophy/no-word-attachments.html /
>>
>>
>>
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>

-- 
/Be the change you wish to see in the world
/ — Mahatma Gandhi —

Dr. Éric Germaneau 
<http://www.linkedin.com/pub/%C3%A9ric-germaneau/30/931/986>

College of Physical Sciences
Graduate University of Chinese Academy of Sciences
Yuquan Road 19A
Beijing 100049
China

/Please, if possible, don't send me MS Word or PowerPoint attachments
Why? See: http://www.gnu.org/philosophy/no-word-attachments.html /

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