[Wien] Structural phase transition

[Peter Blaha]

Finite temperature requires the entropy. So you would not compare 
enthalpies, but free energies.
The main contribution to the entropy comes from the phonons. Thus you 
must calculate the phonons and from the corresponding phonon-DOS you can 
get the "-TS" contribution to the free energy.

Phonons can be calculated with WIEN2k and the help of programs like 
PHONOPY or PHONON (see unsupported software at www.wien2k.at) and one 
would do what is called the "quasiharmonic" approximation (i.e. 
calculate the phonons at various volumes).
This is straight forward and has been done many times in Literature.

However, this approach fails eventually, since in the harmonic 
approximation some phonons might be instable, giving imaginary frequencies.

In that case, anharmonic terms are needed and several different schemes 
are discussed in Literature without consensus about the best method. In 
any case, all these calculations become very expensive.

On 5/18/19 8:57 PM, Subhasis Panda wrote:
> Dear experts,
> I've used Wien2k and subsequently Gibbs2 package to study structural 
> phase transition between two polymorphs of a material. The enthalpy vs 
> pressure plot at T=0K gives me the transition pressure for a particular 
> transition.
> However, when I try to plot difference of Gibbs free energy at P=0 GPa 
> it doesn't cross zero. But, the literature is showing transition 
> temperature for the phase transition.
> So, my questions are
> 1) How to determine the transition temperature at P=0 GPa or at any 
> pressure?
> 2) How do we get the variation of transition pressure with temperature?
> Looking forward for your valuable comments.
> Any reference in this direction is most welcomed.
> Thank you in advance for the help.
> 
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-- 

                                       P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-165300             FAX: +43-1-58801-165982
Email: blaha at theochem.tuwien.ac.at    WIEN2k: http://www.wien2k.at
WWW:   http://www.imc.tuwien.ac.at/TC_Blaha
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