[Wien] Structural phase transition

[Subhasis Panda]

Thank you, Sir, for the help!
I'm still having some doubts.
For studying the structural phase transition at T=0K, I've plotted enthalpy
against pressure and got the transition pressure (T_p) from Gibbs2 data
which is similar to the one mentioned in the attached paper.
Gibbs2 under QHA (Debye-Slater model) provides G by varying p and T at a
prescribed interval. When I plot \deltaG =0 I got many (p,T) points, when
plotted I get the temperature variation of the transition pressure (T_p).

But I am not getting the transition temperature as mentioned in figure 3 of
the attached paper at P=0Gpa.
The Entropy calculated in Gibbs2 can it be used for this study?



On Wed, May 29, 2019, 5:55 PM Peter Blaha <pblaha at theochem.tuwien.ac.at>
wrote:

> 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.
> >
> > _______________________________________________
> > Wien mailing list
> > Wien at zeus.theochem.tuwien.ac.at
> > http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
> > SEARCH the MAILING-LIST at:
> http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html
> >
>
> --
>
>                                        P.Blaha
> --------------------------------------------------------------------------
> 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
> --------------------------------------------------------------------------
> _______________________________________________
> Wien mailing list
> Wien at zeus.theochem.tuwien.ac.at
> http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
> SEARCH the MAILING-LIST at:
> http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://zeus.theochem.tuwien.ac.at/pipermail/wien/attachments/20190530/3a378279/attachment.html>