[Wien] Structural phase transition

[Subhasis Panda]

I forgot to the link of the paper.
https://iopscience.iop.org/article/10.1088/1674-1056/18/1/043

On Thu, May 30, 2019 at 10:10 AM Subhasis Panda <onnyorup.iit at gmail.com>
wrote:

> 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.
>> >
>> > _______________________________________________
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>> >
>>
>> --
>>
>>                                        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
>> --------------------------------------------------------------------------
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>>
>

-- 



Best regards,
------------------------------------------------------------
Subhasis Panda
Assistant Professor
Department of Physics
National Institute of Technology Silchar
Assam, India  - 788010.

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