[Wien] stability issue

[chin Sabsu]

 Dear Sir Xavier and Gerhard,
Thank you very much for making my understanding a little bit clear.
This query is in continuation of my past queries.
I shall mention here that, I am reproducing a paper and I took optimised lattice parameters from the literature paper and the initialisation and scf were performed on exactly same parameters what the author reported. 
[http://pubs.rsc.org/-/content/articlehtml/2016/cp/c6cp04065a]

I have calculated ground state energy of ABO3, AO, and BO2 for a variety of compounds (high-temperature cubic perovskite). The purpose is to study the thermodynamic stability of the bulk system (ABO3) and creation of defect in the system  (A, B, and O; charged and neutral vacancy). 

The energy calculated from Eq 2 is in agreement with the reported one in the literature but none of the authors provided the energy calculated by Eq. 1 and I could not compare it.
So my query is related to stability issue.
As Gerhard sir said, binary reactants may be more stable and thus I am getting the total energy positive from Eq. 1 and this is endothermic. Yes, this may be a case and I am getting a positive energy.
Can we study such a system (high-temperature cubic phase with endothermic nature)  from DFT/Wien2k (at T=0K) to study the defect creation and thermodynamic stability?


Thank you again sir,

Chin





    On Friday 22 December 2017, 3:43:59 PM IST, Fecher, Gerhard <fecher at uni-mainz.de> wrote:  
 
 Isn't it simply possible that the binaries AO + BO2 are more stable than the ternary ABO3 ?

BTW.: you find tons of papers where quaternaries or ternaries are predicted to exist even so the binaries (possibly in combination with elements) are more stable
and a phase separation will occur.

Ciao
Gerhard

DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy:
"I think the problem, to be quite honest with you,
is that you have never actually known what the question is."

====================================
Dr. Gerhard H. Fecher
Institut of Inorganic and Analytical Chemistry
Johannes Gutenberg - University
55099 Mainz
and
Max Planck Institute for Chemical Physics of Solids
01187 Dresden
________________________________________
Von: Wien [wien-bounces at zeus.theochem.tuwien.ac.at] im Auftrag von Xavier Rocquefelte [xavier.rocquefelte at univ-rennes1.fr]
Gesendet: Freitag, 22. Dezember 2017 10:54
An: wien at zeus.theochem.tuwien.ac.at
Betreff: Re: [Wien] stability issue

Dear Chin

Your question is a little aside from the WIENLIST. Indeed, it is thermodynamics.

In equation 1, you evaluate the enthalpie of formation of the compound ABO3 from the reactants AO and BO2 using internal energies. Indeed, you neglect the volumetric effect (P deltaV). From your DFT calculation, if correct, you can simply conclude that the following reaction is endothermic.

Equation 2 gives you a different reaction and in your case is exothermic. We cannot answer in more details based on your message which do not provide any details. I can simply say that depending on the experimental conditions, the atmosphere can be oxygen rich / oxygen poor ... you can start from pure solids A and B ... or other compounds ... In all case you will estimate a total energy difference which will give you an idea of the endo/exo-thermic nature of the reaction. You will not estimate the Gibbs energy (because you also neglect the entropic part T deltaS). However, in many articles we consider that estimation of internal energies are sufficient for solids because both deltaV and deltaS are negligeable (but such an approximation can be really bad).

Hope it will help you.

Cheers

Xavier




Le 22/12/2017 à 10:32, chin Sabsu a écrit :
Dear Sir,

I am working on the system ABO3 .

I have calculated formation energy in two ways:

Delta_E= Et[ABO3]  - Et[AO]  - Et[BO2]  ------ (1)

 Delta_E= Et[ABO3] - Et[A] - Et[B] - 3/2Et[O2]    ----- (2)

The Delta_E is positive 0.04 Ry from Eq (1) while is negative from Eq 2.

Why is the energy difference calculated from Eq (1)  positive?


Thanks in advance!


Regards

Chin




_______________________________________________
Wien mailing list
Wien at zeus.theochem.tuwien.ac.at<mailto: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


_______________________________________________
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/20171222/0b001604/attachment.html>