Re: [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 
 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-boun...@zeus.theochem.tuwien.ac.at] im Auftrag von Xavier 
Rocquefelte [xavier.rocquefe...@univ-rennes1.fr]
Gesendet: Freitag, 22. Dezember 2017 10:54
An: wien@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@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@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@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

Re: [Wien] stability issue

[Fecher, Gerhard]

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-boun...@zeus.theochem.tuwien.ac.at] im Auftrag von Xavier 
Rocquefelte [xavier.rocquefe...@univ-rennes1.fr]
Gesendet: Freitag, 22. Dezember 2017 10:54
An: wien@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@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@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

Re: [Wien] stability issue

[Xavier Rocquefelte]

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@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@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] stability issue

[chin Sabsu]

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@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