A W atom could have other formal valencies than 6+ and an electronic
reconstruction is not impossible (remember the famous LAO/STO interface)
Of course the surface depends a lot on the preparation conditions and on
the vacuum / O partial pressure, H2O traces (or even "water") ...
There could be several possible surface reconstructions. Thus you can
try various supercells with W vacancies to reduce the formal 2+ charge)
or some O on top (but I don't like this "sticking out").
Finally do the thermodynamics and calculate a surface phase diagram as
function of O partial pressure.
In any case, I hope you have followed what Ludmilla said and use the
theoretical lattice parameter (just in case- it could be much larger
with PBE than experiment).
And clear is: if the surface layer lifts off, it is a clear indication
that your model is very "unrealistic".
Am 25.06.2021 um 14:43 schrieb Laurence Marks:
Sorry Peter, that is also wrong.
The valence of a WO2 layer is 2+ ; of the O layer 2-. A valence neutral
surface comes from
a) Adding OH above every W -- 1-
b) Add 1/2 the O above, and letting the structure relax in a 2x1 cell
(or larger) -- the last layer has a valence of 1- per 1x1
c) Adjusting a top WO2 layer so it has a valence of 1+ per 1x1 (tricky)
Note: a) is for wet samples (in air); b) more common for dry but only if
the top O can adequately bond; c) less common but not impossible. There
is some literature, but I expect many of the existing calculations are
dodgy.
_____
Professor Laurence Marks
"Research is to see what everybody else has seen, and to think what
nobody else has thought", Albert Szent-Györgyi
www.numis.northwestern.edu <http://www.numis.northwestern.edu>
On Fri, Jun 25, 2021, 07:16 Peter Blaha <pbl...@theochem.tuwien.ac.at
<mailto:pbl...@theochem.tuwien.ac.at>> wrote:
Remove the O atoms, which are sticking out of the lower surface.
This makes the system inversion symmetric and more realistic.
Since you are now not stoichiometric anymore, eventually test with more
than 5 layers .
Am 25.06.2021 um 13:42 schrieb Dr. K. C. Bhamu:
> Dear Prof. L. Marks,
> Thanks for your suggestions. Yes, WO3 is an insulator.
>
> I am facing the same issue for WC (SG: 187).
>
> I have worked on Ni(OH)2 and ZnO surfaces and I did not face any
> difficulties there. Ni(OH) surface is also polar.
>
> For the present case, I do not have much choice. I was asked to
provide
> the absorbance of some molecules on the (100) surface of WO3. The
> experimental group is not using any substrate/support for it.
>
> I will read up on oxide surfaces.
>
> Thank you very much,
> Bhamu
>
>
>
>
>
>
>
>
> On Fri, Jun 25, 2021 at 12:46 PM Laurence Marks
> <laurence.ma...@gmail.com <mailto:laurence.ma...@gmail.com>
<mailto:laurence.ma...@gmail.com <mailto:laurence.ma...@gmail.com>>>
wrote:
>
> N.B., your cell is polar, and you are not satisfying valence
> neutrality or Pauling's rules. It will never occur in
reality, and
> any results you obtain with it will move science backwards.
>
> Read up on oxide surfaces, much is known.
>
> _____
> Professor Laurence Marks
> "Research is to see what everybody else has seen, and to
think what
> nobody else has thought", Albert Szent-Györgyi
> http://www.numis.northwestern.edu
<http://www.numis.northwestern.edu>
<http://www.numis.northwestern.edu <http://www.numis.northwestern.edu>>
>
> On Fri, Jun 25, 2021, 01:59 Laurence Marks
<laurence.ma...@gmail.com <mailto:laurence.ma...@gmail.com>
> <mailto:laurence.ma...@gmail.com
<mailto:laurence.ma...@gmail.com>>> wrote:
>
> Did you check:
> a) The BVS, to ensure that you do not have a GIGO surface?
> b) Whether the system has a decent gap or not -- I
believe WO3
> should be an insulator.
> c) Whether you are satisfying valence neutrality at the
surface.
>
> I expect you have a GIGO surface, 99.99% probability. You
cannot
> just create an oxide supercell and expect it to lead to a
> realistic surface. Fixing layers won't help, and is
flawed thinking.
>
> GIGO.
>
> _____
> Professor Laurence Marks
> "Research is to see what everybody else has seen, and to
think
> what nobody else has thought", Albert Szent-Györgyi
> http://www.numis.northwestern.edu
<http://www.numis.northwestern.edu>
<http://www.numis.northwestern.edu <http://www.numis.northwestern.edu>>
>
> On Fri, Jun 25, 2021, 01:31 Dr. K. C. Bhamu
<kcbham...@gmail.com <mailto:kcbham...@gmail.com>
> <mailto:kcbham...@gmail.com
<mailto:kcbham...@gmail.com>>> wrote:
>
> Dear Wien2k Users,
>
> I am trying to relax the WO3 surface structure (1x1x5
with
> 15 Ang vacuum). The bulk structure crystallizes in
221 SG.
> Then I created the 1x1x5 layered structure and
relaxed the
> top two layers with the bottom three layers kept fixed.
> I see after relaxation, the top layer detached from the
> system (the W-O bond length increased from 1.9 to 2.3
Ang),
> you can see from the PDF provided with this email.
> I have tried with QE as well and the same problem I
faced there.
> I am wondering if someone has faced a similar
experience and
> would like to share his/her experience with me to tackle
> this issue.
>
> I have used a different strategy: with U, with U, with
> dipole correction, without dipole correction, vacuum up
> to 30Ang.
>
> As the bulk system is a high symmetry system, is it the
> reason due to the symmetry? If so then how one can
handle it?
>
> I am providing the required information here in the tar
> file (PLEASE DOWNLOAD FROM HERE
>
<https://urldefense.com/v3/__https://we.tl/t-0vedkrUS54__;!!Dq0X2DkFhyF93HkjWTBQKhk!Day7KZQBXQaPn8HGxSpldvOiJzObtS73z_8I17izzlA5ZmuvSj27rMzSsjCo-bt6MCzNAQ$ <https://urldefense.com/v3/__https://we.tl/t-0vedkrUS54__;!!Dq0X2DkFhyF93HkjWTBQKhk!Day7KZQBXQaPn8HGxSpldvOiJzObtS73z_8I17izzlA5ZmuvSj27rMzSsjCo-bt6MCzNAQ$>>).
>
> I would appreciate it if someone shares their experience
> with me.
>
> Regards
> Bhamu
>
>
>
>
>
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
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