[Wien] Top surface layer seems to be isolated from the rest of the system
Peter Blaha
pblaha at theochem.tuwien.ac.at
Fri Jun 25 16:19:43 CEST 2021
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 <pblaha at theochem.tuwien.ac.at
> <mailto:pblaha at 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.marks at gmail.com <mailto:laurence.marks at gmail.com>
> <mailto:laurence.marks at gmail.com <mailto:laurence.marks at 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.marks at gmail.com <mailto:laurence.marks at gmail.com>
> > <mailto:laurence.marks at gmail.com
> <mailto:laurence.marks at 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
> <kcbhamu85 at gmail.com <mailto:kcbhamu85 at gmail.com>
> > <mailto:kcbhamu85 at gmail.com
> <mailto:kcbhamu85 at 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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