[Wien] Top surface layer seems to be isolated from the rest of the system

[Laurence Marks]

Agreed, although to make the bulk truncation valence neutral involves W 5+
which is unusual. As you say, the O "sticking out" is unreal, as it would
be 1-. If it moves closer to the W below that becomes 7+ which is silly.

The first thing to do is work out what the actual experimental conditions
are -- air, UHV, some catalytic gas mix, unknown. Then you can start to
construct relevant models.It is easy to construct bad models!

N.B., remember that STO/LAO is constrained, and forced to be different if
there are no O vacancies.

On Fri, Jun 25, 2021 at 9:20 AM Peter Blaha <pblaha at theochem.tuwien.ac.at>
wrote:

> 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
> > http://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 .
> >
> >

-- 
Professor Laurence Marks
Department of Materials Science and Engineering
Northwestern University
www.numis.northwestern.edu
"Research is to see what everybody else has seen, and to think what nobody
else has thought" Albert Szent-Györgyi
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