Hello
from the pdos 2 electrons have /moved /from Pb 6p to fill the S 3p
band, as expected.
The 3p band is hybridized with 6s and 6p orbitals of Pb and vice versa.
So the S 3p is still made by orbitals centered on S ions but is spread
to neighboring Pb ions. If a S ion is displaced it goes closer to some
Pb ion and far from others, same thing happens dispacing a Pb. As
Stefano wrote the 3p orbitals would still be centered close to the S
ion, Bader charge thus would not change, but the orbital will now
lean towards the closer Pb and away from those that are now a little bit
farther, this enhances the dynamical charge with respect to the
oxidation number.
Pietro
On 14/04/20 00:54, Vahid Askarpour wrote:
The attached projected DOS for PbS suggests that the Pb s- and p-
states have significant DOS and so perhaps the 4 electrons of Pb have
not moved to sulphur unless I am reading the DOS incorrectly. The Pb
d-states are lower in energy and not shown.
Cheers,
Vahid
On Apr 13, 2020, at 12:53 PM, Vahid Askarpour <[email protected]
<mailto:[email protected]>> wrote:
Dear Stefano,
Thank you for this quick response. I was following Xavier Gonze’s
argument in PRB58, 6224 (1998) Eq. 20 where
Z*(u)=dp/du, where p is the dipole moment and u is the displacement.
Since p=u.Z(u), we get
Z*(u)=Z(u)+udZ(u)/du, where Z(u) is the static charge.
So here the Born charge is given in terms of change in charge.
I will try the projected DOS to see if the charge transfer is
actually that large.
Best,
Vahid
On Apr 13, 2020, at 12:35 PM, Stefano de Gironcoli
<[email protected] <mailto:[email protected]>> wrote:
CAUTION: The Sender of this email is not from within Dalhousie.
I didnt follow completely your argument about the Bader charge but what
one should keep in mind is that the effective charge is the change in
polarization due to displacement not the change in charge...
think of a core electron shell (as the d orbitals of Pb can
approximately be considered) ... as you move the atom they follow
rigidly (that would make a contribution of 10 not far from your
estimate).
from the PHONON result it looks like Pb gave away 4 all its valence
electrons to S. or rather they are so weakly bound to Pb that they don't
follow it, even if they can still belong its Bader volume.
it looks a bit extreme but this appears to be the result. You could
compute the atomic projected density of states and see if this seems the
case.
stefano
On 13/04/20 17:11, Vahid Askarpour wrote:
Dear QE Community,
I have calculated the Born charges using the PHONON code for PbS.
The only non-zero elements are the diagonal ones and are 4.122 and
-4.168, respectively.
In the zstar_eu.f90, Born charges consist of two terms as seen
below: a part due to polarization calculation (dynamic) and the
other is zv (static) which is the z_valence according to
read_upf_v2.f90.
do ipol = 1, 3
do na = 1, nat
zstareu (ipol, ipol, na) = zstareu (ipol, ipol, na) + zv
(ityp ( na) )
enddo
enddo
The zv values for Pb and S are 14 and 6 given in the PSP. If we
subtract zv from the Born charges, we get the term due to
polarization: -9.878 and -10.168. These values seem too large
because of the argument below.
To estimate the polarization term, I reduce the alat by 1% and
relax the atoms. This shifts the atoms from the unstrained
position. I calculate the Bader charges for the unstrained and the
strained cases. The change in the Bader charge is related to the
atomic displacement. I have also tried keeping alat fixed and
moving the atoms by 1%.
For unstrained PbS, the Bader charges are 12.998 and 7.001.
For the strained PbS, they are 13.004 and 6.995.
So a ~1% change in atomic positions results in a +/-0.006 change in
Bader charge. From this calculation, I expect the contribution from
polarization to be u(dZ/du), where u is interatomic distance, which
amount to +0.6 for Pb and -0.6 for S.
The contribution from polarization I get (0.6 and -0.6) are quite
different from the those of the PHONON code (-9.878 and -10.168). I
am assuming that the code is correct and my logic is flawed. I
would appreciate any thoughts you may have on this discrepancy.
Thank you,
Vahid
Vahid Askarpour
Department of physics and atmospheric science
Dalhousie University
Halifax, NS
Canada
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<http://www.max-centre.eu/quantum-espresso>)
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users mailing list [email protected]
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users mailing list [email protected]
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