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Yes and No ... Such discussion should be based on realistic
situations. The magnetic moments are related to the way the
electrons occupy the bands (or orbitals) and such occupation depends
on many parameters : <br>
- the atomic arrangement / the nature of the magnetic orbital / the
nature of the ligands --> ligand field (high spin, low spin) ...<br>
- spin-orbit contribution ... <br>
- the oxidation degree (formal charge: Ni2+ or Ni3+) of the magnetic
center ...<br>
<br>
Regards<br>
<br>
Xavier<br>
<br>
<br>
On 03/06/2012 06:52 AM, Yundi Quan wrote:
<blockquote
cite="mid:CALsd9rhVVgjYyEovQ-kHvDPkRhsehmNhamae_eL3__rAGoLG0g@mail.gmail.com"
type="cite">Thanks for your explanation. Now I understand that
formal valence is different from real charge. However, can we say
that anything about the charge based on the magnetic moment. For
example, Ni3+ has d7. Now experiments give the magnetic moment of
Nickel in one material to be 1.4 and another material to be 0.5.
Does it mean the charge around the two Nickel ions should be
different?<br>
<br>
<br>
<br>
<br>
<br>
<div class="gmail_quote">On Mon, Mar 5, 2012 at 9:31 PM,
Rocquefelte <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:Xavier.Rocquefelte@cnrs-imn.fr">Xavier.Rocquefelte@cnrs-imn.fr</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt
0.8ex; border-left: 1px solid rgb(204, 204, 204);
padding-left: 1ex;">
Just to add one more comment on this topic. Bond-valence
analysis is very powerful and will also works for other
compounds (beyond oxides).<br>
It will be more tricky if you have hydrogen-bonds, but also in
this case there is possibilities to extract the valence of the
metals and ligands form their coordination shell.<br>
<br>
About Fe2+ and Fe3+, one other simple method could be used to
extract the "FORMAL CHARGE" which is not REAL, because it is
based on a democratic (covalent) distribution of the bonding
electrons between the different partners. You can look at the
partial DOS and see where the Fermi energy cross the 3d bands:<br>
<br>
- if 6 electrons are in the 3d bands --> Fe2+<br>
- if 5 electrons are in the 3d bands --> Fe3+<br>
<br>
The charge in the RMT of iron differ by small numbers although
if the DOS clearly shows a difference between Fe2+ and Fe3+.
It is the reason why the Bader analysis is better to have
access to the "REAL" charge of the atoms, however as mentioned
by Laurence these charges will be different from the "FORMAL"
values. One iron will tend to be 2+ and the other one will
tend to be 3+ (just a tendency) ...<br>
<br>
But definitely the bond-valence analysis is a very easy method
which allows to extract the FORMAL CHARGE based on the
geometry of the ligands around the iron metal.<br>
<br>
Regards<br>
<br>
Xavier
<div>
<div class="h5"><br>
<br>
<br>
On 03/06/2012 05:58 AM, Laurence Marks wrote:<br>
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt
0.8ex; border-left: 1px solid rgb(204, 204, 204);
padding-left: 1ex;">
Charge can be estimated using (as a conventional
measure) the Bader<br>
volume around an atom, for instance by using the aim
command in<br>
Wien2k.<br>
<br>
However, this is not the same as "Fe2+" which has a
strict meaning<br>
that the iron atom is in a 2+ valence state. This is not
the same (but<br>
alas often confused with) the effective charge, which I
prefer to<br>
write as +X, as against valence as X+. You can blame
Linus Pauling for<br>
this as he used the word "charge" in his original
papers, when<br>
referring to what is really valence.<br>
<br>
As an example, strontium titanate has a valence of Sr2+
Ti4+ (O2-)3.<br>
However, the "charges" from a Bader analysis are closer
to Sr+1.6<br>
Ti+2.2 (O-1.2)3 . (The numbers are right, there is a
slight rounding<br>
error in the charges).<br>
<br>
In the limit of completely ionic bonding the two numbers
are the same.<br>
However, in general they are not and a source of endless
confusion.<br>
Depending upon exactly what functional you use the
charges change, for<br>
instance PBE tends to be too covalent and hybrids and/or
LDA+U<br>
improves on this (for d-electrons in Wien2k) making
oxides more ionic.<br>
<br>
For an oxide, you can readily get the valence from a
bond-valence<br>
analysis. I am not sure about other cases.<br>
<br>
And, just to make life more interesting, many
spectroscopies (e.g.<br>
XPS, EELS) have been calibrated versus valence.<br>
<br>
2012/3/5 Yundi Quan<<a moz-do-not-send="true"
href="mailto:quan@ms.physics.ucdavis.edu"
target="_blank">quan@ms.physics.ucdavis.edu</a>>:<br>
<blockquote class="gmail_quote" style="margin: 0pt 0pt
0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204);
padding-left: 1ex;">
Dear Professor Laurence Marks,<br>
Charge ordering has long been a generator of
controversies. I'm wondering<br>
whether charge as measure from DFT can be used to
identify charge ordering.<br>
<br>
Thanks<br>
Yundi<br>
<br>
<br>
2012/3/5 Laurence Marks<<a moz-do-not-send="true"
href="mailto:L-marks@northwestern.edu"
target="_blank">L-marks@northwestern.edu</a>><br>
<blockquote class="gmail_quote" style="margin: 0pt 0pt
0pt 0.8ex; border-left: 1px solid rgb(204, 204,
204); padding-left: 1ex;">
Charge, as measured from DFT, and valence (chemical
bonding) are very,<br>
very different.<br>
<br>
Without more information as to what you are asking,
that is about the only<br>
answer available.<br>
<br>
---------------------------<br>
Professor Laurence Marks<br>
Department of Materials Science and Engineering<br>
Northwestern University<br>
<a moz-do-not-send="true"
href="http://www.numis.northwestern.edu"
target="_blank">www.numis.northwestern.edu</a> <a
moz-do-not-send="true" href="tel:1-847-491-3996"
value="+18474913996" target="_blank">1-847-491-3996</a><br>
"Research is to see what everybody else has seen,
and to think what nobody<br>
else has thought"<br>
Albert Szent-Gyorgi<br>
<br>
On Mar 5, 2012 7:11 PM, "Yundi Quan"<<a
moz-do-not-send="true"
href="mailto:quanyundi@gmail.com" target="_blank">quanyundi@gmail.com</a>>
wrote:<br>
<blockquote class="gmail_quote" style="margin: 0pt
0pt 0pt 0.8ex; border-left: 1px solid rgb(204,
204, 204); padding-left: 1ex;">
Does LAPW method always underestimate the chemical
bonding? For example,<br>
Fe2+ and Fe3+ have the same charge around them.<br>
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