[Wien] chemical bonding
Rocquefelte
Xavier.Rocquefelte at cnrs-imn.fr
Tue Mar 6 07:51:57 CET 2012
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 :
- the atomic arrangement / the nature of the magnetic orbital / the
nature of the ligands --> ligand field (high spin, low spin) ...
- spin-orbit contribution ...
- the oxidation degree (formal charge: Ni2+ or Ni3+) of the magnetic
center ...
Regards
Xavier
On 03/06/2012 06:52 AM, Yundi Quan wrote:
> 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?
>
>
>
>
>
> On Mon, Mar 5, 2012 at 9:31 PM, Rocquefelte
> <Xavier.Rocquefelte at cnrs-imn.fr
> <mailto:Xavier.Rocquefelte at cnrs-imn.fr>> wrote:
>
> Just to add one more comment on this topic. Bond-valence analysis
> is very powerful and will also works for other compounds (beyond
> oxides).
> 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.
>
> 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:
>
> - if 6 electrons are in the 3d bands --> Fe2+
> - if 5 electrons are in the 3d bands --> Fe3+
>
> 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) ...
>
> 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.
>
> Regards
>
> Xavier
>
>
>
> On 03/06/2012 05:58 AM, Laurence Marks wrote:
>
> Charge can be estimated using (as a conventional measure) the
> Bader
> volume around an atom, for instance by using the aim command in
> Wien2k.
>
> However, this is not the same as "Fe2+" which has a strict meaning
> that the iron atom is in a 2+ valence state. This is not the
> same (but
> alas often confused with) the effective charge, which I prefer to
> write as +X, as against valence as X+. You can blame Linus
> Pauling for
> this as he used the word "charge" in his original papers, when
> referring to what is really valence.
>
> As an example, strontium titanate has a valence of Sr2+ Ti4+
> (O2-)3.
> However, the "charges" from a Bader analysis are closer to Sr+1.6
> Ti+2.2 (O-1.2)3 . (The numbers are right, there is a slight
> rounding
> error in the charges).
>
> In the limit of completely ionic bonding the two numbers are
> the same.
> However, in general they are not and a source of endless
> confusion.
> Depending upon exactly what functional you use the charges
> change, for
> instance PBE tends to be too covalent and hybrids and/or LDA+U
> improves on this (for d-electrons in Wien2k) making oxides
> more ionic.
>
> For an oxide, you can readily get the valence from a bond-valence
> analysis. I am not sure about other cases.
>
> And, just to make life more interesting, many spectroscopies (e.g.
> XPS, EELS) have been calibrated versus valence.
>
> 2012/3/5 Yundi Quan<quan at ms.physics.ucdavis.edu
> <mailto:quan at ms.physics.ucdavis.edu>>:
>
> Dear Professor Laurence Marks,
> Charge ordering has long been a generator of
> controversies. I'm wondering
> whether charge as measure from DFT can be used to identify
> charge ordering.
>
> Thanks
> Yundi
>
>
> 2012/3/5 Laurence Marks<L-marks at northwestern.edu
> <mailto:L-marks at northwestern.edu>>
>
> Charge, as measured from DFT, and valence (chemical
> bonding) are very,
> very different.
>
> Without more information as to what you are asking,
> that is about the only
> answer available.
>
> ---------------------------
> Professor Laurence Marks
> Department of Materials Science and Engineering
> Northwestern University
> www.numis.northwestern.edu
> <http://www.numis.northwestern.edu> 1-847-491-3996
> <tel:1-847-491-3996>
> "Research is to see what everybody else has seen, and
> to think what nobody
> else has thought"
> Albert Szent-Gyorgi
>
> On Mar 5, 2012 7:11 PM, "Yundi
> Quan"<quanyundi at gmail.com
> <mailto:quanyundi at gmail.com>> wrote:
>
> Does LAPW method always underestimate the chemical
> bonding? For example,
> Fe2+ and Fe3+ have the same charge around them.
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