[Wien] Crystal field splitting in empty 3d band of Fe2O3

[Pavel Novak]

Dear Yang Ding,

care is needed when estimating the crystal field splitting from the LDA+U 
calculation using the DOS. There are two reasons for it. First, the LDA+U 
lower the energy of more occupied states and increase the energy of less 
occupied states. Even if the bands are above Fermi energy, they contain 
nonzero fraction of electrons (cf :QTL in scf file), which is different 
for eg and t2g states, hence LDA+U distorts the splitting. Second, the 
selfinteraction of the d-electrons is present, again distorting the 
crystal field splitting.

Regards
Pavel Novak

On Tue, 15 Sep 2009, Yang Ding wrote:

> Dear WIEN2k  users,
> 
> I am really new to WIEN2k, and wondering if you could give your advice and
> experience on following question concerning the crystal filed splitting
> calculated from WIEN2k.
> 
> In order to understand if the pre-edge splitting appearing in the Fe K-edge
> spectra (1s-4p transition) measured by emission-XANES on Fe2O3 [Groot et al.
> J. Phys.: Condens. Matter 21 (2009) 104207
> <http://www.iop.org/EJ/abstract/0953-8984/21/10/104207/>], is linked to
> crystal-filed splitting in 3d empty band. We did a very preliminary ground
> state calculation using WIEN2k based on GGA+U (and LSDA+U) with U = 4 eV
> structure to check the crystal field splitting in empty d band above Fermi
> level.
> 
> As a result, we found that above 2-6 eV above Fermi level, the energy of t2g
> is higher than that of eg. This result is similar to what reported by Rollsman
> et al (PHYSICAL REVIEW B 69, 165107 (2004)
> <http://prola.aps.org/abstract/PRB/v69/i16/e165107>) on Fe2O3. In his
> calculation (GGA/LSDA+U , U= 4eV), the energy of t2g is also higher than that
> of eg. So my question is why the t2g and eg are reversed in DFT, but the
> Multiplet calculation gives contradictory results (i.e from Groot et al.).
> 
> I noticed that  Glatzel et al (PHYSICAL REVIEW B 77, 115133 (2008)
> <http://prola.aps.org/abstract/PRB/v69/i16/e165107>) reported that they
> obtained the "right" crystal field splitting using (LDA+U, U=6 eV) from
> WIEN2k.   So we wonder if we might missed something in the calculations?
> 
> Thanks  in advance for your help,
> 

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