[QE-users] Need some suggestion regarding the band gap issue of Fe3O4
Dr. K. C. Bhamu
kcbhamu85 at gmail.com
Mon Nov 15 11:47:16 CET 2021
Dear QE Users,
This email may be large for you all as I am trying to put all the
information so that I do not miss any information in your response.
I sincerely apologize for this long email.
I am dealing with ferrimagnetic Fe3O4 (with QE6.6) and trying to reproduce
its band gap. In the literature, its experimental band gap was reported to
be in the range of 0.14-0.3eV, Table SI-1 [1]
<https://aip.scitation.org/doi/10.1063/1.5138941> for the up-spin state.
For Fe3O4 nanoparticles, the band gap is reported ~2eV.
I have tried with all available PPs (mentioned below in different sets) and
U values from 1eV to 7eV. At lower U (=1-2eV), I am getting this system a
direct band-gap half-metal (a gap is opened in the spin-up channel) and
with increasing the U value, the direct band-gap (=~1eV) nature changes to
the indirect (=~2eV) band gap. At U=7eV with the rrkjus PPs, it showed a
metallic character.
I have also carried out the band structure, by keeping the lattice
parameters fixed at experimental values and only relaxing ionic position,
with varying U-values. But still, the nature of the bang-gap is similar to
what I just mentioned above(band gap varies from ~1eV to ~2eV with U=1eV to
7eV, respectively).
The lattice parameters, magnetic moments, and the charge on Fe_tet and
Fe_oct sites I am getting reasonably in accordance with the Table SI-1
<https://aip.scitation.org/doi/10.1063/1.5138941>.
A sample of my input file is also shown below (the only PPs are different
with respective cutoffs).
Information collected from other research papers:
1. See [2]
<https://www.sciencedirect.com/science/article/abs/pii/S0039602812000544>
(In section 3.1. Bulk Fe3O4: 0.33 eV band gap with VASP with U=3.8eV ).
2. See [3]
<https://iopscience.iop.org/article/10.1088/1468-6996/15/4/044202> (Page-4,
bottom left para: 2.1 eV using VASP with U=4 eV). This paper only shows
the direct band-gap of 2.1eV.
Based on my already finished calculations and the details that I have
covered above, I have a few queries:
1. Do you think my input file (see below) has any issues? I have defined
all tetra-Fe sites dn-spin and all octahedra Fe sites up-spin.
2. What other things I should take care of to reproduce the band gap
(0.14 eV to 0.3 eV)?
3. In the research paper [3]
<https://iopscience.iop.org/article/10.1088/1468-6996/15/4/044202>, the
authors used VASP and the valence configuration for the Fe atom was taken
as 3d64s1. I could not find any PP from the list I mention below which has
this configuration. Does this may be an issue? If anyone has PP of Fe
with 3d64s1 configuration, please provide me.
4. As mentioned in the research paper [4]
<https://pubs.acs.org/doi/full/10.1021/acs.jpcc.7b09387>, the authors
have mentioned that "Our results show that, upon releasing the symmetry
constraint on the density but not on the geometry, charge
disproportionation (Fe2+/Fe3+) is observed, resulting in a band gap of
around 0.2 eV at the Fermi level." How can I release symmetry constrain
on the density? I used nonsym=True also but the calculation was not
converging in this case.
Supporting information for this email:
*Combinations of my PPs:*
Set-1 (ecutwfc/ecutrho = 40/320Ry)
Fe.pbe-nd-rrkjus.UPF
O.pbe-rrkjus.UPF
Set-2 (ecutwfc/ecutrho = 45/450Ry)
Fe.pbe-sp-van_mit.UPF
O.pbe-rrkjus.UPF
*Set-3 (ecutwfc/ecutrho = 65/785Ry) >> similar to yours*
*Fe.pbe-spn-rrkjus_psl.1.0.0.UPFO.pbe-n-rrkjus_psl.1.0.0.UPF*
set-4 (ecutwfc/ecutrho = 64/782Ry)
Fe.pbe-spn-kjpaw_psl.0.2.1.UPF
O.pbe-n-kjpaw_psl.0.1.UPF
*QE input file:*
&CONTROL
calculation = 'scf'
etot_conv_thr = 1.0000000000d-04
forc_conv_thr = 1.0000000000d-03
! disk_io = 'none'
outdir = './tmp/'
prefix = 'pwscf'
pseudo_dir = '~/PPs/'
! tprnfor = .true.
! tstress = .true.
! verbosity = 'high'
/
&SYSTEM
degauss = 0.01
ecutrho = 785
ecutwfc = 65
ibrav = 0
nat = 56
nosym = .false. !! I have tried with .True. also but my calculations
were not converging.
ntyp = 3
occupations = 'smearing' , smearing = 'mp'
starting_magnetization(1) = 1
starting_magnetization(2) = -1
nspin = 2
lda_plus_u=.true.
Hubbard_U(1)=X (tried 1eV to 7eV)
Hubbard_U(2)=X (tried 1eV to 7eV)
/
&ELECTRONS
conv_thr = 1.000000000d-6
electron_maxstep = 200
mixing_beta = 3.0000000000d-01
/
&IONS
ion_dynamics = 'bfgs'
/
&CELL
/
ATOMIC_SPECIES
Fe1 55.845 Fe.pbe-spn-rrkjus_psl.1.0.0.UPF
Fe2 55.845 Fe.pbe-spn-rrkjus_psl.1.0.0.UPF
O 15.9994 O.pbe-n-rrkjus_psl.1.0.0.UPF
ATOMIC_POSITIONS crystal
Fe1 0.6250000000 0.6250000000 0.6250000000
Fe1 0.6250000000 0.3750000000 0.3750000000
Fe1 0.3750000000 0.6250000000 0.3750000000
Fe1 0.3750000000 0.3750000000 0.6250000000
Fe1 0.6250000000 0.8750000000 0.8750000000
Fe1 0.8750000000 0.6250000000 0.8750000000
Fe1 0.8750000000 0.8750000000 0.6250000000
Fe1 0.6250000000 0.1250000000 0.1250000000
Fe1 0.1250000000 0.6250000000 0.1250000000
Fe1 0.1250000000 0.1250000000 0.6250000000
Fe1 0.1250000000 0.3750000000 0.8750000000
Fe1 0.1250000000 0.8750000000 0.3750000000
Fe1 0.3750000000 0.1250000000 0.8750000000
Fe1 0.8750000000 0.1250000000 0.3750000000
Fe1 0.3750000000 0.8750000000 0.1250000000
Fe1 0.8750000000 0.3750000000 0.1250000000
Fe2 0.0000000000 0.0000000000 0.0000000000
Fe2 0.2500000000 0.2500000000 0.2500000000
Fe2 0.0000000000 0.5000000000 0.5000000000
Fe2 0.5000000000 0.0000000000 0.5000000000
Fe2 0.5000000000 0.5000000000 0.0000000000
Fe2 0.2500000000 0.7500000000 0.7500000000
Fe2 0.7500000000 0.2500000000 0.7500000000
Fe2 0.7500000000 0.7500000000 0.2500000000
O 0.3750000000 0.3750000000 0.3750000000
O 0.3750000000 0.6250000000 0.6250000000
O 0.6250000000 0.3750000000 0.6250000000
O 0.6250000000 0.6250000000 0.3750000000
O 0.8750000000 0.8750000000 0.8750000000
O 0.8750000000 0.6250000000 0.6250000000
O 0.6250000000 0.8750000000 0.6250000000
O 0.6250000000 0.6250000000 0.8750000000
O 0.3750000000 0.8750000000 0.8750000000
O 0.8750000000 0.3750000000 0.8750000000
O 0.8750000000 0.8750000000 0.3750000000
O 0.3750000000 0.1250000000 0.1250000000
O 0.8750000000 0.6250000000 0.1250000000
O 0.8750000000 0.1250000000 0.6250000000
O 0.6250000000 0.8750000000 0.1250000000
O 0.1250000000 0.3750000000 0.1250000000
O 0.1250000000 0.8750000000 0.6250000000
O 0.6250000000 0.1250000000 0.8750000000
O 0.1250000000 0.6250000000 0.8750000000
O 0.1250000000 0.1250000000 0.3750000000
O 0.8750000000 0.3750000000 0.3750000000
O 0.3750000000 0.8750000000 0.3750000000
O 0.3750000000 0.3750000000 0.8750000000
O 0.8750000000 0.1250000000 0.1250000000
O 0.3750000000 0.6250000000 0.1250000000
O 0.3750000000 0.1250000000 0.6250000000
O 0.6250000000 0.3750000000 0.1250000000
O 0.1250000000 0.8750000000 0.1250000000
O 0.1250000000 0.3750000000 0.6250000000
O 0.6250000000 0.1250000000 0.3750000000
O 0.1250000000 0.6250000000 0.3750000000
O 0.1250000000 0.1250000000 0.8750000000
K_POINTS automatic
3 3 3 0 0 0
CELL_PARAMETERS angstrom
8.3840000000 0.0000000000 0.0000000000
0.0000000000 8.3840000000 0.0000000000
0.0000000000 0.0000000000 8.3840000000
Thank you very much for having patience while going through this detailed
email.
I look forward to hearing from you.
Regards
K C Bhamu
Postdoctoral Fellow
School of Chemical Engineering
University of Ulsan
South Korea
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