[QE-users] Metal-substituted semiconductor: treat as metal or insulator for phonons

[José C. Conesa]

Hi,

GGA is certainly inadequate to sytudy this system. My recommendation 
would be to carry out a hybrid functional calculation, e.g. with HSE. 
This cell has not too may atoms, it should be feasible. Use at least a 
4x4x4 Brillouin zone sampling. Displacing Fe from the symmetry position 
is of course necessary, then the structure should be relaxed. And a 
spin-polarized calculation will be necessary as well. I find likely that 
the system will have in the end a low spin configuration, with all 
Fe(3d) electrons filling the t(2g) manifold which would be well 
separated from the e(g) manifold, therefore having a clear nonmetallic 
behaviour.

Good luck,

JC Conesa


El 04/03/2018 a las 9:51, Pietro Delugas escribió:
> If the band is flat or almost flat maybe one could try to use LDA+U, ( 
> or some hybrid functional with exact exchange if the cell size is  
> feasible ) displace Fe a little if it is in a  too symmetric position, 
> relax and see if it is possible to split Fe impurity levels. If so one 
> can  compute Fe Born Charges by finite differences.
>
>
> Il 04 mar 2018 9:27 AM, Paolo Giannozzi <p.giannozzi at gmail.com> ha 
> scritto:
>
>     The problem is that the true system is not metallic, but in a band
>     picture it is, even for a supercell of 1m side. Maybe one might
>     compute phonons as for a metal, then use dielectirc constant and
>     effective charges of the insulating host crystal. Not sure what
>     one can use for the effective charges of Fe, though.
>
>     Paolo
>
>     On Sun, Mar 4, 2018 at 8:39 AM, Pietro Delugas <pdelugas at sissa.it
>     <mailto:pdelugas at sissa.it>> wrote:
>
>         Hello Chris
>
>         If the system is metallic, no matter how bad a conductor it
>         can be, in a finite time it will be able to screen any
>         constant electric field. This  is as to say that the static
>         dielectric constant is infinite.
>
>         Regards Pietro
>
>
>         Il 04 mar 2018 8:03 AM, Christoph Wolf
>         <wolf.christoph at qns.science> ha scritto:
>
>             Dear all,
>
>             I have a fairly general question and I hope I can pick
>             someone's brain:
>
>             If an insulator or semiconductor is doped with a metal
>             narrow bands determined by the crystal field emerge and
>             often the fermi level lies within one of the bands, i.e.
>             the "bands cut the fermi level", which is often called a
>             characteristic of a conductor but in the bigger picture no
>             electrons would be able to cross from the VB to the CB,
>             i.e. the host system is an insulator.
>
>             When attempting to calculate the phonons of a Mg7O8Fe
>             supercell the  dieletric constant (in the case of pure MgO
>             eps~3.1) cannot be computed
>
>                  Electric Fields Calculation
>                  ik   1 ibnd   0 linter: root not converged 2.635E-07
>             ....
>             ..
>
>                  End of electric fields calculation
>
>                       Dielectric constant in cartesian axis
>
>             (******************  0.000244141      -0.001708984 )
>                       ( -0.000244141******************      0.000244141 )
>                       ( -0.000732422  0.000732422****************** )
>
>             And I am wondering if that system should be treated as a
>             metal instead (epsil    = .false.,) during the phonon run?
>
>             --
>             &inputph
>               prefix   = 'MgO',
>               epsil    = .true.,
>               alpha_mix(1) =0.4
>               alpha_mix(2) =0.4
>               fildyn   = 'MgO.dyn',
>               ldisp    = .true.
>               fildvscf = 'dvscf'
>               amass(1) = 24.30500
>               amass(2) = 15.99900
>               amass(3) = 55.84500
>               outdir='./',
>               nq1=6,
>               nq2=6,
>               nq3=6,
>               tr2_ph   =  1.0d-12,
>              /
>
>             Any hint is welcome!
>
>             Thanks in advance for your help and a nice Sunday everyone!
>
>             Chris
>             -- 
>             Postdoctoral Researcher
>             Center for Quantum Nanoscience, Institute for Basic Science
>             Ewha Womans University, Seoul, South Korea
>
>
>
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>
>
>
>     -- 
>     Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
>     Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
>     Phone +39-0432-558216, fax +39-0432-558222
>
>
>
>
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-- 
José C. Conesa
Instituto de Catálisis y Petroleoquímica, CSIC
Marie Curie 2, Cantoblanco
28049 Madrid, Spain
Tel. (+34)915854766



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