[Wien] Optimization of an spinel structure
Peter Blaha
pblaha at theochem.tuwien.ac.at
Wed Nov 3 08:50:39 CET 2021
Technically, you seem to do everything correct.
When you split the 4 Fe into 2 sets, you break cubic symmetry. You also
see it from the resulting Sg #74.
Therefore there will be a relaxation ... (even without relaxation, you
already broke cubic symmetry).
Your approximation of an AFM ordering in this specific way, simply
breaks symmetry. And since this is a Kagome lattice, there will probably
be no simple (collinnear) way to introduce AFM without breaking symmetry.
Regards
Peter Blaha
Am 03.11.2021 um 04:40 schrieb delamora:
> Trying to answer your comments;
> SG227 is FCC, that is, the conventional cell has 4 primitive cells
> the first wyckoff positions are;
> 8a 1/8,1/8,1/8
> 8b 3/8,3/8,3/8
>
> 16c 0,0,0
> 16d 1/2,1/2,1/2
>
> 32e x,x,x
>
> 8a and 8b have 2 atoms in the primitive cell
> 16c and 16d have 4
> 32e has 8
>
> MnFe2O4 is a spinel
> Mn; 8a
> Fe; 16d
> O; 32e x=0.2639
> The primitive cell has 2 formula units while the conventional cel has
> 8 formula units
> then the atomic positions in MnFe2O4.struct are;
> F LATTICE,NONEQUIV.ATOMS: 3 227_Fd-3m
>
> 16.059466 16.059466 16.059466 90.000000 90.000000 90.000000
>
> ATOM 1: X=0.12500000 Y=0.12500000 Z=0.12500000
> ATOM 1:X= 0.87500000 Y=0.87500000 Z=0.87500000
> Mn NPT= 781 R0=0.00005000 RMT= 1.8600 Z: 25.000
>
> ATOM -2: X=0.50000000 Y=0.50000000 Z=0.50000000
> ATOM -2:X= 0.50000000 Y=0.75000000 Z=0.75000000
> ATOM -2:X= 0.75000000 Y=0.75000000 Z=0.50000000
> ATOM -2:X= 0.75000000 Y=0.50000000 Z=0.75000000
> Fe NPT= 781 R0=0.00005000 RMT= 2.1500 Z: 26.000
>
> ATOM -3: X=0.26390000 Y=0.26390000 Z=0.26390000
> ATOM -3:X= 0.73610000 Y=0.73610000 Z=0.73610000
> ATOM -3:X= 0.73610000 Y=0.51390000 Z=0.51390000
> ATOM -3:X= 0.26390000 Y=0.98610000 Z=0.98610000
> ATOM -3:X= 0.98610000 Y=0.98610000 Z=0.26390000
> ATOM -3:X= 0.51390000 Y=0.51390000 Z=0.73610000
> ATOM -3:X= 0.51390000 Y=0.73610000 Z=0.51390000
> ATOM -3:X= 0.98610000 Y=0.26390000 Z=0.98610000
> O NPT= 781 R0=0.00010000 RMT= 1.6000 Z: 8.000
>
> Now, if one wants to make it antiferro then one posibility is to take
> two Fe 'up' and two 'dn', and the system becomes;
>
> B LATTICE,NONEQUIV.ATOMS: 5 74_Imma
>
> 11.355757 11.355757 16.059466 90.000000 90.000000 90.000000
>
> ATOM 1: X=0.00000000 Y=0.25000000 Z=0.12500000
> ATOM 1:X= 0.00000000 Y=0.75000000 Z=0.87500000
> Mn NPT= 781 R0=0.00005000 RMT= 1.8600 Z: 25.000
>
> ATOM 2: X=0.25000000 Y=0.75000000 Z=0.25000000
> ATOM 2:X= 0.25000000 Y=0.25000000 Z=0.75000000
> Fe1 NPT= 781 R0=0.00005000 RMT= 2.1500 Z: 26.000
>
> ATOM 3: X=0.00000000 Y=0.52780000 Z=0.26390000
> ATOM 3:X= 0.00000000 Y=0.47220000 Z=0.73610000
> ATOM 3:X= 0.00000000 Y=0.02780000 Z=0.73610000
> ATOM 3:X= 0.00000000 Y=0.97220000 Z=0.26390000
> O 1 NPT= 781 R0=0.00010000 RMT= 1.6000 Z: 8.000
>
> ATOM 4: X=0.77780000 Y=0.75000000 Z=0.48610000
> ATOM 4:X= 0.22220000 Y=0.25000000 Z=0.51390000
> ATOM 4:X= 0.77780000 Y=0.25000000 Z=0.51390000
> ATOM 4:X= 0.22220000 Y=0.75000000 Z=0.48610000
> O 2 NPT= 781 R0=0.00010000 RMT= 1.6000 Z: 8.000
>
> ATOM 5: X=0.00000000 Y=0.00000000 Z=0.50000000
> ATOM 5:X= 0.00000000 Y=0.50000000 Z=0.50000000
> Fe2 NPT= 781 R0=0.00005000 RMT= 2.1500 Z: 26.000
>
> As it can be seen the 8 O atoms in 32e position are now in two groups
> of 4 O atoms
> and as I mentioned before the 'runsp -min' will move them in a way
> that it would break the cubic FCC crystal symmetry.
> The problem is that Fe in this system has the kagome arangement and
> antiferro ordering is just a simplification, but the system should be
> cubic with the SG 227.
>
> Saludos
>
> Pablo
> ------------------------------------------------------------------------
> *De:* Wien <wien-bounces at zeus.theochem.tuwien.ac.at> en nombre de
> Fecher, Gerhard <fecher at uni-mainz.de>
> *Enviado:* martes, 2 de noviembre de 2021 05:07 a. m.
> *Para:* A Mailing list for WIEN2k users <wien at zeus.theochem.tuwien.ac.at>
> *Asunto:* Re: [Wien] Optimization of an spinel structure
> Sorry,
> it depends on the setting for space group 227 whether X=1/4 results in
> a 8- or 32-fold degenerate position, I did not check which one Wien2k
> uses (unfortunately one can not use crystallographic Wykoff positions
> directly).
> if you have already the 32-fold degenerate site (I guess wien2k
> reports a smaller cell with 8 sites) then min should find the correct
> positions by minimizing the forces.
>
> The remarks on the magnetic order are not touched by that.
>
> Ciao
> Gerhard
>
> DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy:
> "I think the problem, to be quite honest with you,
> is that you have never actually known what the question is."
>
> ====================================
> Dr. Gerhard H. Fecher
> Institut of Physics
> Johannes Gutenberg - University
> 55099 Mainz
> ________________________________________
> Von: Wien [wien-bounces at zeus.theochem.tuwien.ac.at] im Auftrag von
> delamora [delamora at unam.mx]
> Gesendet: Dienstag, 2. November 2021 04:49
> An: wien at zeus.theochem.tuwien.ac.at
> Betreff: [Wien] Optimization of an spinel structure
>
> Dear WIEN community,
> I am trying to optimize a spinel structure.
> The normal spinel AB2X4 is FCC with SG 227
> A: 1/8,1/8,1/8
> B: 1/2,1/2,1/2
> X: x,x,x x~1/4
> so it has 2 free parameters; 'volume' and 'x,x,x'
> but in my cases B is magnetic and it has antiferromagnetic coupling, B
> also forms the 3D Kagome structure so it is geometrically frustrated.
> The volume optimization can be easily done with option 'optimize'
> but for the 'x' parameter there is a problem;
> To treat the magnetism the best option, in my opinion, is to put half
> of the B atoms 'up' and the other half 'dn', there are 4 in the
> primitive cell, so 2 are 'up' and 2 'dn'.
> With this, then the FCC cell becomes BCC (SG 74 Imma) and the 8 'X'
> atoms that were in one wyckoff position are now in two wyckoff
> positions with 4 atoms in each.
> So, if the forces are minimized, 'runsp -min', will move these two 'X'
> groups in directions that will break the original cubic symmetry.
> One posibility is to make 4 'struct' files, in the original SG 227,
> with slightly different 'x' positions, for example, the initial x,x,x is;
> x= 0.254, so I make 4 struct files with; 0.251, 0.253, 0.255, 0.257
> after this one should make them antiferromagnetic (SG 74 Imma) then
> run them separatelly, then one can obtain the optimum 'x' parameter.
>
> I tried to use a modified 'optimize.job' but it did not work.
> I also tried to run these 4 'struct' files in the same directory but I
> found that when I put the next case.struct into the directory, I had
> to initiate the system in order to be able to run them.
>
> In other words, if I run the system with 1-struct, then for the next
> 2-struct I cannot just change 1-struct for 2-struct and run.
> For the case of changes of volume then the optimize.job works fine,
> but for small changes in 'x' it does not work.
>
> I anybody has an idea that would make this case simpler I would apreciate.
> Thank you anyway
>
> Pablo
>
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--
-----------------------------------------------------------------------
Peter Blaha, Inst. f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-158801165300
Email: peter.blaha at tuwien.ac.at
WWW: http://www.imc.tuwien.ac.at WIEN2k: http://www.wien2k.at
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