[Wien] Optimization of an spinel structure

Wed Nov 3 04:40:33 CET 2021

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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