[Wien] Optimization of an spinel structure

[Laurence Marks]

Basically....no!

If you have constructed the AFM correctly the structure will have the
symmetry of the Coulomb field, not the magnetism. You met have made a
mistake.

--
Professor Laurence Marks
Department of Materials Science and Engineering, Northwestern University
www.numis.northwestern.edu
"Research is to see what everybody else has seen, and to think what nobody
else has thought" Albert Szent-Györgyi

On Mon, Nov 1, 2021, 10:50 PM delamora <delamora at unam.mx> wrote:

> 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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