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Hello,

I'm trying to do a structure relaxation of a bilayer (Fe/FePt) structure, however, I'm not sure how to arrange my POSCAR because bulk Fe and bulk FePt (obviously) have different lattice constants. At present I'm using Cartesian coordinates ("squashing" the Fe ions in the Fe layer somewhat by fitting it all in an FePt-shaped unit cell!). As an example, for two layers of FePt on top of one layer Fe, my POSCAR presently looks like:

Fe-FePt Bilayers
1.0
3.852 0.0 0.0
0.0 3.852 0.0
0.0 0.0 10.296
4 8
Selective dynamics
Cartesian
1.926 0.0 1.856 T T T #Pt A of FePt layer 1
0.0 1.926 1.856 T T T #Pt B of FePt layer 1
1.926 0.0 5.569 T T T #Pt A of FePt layer 2
0.0 1.926 5.569 T T T #Pt B of FePt layer 2
0.0 0.0 0 T T T #Fe A of FePt layer 1
1.926 1.926 0 T T T #Fe B of FePt layer 1
0.0 0.0 3.713 T T T #Fe A of FePt layer 2
1.926 1.926 3.713 T T T #Fe B of FePt layer 2
0.0 0.0 7.426 T T T #Fe A of Fe layer 1
2.030 2.030 7.426 T T T #Fe D of Fe layer 1
2.030 0.0 8.859 T T T #Fe B of Fe layer 1
0.0 2.030 8.859 T T T #Fe C of Fe layer 1



Is there any way of writing the POSCAR such that the input (not-yet-relaxed) unit cells have different sizes in the Fe and FePt layers? Or is there any other clever way of doing this?

Cheers,
Cecilia

Hello Cecilia,

there is no easy way. It looks like you are after interfaces between FePt and Fe, right?

Some hints:
- you may check literature for already published, low energy interfaces for these systems
- the important information you are looking for is which Fe plane fits best with what FePt plane, e.g. Fe(111) on FePt(210) or so
- the way to figure that out for your own is to measure different distances in, e.g., an Fe bulk and try to find a best fit plane in FePt.
- there was a semiautomatic tool in Accelrys Cerius2 (Interface builder), but I haven't used that.
- btw: I'm not sure if selective dynamics & cartesian coordinates work together. check your output carefully

Hth,

alex

apart from alex's suggetstions please keep in mind:
there will always be some internal stress in layered structures if there is a lattice mismatch of the components (please have eg a look at the papers on epitaxy publised by Alex Zunger's group