[Wien] Help me to solve #194 SG perovskites structure in AFM phase calculation problem

Enamul Haque Mon, 20 Aug 2018 05:55:42 -0700

Dear Wien2k Users and Experts,

We already got sufficient help from Dr. Gavin in this case but we are not 
convinced from the space group point of view.
With the original CIF file (attached), we want to check the ground state energy 
of the AFM case of "Cs3Fe2Cl9" (conventional standard and symmetrized taken 
from MP).
The original structure crystallizes in hexagonal space group (194) with one Fe 
atom having multiplicity 4. 

So to do AFM calculations, we need four Fe atoms (udud configuration is 
reported as a stable structure in AFM case).
We adopted two different ways to make four equivalent Fe atoms (to make udud 
AFM configuration) and in both the ways our space group adopts different 
geometry (SG) which does not fall within the hexagonal family.
Original SG:

H   LATTICE,NONEQUIV.ATOMS:  5 194 P63/mmc
MODE OF CALC=RELA unit=bohr
 14.141194 14.141194 34.315650 90.000000 90.000000120.000000
ATOM   1: X=0.66666733 Y=0.33333267 Z=0.41670700

Labeling all Fe (1-4) in the file we get from cif2struct from conventional 
standard cif file:

blebleble
CXZ LATTICE,NONEQUIV.ATOMS: 22 8 Cm
MODE OF CALC=RELA unit=bohr
 24.493266 42.160217 14.141194 90.000000 90.000000125.517869
ATOM   1: X=0.26213500 Y=0.92880100 Z=0.00000000

>From the supercell approach (from the symmetrized cif file after rounding off 
>for two positions of Cl, ending with 50 and 800):

 
 1. initialize (it should be in 194 SG) 2. x supercell (1x1x1 with no shift) 3. 
cp case_super.struct case.struct 4. open in w2web and split one Fe and save it 
and it will redirect you again on w2web 5. hit edit structure option and again 
click on second Fe to split and split all four Fe atoms 6. again click on edit 
structure and put 1-4 on all four Fe atoms (in the second column) and remove 
original Fe atom (you see now five Fe atoms four that you have split and one 
will still on original position. So delete the original one). 
It gave us (attached case.struct)

H   LATTICE,NONEQUIV.ATOMS: 16 156 P3m1
MODE OF CALC=RELA unit=bohr
 14.141194 14.141194 34.315650 90.000000 90.000000120.000000


Now the issue we are facing is:The pristine structure is having SG 194 and 
after converting multiplicity of Fe atom to single atomic positions by two 
different approaches, we are getting two different SGs which do not fall within 
the hexagonal family.

So, Could someone from the expert list please advice us how to do AFM 
calculations of this kind cases and which SG should be appropriate.It will be a 
great help.




Best regards.
Enamul HaqueMawlana Bhashani Science and Technology UniversityTangail, 
Bangladesh.Email: enamul@mailaps.orgCell: +8801831911133

Cs3Fe2Cl9.cif
Description: Binary data
Cs3Fe2Cl9.struct
Description: Binary data

______________________

Re: [Wien] Help me to solve #194 SG perovskites structure in AFM phase calculation problem

Peter Blaha Thu, 23 Aug 2018 00:31:53 -0700

From the files you sent, I never get Cm space group.

Anyway, if it is antiferromagnetic, it has only TWO Fe sites, one with spin-up and one with dn.

So I used your struct file, but the 4 Fe atoms should probably not be splitted into 1-4, but into two positions, each with multiplicity 2, such that you get the desired udud spin-pattern.

sgroup gives then P63mc.

PS: You have to distinguish between a hexagonal bravais lattice (aac,90,90,120) and a hexagonal space group. If you have a hexagonal lattice, but put atoms in in an arbitrary way, it can easily even go to P1. So the lattice is still hexagonal, but the SG does not belong to a hexagonal SG, since the required symm.ops of a hex.SG are gone.

Reference: https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg17873.html