I guess you are talking about the conventional classification of AFM
ordering used in neutron diffraction. Consider AFM as a standing wave of
spin orientations. Different Types of AFM are then distinguished by the
wave vector of the standing wave in the crystal lattice.
AFM-I is the AFM order with the shortest possible wavelength, that is
moments on nearest neighbor planes are antiparallel. In an fcc structure
the nearest neighbors of an atom at (0,0,0) are on the two planes
'above' and 'below' along the space diagonal of the cube at positions
(1/2,1/2,0) and so on. The wave vector of the standing wave describing
spin orientation is perpendicular to these planes of parallel spins. The
length of the wave vector q in reciprocal space is such that going the
distance d to the next plane with parallel moments (half the space
diagonal) in that direction result in an identical situation in the wave
function cos(qx), that is q*d=2*pi.
For AFM-II moments on planes with next nearest neighbors are
antiparallel. And so on. The longer the wavelength of the standing wave
(or the shorter q in reciprocal space) the more unit cells in the
crystal lattice you will need to represent the AFM structure (depending
on the distance between lattice planes in your structure without AFM
order).
Good luck with figuring out the directions and lengths of wave vectors
in your structure yourself,
Martin Pieper
---
Dr. Martin Pieper
Karl-Franzens University
Institute of Physics
Universitätsplatz 5
A-8010 Graz
Austria
Tel.: +43-(0)316-380-8564
Am 2020-01-26 21:01, schrieb djamel slamnia:
THANKS AGAIN SIR
I NEED TO KNOW WHAT IS THE AFM ORDERS FOR P3M1 (156) ??? TYPE II OR
III
THANKS IN ADVANCE
Le dimanche 26 janvier 2020 à 20:51:41 UTC+1, Gavin Abo
<[email protected]> a écrit :
THE DEFINITION FOR THE COMPOUND A=B = 3.74 A ALPHA = BETA = 90
GAMMA = 120
for AFM type I : i creat superstructure x super cell target
lattice H : x =1, y = 1 , Z =2 then x sgroup, program define
automatically the space group the same of my original space group
156 without warrning
As you have described above (for Z=2), your attempt at creating a
supercell has failed as "x sgroup" collapsed the supercell structure
back to the non-supercell structure.
As mentioned on the FAQ page for supercell construction, you need to
displace an atom, change an atom, or use a special label:
http://susi.theochem.tuwien.ac.at/reg_user/faq/supercells.html
In order to keep the supercell without "x sgroup" reducing it back to
the original structure, refer to previous posts in the mailing list
archive about breaking the symmetry. A few of the many posts about
that as examples are at the three links below:
https://www.mail-archive.com/[email protected]/msg18380.html
https://www.mail-archive.com/[email protected]/msg01866.html
https://www.mail-archive.com/[email protected]/msg15517.html
but when try to do it for type II : x super cell P
: x =1, y = 1 , Z =1 x sgroup could not define the space group
In section "3.12 Setting up a new case" on page 29 in the WIEN2k 19.1
usersguide [
http://susi.theochem.tuwien.ac.at/reg_user/textbooks/usersguide.pdf ],
there is the statement:
"Alternatively with the new StructGen you can specify the spacegroup
and only the inequivalent positions. The equivalent ones will be
generated automatically."
This means spacegroups in WIEN2k are defined according to the
inequivalent positions and not by the equivalent positions.
For the case above (Z=1), it is likely that "x supercell" found some
equivalent positions in the original structure and automatically added
special labels to them changing them into inequivalent positions.
Thus, a supercell structure was successfully created. If you want the
supercell structure to reduce back to the original structure, you
would likely just need to remove all or some of the special labels in
StructGen before running "x sgroup".
For understanding the inequivalent and equivalent positions with
WIEN2k spacegroups, the example in the post at the following link
might helpful:
http://zeus.theochem.tuwien.ac.at/pipermail/wien/2013-January/018171.html
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