[Wien] Why the antiferromagnetic structure of Cr is still cubic

[Juan Manuel Perez Mato]

In order to know the symmetry of a structure you do not have to look at the unit cell, but to the symmetry group of the structure. You can have a triclinic structure with a cubic unit cell. Just look at the symmetry group that is indicated in the database, and you will see that its point-group is monoclinic.

The monoclinic magnetic structure of NiO in the database comes from an experimental report where any deviation of the positional structure  and lattice from the parent cubic was considered negligible. As in general the magnetic induced structural distortions are very weak one has to make a specific effort to detect them and to characterize them experimentally (and I suppose also in calculations as explained by Stefaan Cottenier). A rombohedral strain is just a less general strain than a monoclinic one. If the strain is very weak, a first appoximation to it can be a rombohedral strain, which has only one free parameter to determine, instead of the three that requires the monoclinic strain.

best regards,

Manuel


> El 10/6/2016, a las 12:48, Abderrahmane Reggad <jazairdz at gmail.com> escribió:
> 
> 
> Thanks Mr Khan Manuel for your contribution to enrich the subject of geometrical frustration and magnetism .
> 
> After reading your contribution, I made the following remarks:
> 
> * you have said:In the case of NiO the magnetic order reduces the symmetry to monoclinic, but what we know all is that the magnetic order reduces the symmetry in NiO from cubic to rhombohedral.
> 
> * From theMAGNDATA database, It's mentionned that the magnetic unit cell is cubic 
> 
> Lattice parameters of the magnetic unit cell:
> 8.3500 8.3500 8.3500 90.0000 90.0000 90.0000
> 
> Whould you explain that 
> 
> Best regards
> Mr: A.Reggad                                           
> Laboratoire de Génie Physique
> Université Ibn Khaldoun - Tiaret
> Algerie
> 
> 
> _______________________________________________
> Wien mailing list
> Wien at zeus.theochem.tuwien.ac.at
> http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
> SEARCH the MAILING-LIST at:  http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html

**************************************************
J. Manuel Perez-Mato
Fac. Ciencia y Tecnologia,
Universidad del Pais Vasco, UPV
48080 BILBAO,
Spain

tel. +34 946012473
fax. +34 946013500
***************************************************

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://zeus.theochem.tuwien.ac.at/pipermail/wien/attachments/20160610/0bf8eea9/attachment.html>
-------------- next part --------------
_______________________________________________
Wien mailing list
Wien at zeus.theochem.tuwien.ac.at
http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
SEARCH the MAILING-LIST at:  http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html