In NiO the Ni makes an FCC structure, you cannot have an antiferromagnetic 
arrangement in a triangular lattice (for example in 1,1,1 planes), even less in 
the FCC, which is triangular in many planes, so there is what is called 
"geometrical frustration"

On the other hand, in Cr you have the "Cr up" and "Cr dn" alternated and you 
can have a clear antiferromagnetic arrangement

Pablo
________________________________
De: Wien <wien-boun...@zeus.theochem.tuwien.ac.at> en nombre de Abderrahmane 
Reggad <jazai...@gmail.com>
Enviado: miércoles, 8 de junio de 2016 05:43 a. m.
Para: wien@zeus.theochem.tuwien.ac.at
Asunto: [Wien] Why the antiferromagnetic structure of Cr is still cubic


Dear wien2k users

I have read that the antiferromagnetic ordering is accompanied with a little 
structural distortion as in the case of NiO. This latter is characterized by a 
distortion from NaCl to rhomboherdral structure; but for the chromide Cr the 
structure is still cubic when there is a magnetic transition from paramagnetic 
to antiferromagnetic state.

Best regards
--
Mr: A.Reggad
Laboratoire de Génie Physique
Université Ibn Khaldoun - Tiaret
Algerie


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