This discussion may be very misleading. The concept of frustation is very 
popular, but whatever geometrical frustation may exist in a fcc lattice, the 
magnetic structure of NiO is perfectly collinear (see for instance in the 
MAGNDATA database of the Bilbao crystallographic server: 
http://webbdcrista1.ehu.es/magndata/index.php?this_label=1.6 
<http://webbdcrista1.ehu.es/magndata/index.php?this_label=1.6>) , and each 
triangular plane of spins perpendicular to the <1,1,1>  has NO frustration, as 
the couplings within the planes are FM . Therefore the small structural 
structural strain can not be caused to get rid of the frustration.

In general, any magnetic ordering breaks the symmetry of the system, and 
structural distortions compatible with the new symmetry can happen due to 
magneto-structural coupling. Often this coupling is too weak to be detected, 
but in other cases like NiO is strong enough. In the case of NiO the magnetic 
order reduces the symmetry to monoclinic, and the observed structural 
distortion corresponds to this symmetry break.

best regards,

Manuel

> El 10/6/2016, a las 0:46, delamora <delam...@unam.mx> escribió:
> 
> No, because in NaCl you do not have magnetic moments that want to find order, 
> so there is no frustration
> In NiO the distortion is a way out of this frustration
> De: Wien <wien-boun...@zeus.theochem.tuwien.ac.at 
> <mailto:wien-boun...@zeus.theochem.tuwien.ac.at>> en nombre de Abderrahmane 
> Reggad <jazai...@gmail.com <mailto:jazai...@gmail.com>>
> Enviado: jueves, 9 de junio de 2016 05:26:15 p. m.
> Para: wien@zeus.theochem.tuwien.ac.at <mailto:wien@zeus.theochem.tuwien.ac.at>
> Asunto: Re: [Wien] Why the antiferromagnetic structure of Cr is still cubic
>  
> Thanks Dr Delamora for your reply 
> 
> Now I inderstand that and according to this raison "geometrical frustration" 
> , is there any structural distortion in the planes
>  (100) and (110) for the NaCl structure.
> 
> Best regards
> -- 
> Mr: A.Reggad                                           
> Laboratoire de Génie Physique
> Université Ibn Khaldoun - Tiaret
> Algerie
> 
> 
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**************************************************
J. Manuel Perez-Mato
Fac. Ciencia y Tecnologia,
Universidad del Pais Vasco, UPV
48080 BILBAO,
Spain

tel. +34 946012473
fax. +34 946013500
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