This problem is quite interesting; You have Ni in a FCC arrangement, so you have Ni tetrahedra, so how can you place them in an antiferromangnetic order???
It is frustratng!!! Well this is known as geometric frustration, which is a very interesting field!!! On the opposite stance immagine a simple Ni crystal with an antiferromagnetic order, there is a simple answer; alternating spins in the axis, like the NaCl crystal: Na represents up, Cl dn; Now, the picture that you sent the crystal has Ni atoms alternating in spin in the 111 planes; one plane up, next dn. In each plane the Ni are ordered ferromagnetic!!! This is not the correct structure, but it is good for a approximate calculation. To reproduce this structure you start by defining the structure as shown in your picture; You make a crystal with 2 atoms, cubic "F" with Na at 0,0,0 and O at 1/2,0,0. The conventional structure looks like the one that you sent, but the primitive one is simpler. The problem is that when you try to put one Ni plane up and the next dn then the Ni at 000 is up and the one at 111 is dn, then you have to double the structure in the 111 direction: You make the cell with a=b=c=4.17A, alfa=beta=gamma=89.999 (not 90, but close) and put Ni 000 0,1/2,1/2 1/2,0,1/2 1/2,1/2,0 O 1/2,0,0 0,1/2,0 0,0,1/2 1/2,1/2,1/2 with sgroup it will convert it to rhombohedral which with supercell you can now convert it hexagonal and you can expand in the 001 direction (wich in the original cell was 111) ________________________________ De: [email protected] <[email protected]> en nombre de Abed Reg <[email protected]> Enviado: domingo, 30 de noviembre de 2014 04:05 p.m. Para: [email protected] Asunto: [Wien] What's the crystal structure of NaCl in antiferromagnetic calculation Hello again How to reproduce this structure of NiO if we consider the two atoms of Ni (up and dn ) as inequivalent. Here is the structure NiO structure<http://4.bp.blogspot.com/-EzItbno5RvU/VHi-n_0yT5I/AAAAAAAAAJk/Hyf7Qhg_3ao/s1600/Antiferro-NiO.png> -- Mr: A.Reggad Laboratoire de Génie Physique Université Ibn Khaldoun - Tiaret Algerie
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