An Inverse Spinel - NiFe2O4
Spinel is the name given to the mineral MgAl2O4. It has a common structural arrangement shared by many oxides of the transition metals with formula AB2O4.
In the normal pattern the oxygens form a cubic close packed (face centred) array and the Mg(II) and Al(III) sit in tetrahedral (1/8 occupied) and octahedral (1/2 occupied) sites in the lattice, giving a Unit Cell with 8 Mg's, 16 Al's and 32 O's.
An inverse spinel is an alternative arrangement where half of the trivalent ions swap with the divalent ions so that the M(II) now occupy octahedral sites ie B(AB)O4. In this case Ni(II) is octahedral and half of the Fe(III) are tetrahedral.
Highlight the ccp (ABCABC) layers of O's.
Show 1st layer of O's Show 2nd layer of O's
Show 3rd layer of O's Show 4th layer of O's
Show layers 3 and 4 Show layers 2 and 5
Highlight a tet. Fe site Oct. Ni, Fe site
Highlight an O site Ball and stick

<a href="invspinel.pdb"></a>

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The display shows an idealised unit cell (Space group 216 F-43m). The cubic cell lengths used were a=b=c=8.0858 A.

One method of predicting whether a structure will be normal or inverse is to calculate Octahedral Site Preference Energies (OSPE) for both the A(II) and B(III). The ion with the more stable energy is predicted to fit into the octahedral site.
However, see J. Am. Chem. Soc., 1982, 104, 92-95 for a different method that does NOT involve OSPE calculations.

In this case, Ni(II) has a high OSPE whilst Fe(III) has zero OSPE so that it would be predicted that Ni(II) would have a stronger preference for the octahedral sites.
For alternate views of spinels,
pictures of spinel layers

Information is available to show how the individual structural pages were created.
Return to the structure of binary salts introductory page.
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Copyright © 2005 by Robert John Lancashire, all rights reserved.

Created and maintained by Prof. Robert J. Lancashire,
The Department of Chemistry, University of the West Indies,
Mona Campus, Kingston 7, Jamaica.
Created March 2000. Links checked and/or last modified 2nd November 2005.
URL http://wwwchem.uwimona.edu.jm/courses/invspinel.html