Spinels, normal and inverse structure of spinels.

 Spinels are mixed oxides having the general formula AB2O4 or we can break it as AOB2O3 where A is a di-positive metal cation and B is a tri-positive metal cation. in these mixed oxides, oxide ions form a fcc arrangement, as A stays in octahedral void and B remains in the tetrahedral void. This is called normal spinel structure.

In  contrast, if A goes to tetrahedral void and half of B goes to octa hedral void then that mixed oxide will be called as inverse spinel. That means half of B will be in octahedral void and other half of  B will be in tetrahedal void.

In a spinel both A and B can be same or different. for example : Mn3O4 and ZrCr2O4.

How to determine whether the spinel will be normal or inverse spinel?

It is all about stability of the ions in corresponding voids, that means if the di-positive ion A has more CFSE in octahedral field then it will stay in octahedral void as a result the spinel will be an inverse spinel and if  B has more CFSE in octahedral field  then it will stay in octahedral void and the spinel will be normal spinel.

for example.

from the above example the Co2+ has lower cfse than Co3+ in octahedral field. As a result Co3+ will stay in octahedral void and the above mixed oxide will be normal spinel.

How lattice energy affects the general trend?

There are certain examples where the structure of the mixed oxides is different from what we predict. In these cases lattice energy plays a major role.

1. MAl2O4 

    Here Al3+ has too high lattice energy, that stabilizes it in the octahedral void and the spinel is normal. But if M=Ni2+ then due to very high lattice energy and high CFSE of Ni2+ it is stabilized in octahedral void and the spinel is inverse.

2. MFe2O4 where M=Mg and Mn

    Due to high lattice energy of M2+ these mixed oxides are inverse i.e opposite to prediction.