Composition and structure of ferrites

Ferrites are mixed metal oxides with general chemical composition MO.Fe2O3 where M is a divalent metal such as Fe, Mn, Mg, Ni, Zn, Cd, Co, Cu, Al or a mixture of these. Three different crystal types exist for ferrites and accordingly they are classified into three groups. They are
1. Spinel type with cubic crystal structure and general formula M2+Fe2O4 where M = Fe, Mn, Mg, Ni, Zn, Cd, Co, Cu, Al or a mixture of these.
2. Garnet type with cubic crystal structure and general formula Ln33+Fe5O12 where Ln = Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, or Lu.
3. Magnetoplumbite type with hexagonal crystal structure and general formula M2+ Fe12 O19 where M = Ba or Sr.
The structural, magnetic and electrical properties of these compounds are governed critically by their chemical composition. Hence preparation of these ferrites with specific properties has gained much importance. Ferrites can be prepared by conventional solid-state method and numerous chemical routes, such as reverse micelle synthesis, coprecipitation, thermal decomposition, solgel and aerogel process.

Spinel ferrites crystallize into the spinel structure, which is named after the mineral spinel, MgAl2O4. Primarily the oxygen ions lattice determines the spinel crystal structure. The radii of the oxygen ions are several times larger than the radii of the metallic ions in the compound. Consequently, the crystal structure can be thought of as being made up of the closest possible packing of layers of oxygen ions, with the metallic ions fit in at the interstices A and B.

A metallic ion located at the A site has four nearest oxygen ion neighbors in tetrahedral coordination. The metallic ion, which is situated at site B, has six nearest oxygen ion neighbors in octahedral coordination.
Thus the spinel ferrites have the general formula {(M) d(Fe) 1-d}[(M) 1-d(Fe) 1+d] O4. The divalent metal ion M (eg:- Zn, Mg, Mn, Fe, Co, Ni or a mixture of them) can occupy either tetrahedral(A) or octahedral(B) sites as depicted by curled and square brackets respectively. In the above formula when d =1,it is called normal spinel. ZnFe2O4 and CdFe2O4 are examples of normal ferrites. When d =0, it is called inverse spinel and examples are NiFe2O4, CoFe2O4, CuFe2O4 etc. When d =1/3, it is called a random spinel.