Magnetite Formula unit

The permanent moments of the cations in the octahedral and tetrahedral sites are antiparallel. Thus, the moments of the trivalent cations (Fe +) cancel out, and the net moment is due to the divalent cations. If the individual divalent cation moments are known, then it is possible to calculate the net moment per formula unit (see Cooperative Learning Exercise 6.7). In a formula unit of magnetite, there is one divalent iron ion that has a total moment of 4/tb resulting from four unpaired electrons, which is very close to the observed value. Similarly, in NiFc204, the moment per formula unit is 2/j.b identical to that of the divalent nickel ion. 

The solid solution (ss) on the left has been represented as a mixture consisting of 1 — X moles of magnetite and x moles of zinc ferrite per formula unit of ss. It is assumed that the zinc ferrite moiety remains passive while any changes relating to the oxygen/cation composition occur exclusively in the magnetite component. 

Magnetite structure is an inverse spinel with a face center cubic unit based on 32 O2- ions with a regularly cubic dose packed along the [111] direction. There are eight formula units per unit cell. Magnetite differs from other iron oxides in that it contains both divalent and trivalent iron. Its formula is written as Fe > [Fe Fe in]04and the brackets denote octahedral sites, tetrahedral Fe spans are directed antiparallel to octahedral Fe 3+ and Fe 2+ spins so... 

In the case of magnetite, Fe[Fe2]04, insertion of one Li per formula unit produces an ordered rock-salt array with Fe remaining on aU the 16d sites and Li and Fe randomly distributed over the 16c sites. Further insertion of Li extrudes Fe reversibly... 

Magnetite is an inverse spinel, with iron atoms occupying interstitial sites in the close-packed arrangement of oxygen atoms. The tetrahedral sites (A) are occupied by ferric cations, and the octahedral sites (B) half by ferric and half by ferrous cations. In the unit cell there are twice as many B-sites as A-sites. The structural formula can therefore be written (Fe(lII)),c,[Fe(ll)Fe(III)],c., 04. There is little distortion in the cubic close packing of the oxygens, so that the structure is close to ideal. 

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