Structures of Oxides of Iron

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Iron forms three different types of oxides, which are FeO, Fe₂O₃ and Fe₃O₄. These are non-stoichiometric (the ratio of cations and anions may vary from that given in the formula) and are easily oxidized or reduced into each other. These aspects can be explained on the basis of their crystal structure.

Structure of FeO

O^2- form a cubic close packing. All the octahedral sites are not occupied by Fe²⁺, because a small number of Fe²⁺ are replaced by Fe³⁺ ions.

For every three Fe²⁺ replaced, there are two Fe³⁺ ions and one octahedral site is vacant. Electrical neutrality is maintained. Therefore this oxide is non-stoichiometric with the formula Fe_O.95O called wustite.

Structure of Fe₂O₃

If all the Fe²⁺ ions of FeO are replaced by Fe^3+. For every three Fe²⁺ there will be two Fe³⁺ to maintain electrical neutrality. The ratio of Fe to O is 2:3 and no longer be 1:1. Hence, we get the oxide Fe₂O₃.

Structure of Fe₃O₄ (Magnetite)

In the FeO structure two thirds of Fe²⁺ ions (in the structure of FeO) are replaced by Fe³⁺ ions.

The remaining Fe²⁺ ions and 50% of the Fe³⁺ ions occupy the octahedral voids. The other 50% of the Fe³⁺ ions occupy the tetrahedral voids.

Ferrites

They are compounds having the general formula AFe₂O₄ where A is a divalent cation like Mg²⁺ or Zn²⁺. They are obtained from Fe₃O₄ by replacing Fe²⁺ by divalent cations. These belong to a category with a general formula AB₂O₄ where A is a divalent cation and B is a trivalent cation.

Examples: B³⁺, Fe³⁺ or Al³⁺.

In these compounds divalent cations occupy tetrahedral voids where as the trivalent cations occupy octahedral sites. This is called the spinel structure.

Examples: Mg₂Al₂O₄

Ferrites are used in telephone as well as in memory loops of computers.

Inverse Spinel Structure

Divalent ions are in the octahedral voids while the trivalent ions are distributed equally between tetrahedral and octahedral voids.

Example: Fe₃ O₄

Fe²⁺ are in the octahedral voids while Fe³⁺ are equally distributed between tetrahedral and octahedral voids.

Points to Remember

• Number of octahedral voids = Number of atoms present in the close packed arrangement

• Number of tetrahedral voids

= 2 x Number of octahedral voids

= 2 x Number of atoms.