Question

Upon exposure to heat, the FeTi compound converts to FeO. Due to the temperature and time of heating, the compound ends up with 5 at.% of the Fe ions of
valence of +3. All the other Fe ions have a valence of +2. In order to accommodate the valence difference,
what defect would need to be produced? What is the non-stochiometric chemical formula of this compound?

Answer 1

Since the molecular formula of FeO indicates a 1:1 molar ratio, let us assume the sample of FeO initially had 100 Fe2+ ions and 100 O2- ions.

Since 5 % of Fe2+ ions get converted to Fe3+ ions, final number of Fe3+ ions = 5 % of 100 = 5.

Then the remaining number of Fe2+ ions must balance the total negative charge of 100 oxide ions to maintain electrical neutrality of the compound.

Suppose number of Fe2+ ions finally present is x.

Then , total positive charge of Fe ions = total negative charge of oxide ions

5(+3) + 2x = 2(100); do not consider negative sign of oxide ion charge

Hence 15 + 2x = 200

Or x = 92.5

Total number of Fe2+ and Fe3+ = 92.5 + 5 = 97.5

Hence the compound contains 97.5% of Fe ions and 100% of oxide ions.

Thus, the non-stoichiometric formula of the compound is Fe_0.975O

Since this compound contains less amount of the metal(iron), the defect introduced is metal deficiency defect.

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