Question

Suppose that some of the iron atoms in the ceramic FeO are oxidized from Fe 2+ to Fe 3+.

a) Would the net charge of the crystal be positive or negative?

b) Explain why each of the following defects would or would not be reasonable to regain charge balance:
O 2- vacancies
O 2- interstitials
Fe 2+ vacancies 
Fe 2+ interstitials

c) what ratio of these defects above to Fe 3+ is needed for charge neutrality ?

d) Ionic materials such as this are often reffered to as non stoichiometric compounds and have chemical formuals written in the manner Fe(1-x)O. if the overall formula of an iron oxide is Fe 0.95 O what percentage of cation sites are Fe 2+, Fe 3+, and vacancies?

Answer 1

Some of the iron atoms in the ceramic FeO are oxidized from Fe 2+ to Fe 3+.

a) The net charge of the crystal would be positive or negative as Fe 3+ ions have greater positive charge than Fe 2+ ions.

b) Each of the following defects would not be reasonable to regain charge balance:
O 2- vacancies : The number of positive charges is greater than the number of negative charges. By removing some negative charges, this charge discrepancy is only going to increase.
O 2- interstitials: This will not alter the ratio of positive and negative charges. The number of oxide ions does not change. Only their position changes.
Fe 2+ interstitials This will not alter the ratio of positive and negative charges. The number of ferrous ions does not change. Only their position changes.

The following defect would be reasonable to regain charge balance:

Fe 2+ vacancies

The net charge of the crystal is positive. It means total number of positive charges is greater than the total number of negative charges. When some Fe 2+ ions are missing, some positive charges are missing. This will nullify the effect of excess positive charge.

c) For Fe 2+ vacancies, two iron atoms that are oxidized from Fe 2+ to Fe 3+ should be accompanied by vacancies of one Fe 2+ ion

d)

For 100 formula units, Fe 2+ cations are missing.

cation sites are Fe 3 +

cation sites are Fe 2+

Hence, 85%, 10% and 5% of cation sites are Fe 2+, Fe 3+, and vacancies respectively.