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

a) the net charge will be Zero. The crystal will be in neutral state only. Neither +ve or -ve

b) This is an example for metal defficient defects in crystal. Due to the vaccancy created by Fe2+, oxidation state o some o Fe2+ changes to Fe3+.

O2- vacency not possible as the size and charge of anion ; it is not the good case for escaping from the lattice.

O2- occupiening in interstetial state also not possible because all the octaedral spaces were already occupied y the oxygen atoms and the network in Fe-O-Fe will not be able to form. And   anions size is not in good agrimment with interstitial site, so they can not occupy interstitial site.

Fe2+ defficiency can be seen in this type of metla oxides. The small size cation can be escape while heating from the lattice.

Fe2+ is avoided in the interstitial spaces. As we know that, the oxidation change of Fe can change from 2+ to 3+. The change can be ade easily without changing the position of ions. And the presence of Fe2+ in interstitial site distrube the coordination no of anion, which may lead to unstablity to the crystal.

c) The ain objective of change in the charge from Fe2+ to Fe3+ is to maintain neutrality. SO if 1 molecule of Fe2+ miss from the lattice, 2 Fe2+ ions convert to 2 Fe3+ ions. So it will be 1: ratio.

d) Fe(1-x)O

that means 0.0X % of Fe2+ is missing. In order to maintain neutrality, 2*0.0X Fe2+ convert to Fe3+ ions.

So total no of Fe 3+ is 0.02X.

Fe_0.95O

that means 0.05 % o Fe2+ is missing. In order to maintain neutrality, 2*0.05 Fe2+ convert to Fe3+ ions.

So total no of Fe 3+ is 0.1.

Fe3+ % is = (0. 1/0.95)*100 = 1.06%

ramianing 98.94 % will be Fe2+.

No of Fe2+ missing = no of vcancies = 0.05 %% of vacancies