20
1 – Description of ionic crystals
Exercise 1.7 – The non-stoichiometry of iron monoxide
Iron monoxide is a non-stoichiometric iron-deficient compound. A chemical
analysis of a sample of iron monoxide gives a molar fraction of iron of 0.4767.
1. Give two formulas for the compound susceptible to account for this non-stoichiometry and comment on them with respect to a perfect FeO crystal.
2. Iron monoxide crystallizes in the cubic NaCl-type structure shown in figure 4.
Figure 4 – Representation
of NaCl-type structure of iron monoxide.
Table 2 gives the lattice parameter a and the density ρ of the sample analyzed.
Table 2 – Results of the analysis
of a sample of iron monoxide.
What is the formula for this oxide?
Data
Molar masses: M(Fe) = 55.85 g mol
−1
 M(O) = 16 g mol
−1
3. Write the equilibrium reaction involving this oxide and oxygen in the ambient atmosphere.
4. Explain the presence of the trivalent cation Fe
3+
in the monoxide under
study.
5. Write the formula for non-stoichiometric iron monoxide Fe 1−α O in the form
Fe
2+
x Fe
3+
y V z O, with x, y, and z expressed as functions of α. V denotes an iron
vacancy.
)H
2
<
a A c
6 @
4.371
ρ [g cm
−3
]
5.312