Callister - An introduction - 8th edition

12.5 Imperfections in Ceramics • 473
Figure 12.21 Schematic diagram
showing Frenkel and Schottky defects
in ionic solids. (From W. G. Moffatt,
G. W. Pearsall, and J. Wulff, The
Structure and Properties of Materials,
Vol. I, Structure, p. 78. Copyright ©
1964 by John Wiley & Sons, New York.
Reprinted by permission of John Wiley
& Sons, Inc.)
Schottky defect
Frenkel defect
1:1. A ceramic compound is nonstoichiometric if there is any deviation from this
exact ratio.
Nonstoichiometry may occur for some ceramic materials in which two valence
(or ionic) states exist for one of the ion types. Iron oxide (wüstite, FeO) is one such
material, because the iron can be present in both Fe 2 and Fe 3 states; the number
of each of these ion types depends on temperature and the ambient oxygen pressure.
The formation of an Fe 3 ion disrupts the electroneutrality of the crystal by
introducing an excess 1 charge, which must be offset by some type of defect. This
may be accomplished by the formation of one Fe 2 vacancy (or the removal of two
positive charges) for every two Fe 3 ions that are formed (Figure 12.22). The crystal
is no longer stoichiometric because there is one more O ion than Fe ion; however,
the crystal remains electrically neutral. This phenomenon is fairly common in
iron oxide, and, in fact, its chemical formula is often written as Fe 1–x O (where x is
some small and variable fraction substantially less than unity) to indicate a condition
of nonstoichiometry with a deficiency of Fe.
Concept Check 12.2
Can Schottky defects exist in K 2 O? If so, briefly describe this type of defect. If they
cannot exist, then explain why.
[The answer may be found at www.wiley.com/college/callister (Student Companion Site).]
Fe 2+
Vacancy
Figure 12.22 Schematic
representation of an Fe 2
Fe 3+ vacancy in FeO that results
from the formation of two
Fe 3 ions.
Fe 2+
O 2–