Callister - An introduction - 8th edition

12.5 Imperfections in Ceramics • 475
Interstitial impurity atom
Substitutional impurity ions
Figure 12.23 Schematic representations of interstitial, anion-substitutional, and cationsubstitutional
impurity atoms in an ionic compound. (Adapted 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.)
Impurities in Ceramics
Impurity atoms can form solid solutions in ceramic materials much as they do in
metals. Solid solutions of both substitutional and interstitial types are possible. For
an interstitial, the ionic radius of the impurity must be relatively small in comparison
to the anion. Because there are both anions and cations, a substitutional impurity
will substitute for the host ion to which it is most similar in an electrical sense:
if the impurity atom normally forms a cation in a ceramic material, it most probably
will substitute for a host cation. For example, in sodium chloride, impurity
Ca 2 and O 2 ions would most likely substitute for Na and Cl ions, respectively.
Schematic representations for cation and anion substitutional as well as interstitial
impurities are shown in Figure 12.23. To achieve any appreciable solid solubility of
substituting impurity atoms, the ionic size and charge must be very nearly the same
as those of one of the host ions. For an impurity ion having a charge different from
the host ion for which it substitutes, the crystal must compensate for this difference
in charge so that electroneutrality is maintained with the solid. One way this is accomplished
is by the formation of lattice defects—vacancies or interstitials of both
ion types, as discussed previously.
EXAMPLE PROBLEM 12.5
Determination of Possible Point Defect Types in NaCl Due
to the Presence of Ca 2 Ions
If electroneutrality is to be preserved, what point defects are possible in NaCl
when a Ca 2 substitutes for an Na ion? How many of these defects exist for
every Ca 2 ion?