Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

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5.3 Point Defects in Ceramics • 131

Cation interstitial

Cation vacancy

Anion vacancy

Figure 5.2 Schematic representations of cation and anion vacancies and a cation

interstitial. (From W. G. Moffatt, G. W. Pearsall, and J. Wulff,The Structure and Properties

of Materials,Vol. I,Structure,p. 78. Copyright©c1964 by John Wiley & Sons, New York.

Reprinted by permission of John Wiley & Sons, Inc.)

stoichiometry stoichiometric.Stoichiometrymay be defined as a state for ionic compounds wherein

there is the exact ratio of cations to anions predicted by the chemical formula. For

example, NaCl is stoichiometric if the ratio of Na+ions to Cl−ions is exactly 1:1. A

ceramic compound isnonstoichiometricif 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 (w ̈ustite, FeO) is one such

material, for 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 intro-

ducing 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 pos-

itive charges) for every two Fe^3 +ions that are formed (Figure 5.4). 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

Schottky defect

Frenkel defect

Figure 5.3

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 Mate-

rials,Vol. I,Structure,

p. 78. Copyright©c

1964 by John Wiley

& Sons, New York.

Reprinted by

permission of John

Wiley & Sons, Inc.)