We note in Section 12.5 (Figure 12.20) that in FeO (wüstite), the iron ions can exist in both Fe2+ and Fe3+ states. The number of each of these ion types depends on temperature and the ambient oxygen pressure. Furthermore, we also note that in order to retain electroneutrality, one Fe2+ vacancy is created for every two Fe3+ ions that are formed; consequently, in order to reflect the existence of these vacancies, the formula for wüstite is often represented as Fe(1-x)O, where x is some small fraction less than unity. In this nonstoichiometric Fe(1-x)O material, conduction is electronic and, in fact, it behaves as a p-type semiconductor—that is, the Fe3+ ions act as electron acceptors, and it is relatively easy to excite an electron from the valence band into an Fe3+ acceptor state with the formation of a hole. Determine the electrical conductivity of a specimen of wüstite with a hole mobility of 1.0 × 10–5 m2/V•s, and for which the value of x is 0.040. Assume that the acceptor states are saturated (i.e., one hole exists for every Fe3+ ion). Wüstite has the sodium chloride crystal structure with a unit cell edge length of 0.437 nm.
Figure 12.20 Schematic representation of an Fe2+ vacancy in FeO that results from the formation of two Fe3+ ions.
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