We noted in Section 12.5 (Figure) that in FeO (wstite), the iron ions can exist in both

We noted in Section 12.5 (Figure) 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 noted that in order to retain electroneutrality, one Fe2+ vacancy will be 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 that has a hole mobility of 1.0 ?? 10???5 m2/V-s and for which the value of x is 0.060. 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 0.437nm.

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Fer+ Vacancy