Perovskite oxides oxygen pressure dependence, electronic

Oxides play many roles in modem electronic technology from insulators which can be used as capacitors, such as the perovskite BaTiOs, to the superconductors, of which the prototype was also a perovskite, Lao.sSro CutT A, where the value of x is a function of the temperature cycle and oxygen pressure which were used in the preparation of the material. Clearly the chemical difference between these two materials is that the capacitor production does not require oxygen partial pressure control as is the case in the superconductor. Intermediate between these extremes of electrical conduction are many semiconducting materials which are used as magnetic ferrites or fuel cell electrodes. The electrical properties of the semiconductors depend on the presence of transition metal ions which can be in two valence states, and the conduction mechanism involves the transfer of electrons or positive holes from one ion to another of the same species. The production problem associated with this behaviour arises from the fact that the relative concentration of each valence state depends on both the temperature and the oxygen partial pressure of the atmosphere. 

A characteristic feature of perovskite-type oxide ion conductors is that they are often accompanied with p-type electronic conduction under an oxidizing atmosphere such as air at elevated temperatures. As described in Section 3.2, the contribution of electronic conduction depends on P02 in the atmosphere and temperature. As a typical example, Fig. 3.4 shows the P02 dependence of conductivities of CaTiOs- and SrTiOs-based solid solutions at 800°C [16]. P-type electronic conduction appears in the region of high P02 and n-type one under low oxygen partial pressure, i.e., a reducing atmosphere. The shape of the curve Incr lnPo2 is essentially the same as that shown schematically in Fig. 3.1. In many fluorite-type oxide ion conductors such as stabilized zirconias and... 

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