Materials displaying high oxide-ion conductivity have attracted considerable interest due to technological applications in solid oxide fuel cells (SOFCs), oxygen sensors and separation membranes. This has driven research into the identification of new classes of oxide-ion conductors, and in this review, work on the recently discovered apatite-type silicate/germanate oxide-ion conductors is presented. In contrast to the traditional perovskite- and fluorite-based oxide-ion conductors, in which conduction proceeds via oxygen vacancies, the research on these apatite systems suggests that the conductivity involves interstitial oxide-ions. In addition, the flexibility of the tetrahedral (Si/GeO4) framework also plays a crucial role in facilitating oxide-ion migration. Detailed doping studies have shown that the apatite structure is able to accommodate a large range of dopants (in terms of both size and charge state), and the influence of these dopants on the conductivity is discussed.
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