Development of apatite-type oxide ion conductors.

Research into materials displaying oxide ion conductivity is attracting considerable attention due to their potential technological applications in devices such as Solid Oxide Fuel Cells. In this paper, recent work on apatite-type oxide ion conductors is reviewed, showing that a wide range of cation substitutions are possible, due to the flexibility of the apatite structure in accommodating a range of ion sizes. The conductivity studies on these doped samples show that to achieve high oxide ion conduction, non-stoichiometry in terms of cation vacancies and/or oxygen excess is required, with the latter resulting in the highest conductivities. In contrast to most common oxide ion conductors, e.g. perovskite and fluorite in which oxide ion conduction proceeds via oxygen vacancies, the research on these apatite systems suggests that the conductivity involves interstitial oxide ions. With further optimization of these materials, particularly in terms of the Ge-containing systems, significant improvements in conductivity are likely, leading to the very real possibility of the application of apatite-type electrolytes in fuel cell and other applications.