Fast oxygen ion conductors - from doped to ordered systems

This contribution briefly introduces the traditional acceptor doping of oxides, charge compensated by oxygen vacancies to bring about the oxygen ion conduction utilised in solid oxide electrolytes. It is next shown how a simple defect-chemical approach to defect-defect interactions is a useful first-approximation tool in the interpretation of data and design of improved materials, e.g. by optimising dopants. Moreover, it is shown how the trapping of vacancies at acceptor defects can be avoided altogether if the cation sublattice can be made to order completely. This is realised in a few known systems and we illustrate the principle using the oxygen deficient complex perovskite Sr-4(Sr2Nb2)O-11 as an example, in which the Sr and Nb on the B-site sublattice can be perfectly ordered into a NaCl-structure-type arrangement, leaving all oxygen ion sites equivalent, without trapping sites. The design of new materials should thus aim at cation order, and the possibility of achieving this is discussed also for pyrochlore type oxides. A Kroger-Vink type notation for an inherently deficient sublattice without defects in the other sublattices is proposed.