In oxygen ion conductors, current flow occurs by the
movement of oxide ions through the crystal lattice. This
movement is a result of thermally-activated hopping of the
oxygen ions, moving from crystal lattice site to crystal lattice
site, with a superimposed drift in the direction of the electric
field. The ionic conductivity is consequently strongly
temperature dependent, but at high temperatures can
approach values close to 1 S cm-1 (Fig. 2), comparable to the
levels of ionic conductivity found in liquid electrolytes.
This is clearly a remarkable property of these solids and to
understand its origins we must make several important
observations. The first observation is that the crystal must
contain unoccupied sites equivalent to those occupied by the
lattice oxygen ions. Secondly, the energy involved in the
process of migration from one site to the unoccupied
equivalent site must be small, certainly less than about 1 eV.
For an oxygen ion conductor, this small barrier to migration
would seem at first sight to be difficult to attain. The oxygen
ions are the largest components of the lattice, with an ionic
radius of 1.4 Å. Intuitively, it would be expected that the
smaller metal ions would be more likely to have an
appreciable mobility in the lattice and, hence, carry the
current. However, in certain very special and open crystal
structures, this is not the case and it is the oxygen ions that
migrate in the electric field. It is thus clear from these initial
considerations that the materials displaying oxygen ion