Mixed ionic-electronic conductors have both ionic and electronic charge carriers, whose relative importance for electrical transport depends upon the boundary conditions imposed on the charge carriers by the electrodes. Timely examples are ${\mathrm{La}}_2{\mathrm{CuO}}_{4+x}$ and ${\mathrm{La}}_2{\mathrm{NiO}}_{4+x}.$ When these systems are oxygenated electrochemically, oxygen enters as ${\mathrm{O}}^{2-}$ at one electrode and “holes” enter at the other electrode. For slow steady transport, the electric field, rather than being uniform, varies linearly across a cylindrical sample. For pairs of electrodes that pass only holes or only ${\mathrm{O}}^{2-}$ ions, the electric field is uniform, but the steady-state conductivity is not that due to pure conduction by the charge carrier; it is enhanced by diffusion of that charge carrier. The short-time, or ac, conductivity is independent of the electrodes and is the sum of the individual charge-carrier conductivities.

• Received 4 December 1998

DOI:https://doi.org/10.1103/PhysRevB.59.15160