Ionic Conductivity

Abstract

When in 1928 the article by v. Hevesy appeared in the Handbuch der Physik of Geiger and Scheel [1] the subject of ionic or electrolytic con-ductivity in solids was scarcely more than a descriptive study. It can now properly be regarded as an integral part of the wider, closely interrelated, subject of imperfections in all crystals, metallic and non-metallic; by imperfections we mean departures from the perfect crystalline arrangement of atoms envisaged by the early X-ray crystallographers. It is clear, for example, that the diffusion of atoms from one part of a perfect crystal to another part is impossible so long as we visualize the heat motion of the atoms as being merely vibrations about fixed lattice points. Likewise the drift of ions through an ionic crystal under the action of an electric field is inconceivable if the crystal is perfect. But whereas we might explain diffusion in crystalline solids on the basis of a direct exchange of places between neighbouring atoms, this mechanism is not adequate to explain ionic conduction. Transport of charge could only take place if anion-cation exchanges were possible. But in NaCl and AgBr, for example, the energy which would have to be supplied to effect an exchange of places between an anion and a cation is so large—the increase in electrostatic (Madelung) energy of the final state is 15 ev—that in one gram-molecule such an event would occur only once in some fantastically long time such as 1030 years.