Drift and Conductivity

When applied to the motion of ions in a crystal, the term drift applies to motion of ions under the influence of an electric field. Although movement of electrons in conduction bands determines conductivity in metals, in ionic compounds it is the motion of ions that determines the electrical condu-ctivity. There are no free or mobile electrons in ionic crystals. The mobility of an ion, ji, is defined as the velocity of the ion in an electric field of unit strength. Intuitively, it seems that the mobility of the ion in a crystal should be related to the diffusion coefficient. This is, in fact, the case, and the relationship is 

As the electric current passes through this system, the cathode (negative electrode) grows in thickness while the anode (positive electrode) shrinks. At the cathode, M+ ions are converted to M atoms, which results in growth of the cathode. From this observation, it is clear that the cations are primarily responsible for conductivity, and this is the result of a vacancy type of mechanism. In this case, the positive ion vacancies have higher mobility than do the vacancies that involve negative ions. 

In this chapter, we have described some of the types of transformations in solids that involve rate processes. This is an immensely practical area because many industrial processes involve such changes in inorganic substances, and they are an essential part of materials sciences. For a more complete discussion of these important topics, the references given below should be consulted. 

and Dienes, G. J. (1988). An Introduction to Solid State Diffusion. Academic Press, San Diego, CA. Athorough treatment of many processes in solids that are related to diffusion. 

(1961). Definition of Rate Constant and Activation Energy in Solid State Reactions, Nature (London) 192, 965. An article discussing the difficulties associated with interpreting activation energies for reactions in solids. 

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