Vacancies and Self-Interstitials

Abstract

This article aims to review in rather cursory fashion the ways the concepts of lattice defects have contributed at an early stage to our understanding of solid-state kinetics in metals and insulators.The importance of point defects in solid-state kinetics was just beginning to be widely recognized in the late 1930s. The crucial experiments of Kirkendall and others were still to come. Many of the leaders in the field of metallurgy believed almost tacitly that diffusion in substitutional alloys occurred by direct interchange or perhaps a ring mechanism.For the ionic salts, however, basic calculations were further advanced, and it was possible to figure quite confidently the role of Schottky and Frenkel defects in facilitating atom movements. In their seminal paper, Mott and Littleton made specific calculations as to the energies involved in diffusion by the various mechanisms and hence to the relative importance of these mechanisms in the kinetics of these materials. They began by taking over the Born-Mayer short-range formula for ionic repulsion. Next they treated in detail the polarization response of the salt to an extra charge in the lattice, whether interstitial or vacancy. This polarization included the individual polarizabilities of the ions and, for the static case, the ion displacements. Application of this analysis gave good quantitative results for the activation energies to be expected for diffusion and ionic conductivity. For the alkali halides it was made clear that the Schottky defect would dominate and that Frenkel defects would be few.