Within the last few decades though its foundations were laid over 60 years ago-a direction of research has developed almost unnoticed by the classical chemical disciplines, such that one can now recognize a chemistry within the solid state that is analogous to the long-familiar chemistry in the liquid state. It arises from those departures from the ideal structure that are thermodynamically unavoidable, the point defects, and is referred to as defect chemistry. It includes the description of ionic and electronic effects, and it considers diffusion as a special step of the overall reaction. This area of chemistry enables one to describe and treat in a unified way many widely different phenomena such as ionic conduction in crystals, doping effects and p-n junctions in semiconductors, color centers in alkali metal halides, image development in photography, passivation and corrosion of metals, the kinetics of synthesis and sintering of solid materials, problems of rock formation during the earth's evolution, the mechanisms of gas sensors and high temperature fuel cells, the performance of photosensitive electrodes, variations of the electron balance in high-temperature superconductors, elementary processes of heterogeneous catalysis, nonequilibrium transitions and oscillations in semiconductors in electric fields, and many more. In such phenomena the equilibrium concentration of defects has an important double role: it not only determines the disorder and the departure from the Dalton composition in the equilibrium state, but also, together with the mobility as the kinetic parameter, is the key parameter concerning the rates of physicochemical processes. Accordingly, in this first part of the review the emphasis will be on the equilibrium thermodynamics of point defects, whereas the second part will be specifically concerned with the kinetic aspects. Both parts will emphasize the fact that the defect chemistry of solids, as well as being the counterpart of solution chemistry in the liquid state, also provides a unifying approach in which electronic and ionic charge carriers are treated by analogous methods, both in the bulk and in boundary layers, and allows diffusion to be incorporated naturally into the overall kinetics of reactions as an elementary chemical process.
