Plea for Defect Chemistry: The Fundamental Role of the Chemically Excited States in Solid‐State Chemistry and Beyond

In solids, the point defects take the role that in water is held by H 3 O + , OH − , and solutes. While the chemical ground structure (crystallographic structure) plays the role of the matrix, it is the chemically excited states (dissociation, disorder) that breathe life into the matter. Point defects, such as interstitial particles, vacancies, and substitutional particles, are the centers that are responsible for the chemical kinetics and for the vast majority of chemical and physical functionalities. Subsuming excess electrons and electron holes under the same umbrella, the well‐developed field of defect chemistry has been constructed. In spite of a long tradition and of many examples of high technological relevance, it has not yet become part of the chemists’ or physicists’ basic education. Without using the sometimes scary nomenclature and the adequate mathematics, the relevance of the field is highlighted by using a variety of characteristic examples (solid‐state reactions, acidity and basicity, kinetics and catalysis, fuel cells, batteries, and solid‐state physics).