The mechanism of solid-state decompositions in a retrospective

This is an overview of the early investigations into the mechanism of solid decompositions since the fundamental studies by Ostwald on catalysis during 1890-1902 and the first experimental study of the autocatalytic decomposition of Ag2O by Lewis in 1905. In order to explain the formation mechanism of the solid product, Volmer suggested in 1929 that the decomposition of Ag2O includes two sequential stages: first, a thermal decomposition of the oxide into gaseous silver atoms and oxygen molecules and second, the condensation of the supersaturated silver vapor. This revolutionary idea was immediately used by Schwab to explain the autocatalytic peculiarity of solid-state decomposition reactions. However, this mechanism did not receive the acceptance of the scientific community. On the contrary, as can be seen from the results presented at the conference "Chemical reactions involving solids" in Bristol in 1938, this model was dismissed as unrealistic and, as a result, was since forgotten. Instead, considerable attention at this conference was devoted to the disorder theory proposed earlier by Wagner and Schottky, and to the mechanism of ion transport in the solid crystals. During the subsequent 70 years, decomposition mechanisms have been interpreted, without visible progress, on the latter basis. The mechanism of congruent dissociative vaporization proposed independently in 1990 turned out to be in complete agreement with the Volmer-Schwab model. It has been treated with the same distrust. These historical events, in the author's opinion, are responsible for the prolonged stagnation in the development of solid-state decomposition theory.