What Is Unique About Mechanochemical Reactions?

Mechanochemical reactions can provide compounds, phases, and microstructures that are essentially different from the products of ordinary reactions. In this paper, the origin of this uniqueness is discussed in light of the recent advances of the field. It is claimed that the local availability of large batches of energy, well above kT, is the key feature of mechanochemical reactions. As a consequence, reactions that cannot occur thermally become possible, similarly to the reactions induced by the energy of photons in photochemistry. However, the situation is more complex, as macroscopic deformation affects many defect sites simultaneously. The direction of the mechanical load relative to the orientation of a molecule or the crystallographic axes of a solid can be important. Many mechanochemical reactions of organic compounds take place at low milling energy that is not sufficient to break primary bonds, but the gentle mechanical grinding can influence the relative position of macromolecules, leading to the formation of unique cocrystals and compounds. In inorganic systems, unusual products form due to forced mixing and the high defect density generated by intense milling.