Chemical Reactions followed by in situ Neutron Powder Diffraction

In situ neutron powder diffraction is a very convenient tool for the investigation of chemical reactions, whenever crystalline solids are involved. It may give insight into reaction pathways, thus allowing for strategic synthesis planning, revealing particular intermediate steps and quantifying the reaction kinetics. Although the method is rather straightforward, one is bound to a small number of available neutron sources and the few sufficiently high intensity powder diffractometers there, most of which are briefly described herein. The high penetration depth of thermal neutrons allows for a large range of complex sample environments; however neutron absorption and parasitic scattering have to be carefully considered in the construction of dedicated in situ equipment. Some specific types of sample environment, specially designed for in situ neutron powder diffraction will be discussed such as electrochemical cells, autoclaves for the study of solvothermal reactions, and gas-solid reaction cells. Several examples from inorganic chemistry are being presented, ranging from metal hydrides, nitrides, oxides, and halides, to intermetallic compounds or metal organic frameworks, many of them of importance in applications, e.g. in lithium ion batteries, as magnetic or hard materials, high strength alloys, hydrogen storage, ferroelectric or superconducting materials, or as solid catalysts. In situ neutron powder diffraction often paves the way to a better reaction control in chemical synthesis, to the discovery of metastable compounds or gives insight into technologically relevant processes. These successful applications in inorganic chemistry demonstrate the usefulness of the method of in situ neutron powder diffraction and may encourage the reader to consider neutron diffraction in the repertoire of analytic tools.