Mechanistic Insights into Materials Synthesis from Operando Studies

Solid-state synthesis of crystalline inorganic materials has long been accepted as being intrinsically slow, requiring heating at high temperatures for long periods to compensate for the limiting rate of diffusion in solids. Reaction mixtures of metal oxides, carbonates, etc. are often heated in a furnace at temperatures that are high enough for diffusion to occur, and only cooled and checked for products, byproducts, and impurities after hours, days, or weeks. While syntheses in the laboratory are blind to the transformations that occur behind the closed furnace door, in situ synchrotron-based powder diffraction experiments are increasingly being used to gain mechanistic insight into solid-state reactions. Based on the expectation that the reactions are slow, occurring over hours-days, these in situ synchrotron experiments typically use a variable temperature measurement strategy, wherein the temperature of the reaction mixture is progressively increased. However, recent studies have suggested that some solid-state reactions may occur much faster, once initiated, for example, once a threshold temperature for reaction is surpassed. Using a custom-designed reactor to rapidly initiate solid-state syntheses, we explore the earliest stages of a reaction using in situ X-ray scattering. These early stages are formative and set the course for the eventual product and phase selectivity.