Observable Two-Step Nucleation Mechanism in Solid-State Formation of Tungsten Carbide.

The nucleation of crystals from ubiquitous solid-state reactions impacts a wide range of natural and synthetic processes and is fundamental to physical and chemical synthesis. However, the microscopic organization mechanism of amorphous precursors to nanoscale clusters of ordered atoms (nucleus) in an all-solid environment is inaccessible by common experimental probes. Here, by using in situ transmission electron microscopy in combination with theoretical simulations, we show in the reactive formation of a metal carbide that nucleation actually occurs via a two-step mechanism, in which a spinodal-structured amorphous intermediate reorganizes from an amorphous precursor and precedes the emergence of a crystalline nucleus, rather than direct one-step nucleation from classical consideration.