Single-step nucleation refers to the direct formation of crystals from their primary building blocks in solution via a single energy barrier in the free energy landscape (i.e., the induction time is associated with a single bottleneck towards crystallization). In contrast, multi-step nucleation encompasses a variety of crystallization pathways, which can involve the formation of intermediate precipitate phases, such as amorphous liquids, solids or other crystalline phases. The pathway to the most stable crystal form from solutions may involve multiple intermediates occurring in sequence reproducibly. If the relative stability of the intermediates increases with each step, this type of mechanism is consistent with the Ostwald-Lussac empirical rule of stages .