Polymorphism (materials science)#Ostwald.27s rule

The Ostwald step rule states that a system does not pass from one energy state directly into the lowest energy state, but usually takes one or more metastable intermediates. The rule was erected at the end of the 19th century by Wilhelm Ostwald.

The rule can be applied to both chemical reactions and physical processes to. So go substances during crystallization from the melt immediately in the most stable solid modification over, but in a, which are in terms of their free energy of the melt at the next. By converting the low-energy monotropic modifications may be achieved. In addition, often considered the Ostwald -Volmer rule that in energetically similar systems first modification is preferred with lower density. For example, phosphorus separates on cooling of the initially vapor phase metastable white phosphorus from which slowly changes into red phosphorus.

The Ostwald rule does not have universal applicability. Under certain conditions, the possible intermediate steps are skipped and immediately formed the most stable phase or the end product of a reaction.