We always hear about the different arrangements of molecules that lead to the formation of polymorphs, my question here is why would a polymorph form in the first place? The material was given enough time to solidify in a proper arrangement, why did it choose to solidify in a form that isn't the most stable?
This is a very broad and very interesting question, but the basic point is: polymorphs exist either because
- their formation is controlled by kinetics (rather than thermodynamics, which would favor the most stable phase), or
- the thermodynamic equilibrium between the various polymorphs is influenced by the synthesis conditions (temperature, pressure, presence of templating agents and other species in solutions…), and once the material has crystallized in a given form, it does not spontaneously undergo a transition to the most stable phase (again, because of kinetics)