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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?

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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)
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  • $\begingroup$ Thanks for your reply, I actually understand the second part of your answer, but I am unaware of how the kinetics would control the formation of the polymorph crystal. Materials solidify in a way to get to the most stable form (from a thermodynamic point of view). How would kinetics change that rule? $\endgroup$ – Error404 Oct 23 '13 at 10:45
  • $\begingroup$ @Error404 “Materials solidify in a way to get to the most stable form” — not always, the most notorious counter-example being glassy materials. Polymorphs are also example where the thermodynamically favored structure is not formed, for example because another phase crystallizes more rapidly $\endgroup$ – F'x Oct 23 '13 at 12:40
  • $\begingroup$ That's absolutely correct. An amorphous forms when the rate of cooling, or solidification is higher than the rate of crystallization. But what I am wondering about here is that polymorphs (not amorphous) are given enough time to re-arrange themselves, why have they chosen to be in a metastable form (high energy state) when they can crystallize into the crystalline form (lower energy state)? lets not assume any special conditions (pressure or other factors) during recrystallization. $\endgroup$ – Error404 Oct 23 '13 at 13:07
  • $\begingroup$ @Error404 There's not a simple unique answer. Maybe an example of a similar phenomenon is crystal growth: although it is a crystallization process, not all facets of a crystal grow at the same speed, giving rise to specific crystal shapes depending on the material (and synthesis conditions). $\endgroup$ – F'x Oct 23 '13 at 13:21

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