I recently learned about disappearing polymorphs. I understand the general concept behind it and why the morphs sometimes happen or spread. What I don’t understand however is why the new, more efficient polymorph prevents the less efficient one from ever being created again. What I mean is this: A batch of crystals is made and polymorphs into a new structure. Ok fine. That happened. So I throw out that polymorphed batch and make it again. But when I try to re-make the batch, it comes out as the new morph, not the original structure. What is physically preventing a the old morph from being made again? If the manufacturing area is thoroughly cleaned and completely wiped of any trace of the original structure, what influences any further batches into being created in the new structure? It also seems like ‘magic’ for lack of a better word that is compelling any batch going forward to take on a new structure rather than the old one.
In: Chemistry
So crystal structures are low energy states, meaning their creation releases energy and it takes energy to break apart. In other words, you may have an atom or molecule that makes up this crystal structure, pushing it in any direction inside of its crystal structure will cause other atoms or molecules in said crystal structure to push back on it into its original place. This is for example why solids are solid and liquids/gases are not, the components of the solid cannot move without significant energy being applied to them as they are being held in by their surrounding parts, meanwhile liquids have sufficient energy in their particles (heat) to free them of their crystal structure.
If we were to graph the energy of the structure, the crystal would take up what we call a local minimum, you can think of it as the low points in a parabola or some more complex graph. What this means is that changing structure in any way would require energy to “go up the slope.” However, just like graphs can have multiple local minima, so can solids. [The graph on the wikipedia page for this topic actually has a very nice graph of this.](https://en.wikipedia.org/wiki/Disappearing_polymorphs)
A lower local minimum is generally the preferred structure for a crystal, it will try to attain the lowest energy state. **It just happens, by the random arrangement prior and during crystallization, that a higher energy crystal can form sometimes.** Once part of a crystal is formed, be it the higher energy polymorph or not, the crystal can continue building by taking particles out of the surrounding liquid or gas as near the existing crystal as near the crystal, as near the existing crystal, the lowest energy state a particle can achieve is becoming part of that crystal, not creating a new one, even if that crystal is less energetic in total.
But theres no real good way to tease the creation of a specific polymorph beside using a seed crystal (transplanting a part of the original polymorphed crystal). It was originally created due to the random arrangement of atoms in the fluid that just by chance came together to form the original polymorph. Without a seed crystal, its not guaranteed this can occur again and it can be vastly more likely that the random arrangement generates some other crystal.
Also removing all traces of crystals is really really hard. The seed crystal could be just a dozen atoms, you can practically never clean your workbench well enough to do that.
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