Polymorphism is defined as the ability of a solid material to exist in more than one form or crystal structure, whereas allotropy is defined as the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of these elements. (From wikipedia).

In the case of crystal solids, it seems clear that allotropy is a particular case of polymorphism... but this is always true? That is, both terms, polymorphism and allotropy refer always to crystal structures? Polymorphism definition mentions "crystal structures" but "crystal" does not appears in the definition of allotropy...

Thanks in advance.

UPDATE Thank you for your answer Then, in some cases allotropy is more general than polymorphism. For instance, dioxygen and ozone are allotropes but not polymorphisms. But... my question arises from my TRUE/FALSE set of questions which I have to answer, and one of them is:

In a non-crystal material the concept of allotropy or polymorphism does not exist.

What would you answer to that? My conclusion is that the question is not clear... For polymorphism is true whereas for allotropy it is not.

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  • $\begingroup$ I took the liberty to merge your comment that you added in an answer back into your question. You can always extend your own question or add comments to it. I agree that the question is unclear. Btw, unless I'm wrong, greetings to Catalunya. $\endgroup$ – Klaus-Dieter Warzecha Apr 5 '15 at 20:00

In my own work, I've never used the term polymorph for anything but molecular crystals in a highly but differently ordered solid state.

In the case of crystal solids, it seems clear that allotropy is a particular case of polymorphism.

Yes, allotropy is the polymorphism of elements in the same state. This does not mean that each solid allotrope has to be highly ordered: amorphous forms of carbon, phosphorus or selenium are considered as allotropes too.

The definition of allotropy isn't confined to the solid state: ozone is an allotrope of dioxygen.

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