Polymorphism phase changes in the solid state

It is generally convenient to consider the structures of metals in terms of the observed stracture type at 298 K and 

It is generally convenient to consider the structures of metals in terms of the observed structure type at 298 K and atmospheric pressure, but these data do not tell the whole story. When subjected to changes in temperature and/or pressure, the structure of a metal may change each form of the metal is a particular polymorph. For example, scandium undergoes a reversible transition from an hep lattice (a-Sc) 

If a substance exists in more than one crystalline form, it is 

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