In materials science polyamorphism is the ability of a substance to exist in several different amorphous modifications. It is analogous to the polymorphism of crystalline materials. Even though amorphous materials exhibit no long-range periodic atomic structure, different phases can vary in other properties, such as the density. In several cases these transitions are expected to end in a second critical point.
Examples of polyamorphism are the existence of six-coordinated and four-coordinated amorphous silicon, and the different density phases of amorphous ice. In the case of amorphous ice the transition occurs below zero Celsius, so it is not a true equilibrium transition, the stable crystalline ices are the equilibrium phases. There is also continued debate over whether the transformations between the various high density and low density amorphous ices are first order-like or continuous-like transitions.
Yttria-Aluminates are another system believed to exhibit polyamorphism in the supercooled liquid and glassy states.
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