Polyamorphism
![Pressure–temperature phase diagram, including an illustration of the liquid–liquid transition line proposed for several polyamorphous materials. This liquid–liquid phase transition would be a first order, discontinuous transition between low and high density liquids (labelled 1 and 2). This is analogous to polymorphism of crystalline materials, where different stable crystalline states (solid 1, 2 in diagram) of the same substance can exist (e.g. diamond and graphite are two polymorphs of carbon). Like the ordinary liquid–gas transition, the liquid–liquid transition is expected to end in a liquid-liquid critical point. At temperatures beyond these critical points there is a continuous range of fluid states, i.e. the distinction between liquids and gasses is lost. If crystallisation is avoided the liquid–liquid transition can be extended into the metastable supercooled liquid regime.](/Images/godic/202502/03/Liquid-liquid4332.gif")
![Schematic of interatomic pair potentials. The blue line is a typical Lennard-Jones type potential, which exhibits the ordinary liquid–gas critical point. The red line is a double well type potential, which is proposed for polyamorphous systems.[1] The grey line, is a representative of the soft core square well potentials, which in atomisitc simulations exhibit liquid–liquid transitions and a second critical point.[2] The numbers 1 and 2 correspond to the 1st and second minima in the potentials.](/Images/godic/202502/03/Liq-liq-potential2.svg4332.png")
Polyamorphism is the ability of a substance to exist in several different amorphous modifications. It is analogous to the polymorphism of crystalline materials. Many amorphous substances can exist with different amorphous characteristics (e.g. polymers). However, polyamorphism requires two distinct amorphous states with a clear, discontinuous (first-order) phase transition between them. When such a transition occurs between two stable liquid states, a polyamorphic transition may also be referred to as a liquid–liquid phase transition.