Supercooled water, polyamorphism

Water is a very structurally versatile molecule. Water exists in all three physical states solid, liquid, and gas. Under extremely high temperature and pressure conditions, water can also become a supercritical fluid. Liquid water can be cooled carefully to below its freezing point without solidifying to ice, resulting in two possible forms of supercooled water. In the solid state, 13 different crystalline phases (polymorphous) and 3 amorphous forms (polyamorphous) of water are currently known. These fascinating faces of water are explored in detail in this section. 

There are a few experiments supporting the presence of a second CP in water [60,76]. Up to date, the existence of a second CP in water has not been confirmed. We note that the second CP hypothesis is not the only explanation of water polyamorphism theoretical interpretations of water polyamorphism that do not consider the presence of a second CP in supercooled water have also been proposed (see, e.g.. Ref. [2]). 

In addition, for both substances, when the amorphous solid formed at ambient pressure is compressed, the material collapses to a higher density amorphous solid over a relatively narrow range of pressure [3-5]. Such polyamorphic behavior of the amorphous solid has been interpreted, in the case of water, as a subglass transition manifestation of an underlying thermodynamic instability in the supercooled liquid, specifically, a liquid-liquid phase transition (LLPT) occurring without a change in the chemical composition [6 8]. 

Polyamorphism has been promoted as a means for understanding the anomalous thermodynamics and dynamics of water. It has been proposed that the increase of compressibility with the decrease of temperature is related to a second critical point at the end of a coexistence line between two liquid phases, a low-density liquid (LDL) and a high-density liquid (HDL) [6]. This critical point would be located in the supercooled experimentally inaccessible region [7-9], In contrast, it is possible to explain the existing anomalies without invoking the presence of a critical point [9-11] and support the presence of a second critical without the need of polyamorphism [12],... 

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