On a temperature-volume phase diagram for water and three-phase invariant reactions in pure substances.

Phase diagrams are integral to the application and interpretation of materials thermodynamics, and none is more ubiquitous than the common Temperature-Pressure diagram of water and its many icy polymorphs. Inspired by recent advances in isochoric thermodynamics, in this Communication we present a new Temperature-Volume phase diagram for water and five of its solid polymorphs. Fundamental similarities between this T-V diagram and conventional binary temperature-concentration (T-x) diagrams are demonstrated and explained, the notion of isochoric three-phase invariant reactions in pure substances (e.g. a "volumetric eutectic") is introduced, and experimental implications are discussed.