A universal representation of the states of chemical matter including metastable configurations in phase diagrams.

Encoding equilibrium states of matter in the format of phase diagrams is among the basic, and at the same time most fruitful concepts in solid state and materials chemistry. Its appealing stringency is related to the fact that any equilibrium state of matter, as characterized by its phase content (defined by the phases present, their compositions, and concentrations) and the respective components' activities, is unambiguously determined by fixing the set of independent variables of state, p (or V), T, xi-1 for a system constituted of i components. In many fields of science and technology, analyzing chemical processes in terms of phase diagrams and equilibrium thermodynamics has proven an invaluable tool. In spite of the impressive success of such an approach in specific areas, it fails seriously in addressing the full "real" materials world, since most of the matter around us stays trapped in metastable, in many cases quite persistent, states. Even at demanding thermal conditions, e.g. prevailing in gas or steam turbines, many of the materials employed are not in equilibrium. Quite generally, it is easy to demonstrate that metastable materials are of substantial relevance: durable diamond is metastable at ambient conditions, glasses constitute an economically significant class of matter, and (metastable) amorphous ceramics can outperform thermodynamically stable crystalline ones with respect to the overall set of properties relevant for high-temperature applications. As a consequence, a holistic consideration of a given chemical system, e.g. for the purpose of synthesis planning, needs to include both the equilibrium and the metastable states.