The constituents of a phase have a stoichiometry that can be expressed in terms of the components and possibly a charge. For instance, condensed phases may have an internal structure like sublattices or clusters, which may be modeled as constituents.
The constituents can either be chemical elements (Fe, Ni, Al, Ti, O, S, N, etc) or molecular-like aggregates that can be either electrically neutral (H2O, Fe3C, Cr2O3, etc) or charged (Fe+2, Fe+3, OH-1, etc). All such constituents are referred to as species.
Thermo-Calc makes a distinction between the components of a system and the constituents of a phase in the system. By default, the elements are defined as the components of the system. This can be changed with the POLY command DEFINE_COMPONENT. For example, if the elements are Fe, Mn, and O, one could also define FeO, MnO and O2 as a set of components.
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