Exsolution

Solid in which components are compatible and form a unique phase.

Note 1: The definition “crystal containing a second constituent which fits into and
is distributed in the lattice of the host crystal” given in refs., is not general
and, thus, is not recommended.

Note 2: The expression is to be used to describe a solid phase containing
more than one substance when, for convenience, one (or more) of the substances,
called the solvent, is treated differently from the other substances, called solutes.

Note 3: One or several of the components can be macromolecules. Some of
the other components can then act as plasticizers, i.e., as molecularly dispersed
substances that decrease the glass-transition temperature at which the amorphous
phase of a polymer is converted between glassy and rubbery states.

Note 4: In pharmaceutical preparations, the concept of solid solution is often
applied to the case of mixtures of drug and polymer.

Note 5: The number of drug molecules that do behave as solvent (plasticizer)
of polymers is small.

A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase. This often happens when the two elements (generally metals) involved are close together on the periodic table; conversely, a chemical compound generally results when two metals involved are not near each other on the periodic table.

The solid solution needs to be distinguished from a mechanical mixtures of powdered solids like two salts, sugar and salt, etc. The mechanical mixtures have total or partial miscibility gap in solid state. Examples of solid solutions include crystallized salts from their liquid mixture, metal alloys, moist solids. In the case of metal alloys intermetallic compounds occur frequently.

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Wikipedia