Solid solution
In general if two compounds are isostructural then a solid solution will exist between the end members (also known as parents).
For example sodium chloride and potassium chloride have the same cubic crystal structure so it is possible to make a pure compound with any ratio of sodium to potassium (Na1-xKx)Cl by dissolving that ratio of NaCl and KCl in water and then evaporating the solution.
The pure minerals are called halite and sylvite; a physical mixture of the two is referred to as sylvinite.
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.
In pharmaceutical preparations, the concept of solid solution is often applied to the case of mixtures of drug and polymer.
The number of drug molecules that do behave as solvent (plasticizer) of polymers is small.
[6] On a phase diagram a solid solution is represented by an area, often labeled with the structure type, which covers the compositional and temperature/pressure ranges.
Where the end members are not isostructural there are likely to be two solid solution ranges with different structures dictated by the parents.
In metallurgy alloys with a set composition are referred to as intermetallic compounds.
A solid solution is likely to exist when the two elements (generally metals) involved are close together on the periodic table, an intermetallic compound generally results when two metals involved are not near each other on the periodic table.
Solid solution of pseudo-binary systems in complex systems with three or more components may require a more involved representation of the phase diagram with more than one solvus curves drawn corresponding to different equilibrium chemical conditions.
[10] Solid solutions have important commercial and industrial applications, as such mixtures often have superior properties to pure materials.
Even small amounts of solute can affect the electrical and physical properties of the solvent.
In the phase diagram, at three different concentrations, the material will be solid until heated to its melting point, and then (after adding the heat of fusion) become liquid at that same temperature: At other proportions, the material will enter a mushy or pasty phase until it warms up to being completely melted.
In contrast, when lead-tin mixtures were used to solder seams in automobile bodies a pasty state enabled a shape to be formed with a wooden paddle or tool, so a 70–30 lead to tin ratio was used.
When a solid solution becomes unstable—due to a lower temperature, for example—exsolution occurs and the two phases separate into distinct microscopic to megascopic lamellae.
[11] Alkali feldspar minerals, for example, have end members of albite, NaAlSi3O8 and microcline, KAlSi3O8.
In the case of the alkali feldspar minerals, thin white albite layers will alternate between typically pink microcline,[11] resulting in a perthite texture.
