A solid solution is a uniform mixture of two crystalline solids that share a common crystal lattice. Solid solutions often consist of two or more types of atoms or molecules that share a crystal lattice, as in certain metal alloys. The component that is present in the greatest amount is called the solvent, and the other components are called the solutes.

Solid solutions can be formed by combining the two solids when they have been melted into liquids at high temperatures and then cooling the result to form the new solid, or by depositing vapours of the starting materials onto substrates to form thin films. The degree of solubility of the components depends on their chemical properties and crystalline structure, which determine how their atoms fit together in the mixed crystal lattice. The mixed lattice may be substitutional, in which the atoms of one starting crystal replace those of the other, or interstitial, in which the atoms occupy positions normally vacant in the lattice.

The substances may be soluble over a partial or even complete range of relative concentrations, producing a crystal whose properties vary continuously over the range. This provides a way to tailor the properties of the solid solution for specific applications. For example, the olivine mineral group, particularly the forsterite-fayalite series, whose members vary from forsterite (Mg₂SiO₄) to fayalite (Fe₂SiO₄), has a substitutional solid solution that can range from 100 percent magnesium (Mg) to 100 percent iron (Fe), including all proportions in between, with physical properties that vary smoothly from those of forsterite to those of fayalite. Another example is the combination of gallium arsenide (GaAs) with gallium phosphide (GaP), aluminum arsenide (AlAs), or indium arsenide (InAs), which are semiconductors with interstitial solid solutions. The properties of these solid solutions can be tuned to values between those of the end compounds by adjusting the relative proportions of the compounds; for instance, the band gap for combinations of InAs and GaAs can be set anywhere between the value for pure InAs (0.36 electron volt [eV]) and that for pure GaAs (1.4 eV), with corresponding changes in the materials’ electrical and optical properties.

Solid solutions are important in many fields of science and engineering, such as metallurgy, geology, solid-state chemistry, materials science, and electronics. They offer a way to create new materials with desired properties by mixing existing ones in a controlled manner.

There are two main types of solid solutions: substitutional and interstitial. A third type, called omission, is less common and occurs when some atoms are missing from the lattice.

In a substitutional solid solution, the solute atoms replace some of the solvent atoms in the lattice. This type of solid solution requires that the solute and solvent atoms have similar crystal structures and atomic radii (less than 15% difference). The solubility of the solute atoms also depends on their valence and electronegativity relative to the solvent atoms. Substitutional solid solutions are common in metals and minerals, such as brass (zinc in copper), bronze (tin in copper), and olivine ((Mg, Fe)₂SiO₄). Substitutional solid solutions can have complete or partial solubility ranges, depending on the phase diagram of the system.

In an interstitial solid solution, the solute atoms occupy the spaces between the solvent atoms in the lattice. This type of solid solution requires that the solute atoms have much smaller atomic radii than the solvent atoms (less than 59% of the solvent radius). The solubility of the solute atoms also depends on their valence and size relative to the interstitial sites. Interstitial solid solutions are less common than substitutional ones, but they are important for some materials, such as steel (carbon in iron), silicon carbide (carbon in silicon), and ruby (chromium in aluminum oxide). Interstitial solid solutions usually have limited solubility ranges and tend to cause distortion and strain in the lattice.

The properties of solid solutions depend on the type, concentration, and distribution of the solute atoms in the lattice. In general, solid solutions tend to have higher melting points, lower electrical conductivity, higher hardness, and lower ductility than pure metals. These effects are due to the disruption of the lattice order and symmetry by the solute atoms, which create lattice defects, impede dislocation movement, and alter bonding interactions. Some properties, such as color, optical transparency, magnetic behavior, and catalytic activity, may also change depending on the nature and amount of the solute atoms.