SOLIDSOLUTION

A solid solution is a homogeneous crystalline phase that contains two or more chemical species within a single lattice. It forms when solute atoms occupy positions in the crystal lattice of a solvent, either by substitution for solvent atoms (substitutional solid solution) or by occupying interstitial sites between lattice atoms (interstitial solid solution). The result is a single phase with uniform composition, in contrast to mixtures of distinct phases.

Two primary types are recognized. Substitutional solid solutions occur when solute atoms replace solvent atoms in the lattice and are typically possible when the atoms are similar in size and chemical character. Interstitial solid solutions form when small atoms fit into the interstices of the lattice, often occurring with a metal host and small interstitial species such as carbon or nitrogen.

Solubility and phase behavior are described by phase diagrams. At high temperatures, many systems show complete solid solubility, while upon cooling a solvus line may appear, signifying limited solubility and the possible formation of additional phases or exsolution into a two-phase mixture. The lattice parameters generally change with composition, producing measurable shifts in X-ray diffraction patterns.

Common examples include substitutional copper-nickel and zinc in gold alloys, and interstitial carbon in iron (steel). In geology, minerals such as the plagioclase feldspar series (albite to anorthite) illustrate continuous solid solutions. Solid solutions influence properties such as density, hardness, color, and melting point, and are central to alloy design and mineralogical interpretation.