Solid Solutions

Solid solutions are homogeneous solid mixtures formed when one solid substance (the solute) is dissolved throughout the crystal lattice of another (the solvent), producing a single solid phase with properties distinct from either pure component. Unlike a liquid solution, where solvent and solute molecules can move freely past one another, atoms in a solid solution are essentially fixed at lattice sites, so the way a solute atom fits into the solvent's crystal structure determines the type of solid solution formed.

A substitutional solid solution forms when solute atoms directly replace solvent atoms at regular lattice sites of the solvent's crystal structure. This substitution is favourable chiefly when the solute and solvent atoms are similar in atomic size (within roughly 15% of each other, by the empirical Hume-Rothery rules) and have similar chemical character, so that the substitution does not badly distort the lattice. Brass, an alloy of copper and zinc in which zinc atoms occupy sites in the copper lattice that would otherwise hold copper atoms, is a classic substitutional solid solution; other examples include bronze (copper and tin) and many other common structural alloys. …