Solid solution

A solid solution, a term commonly used for metals, is a homogeneous mixture of two compounds in solid state and having a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word "solution" is used to describe the intimate mixing of components at the atomic level and distinguishes these homogeneous materials from physical mixtures of components. Two terms are mainly associated with solid solutions – solvents and solutes, depending on the relative abundance of the atomic species. The solute may incorporate into the solvent crystal lattice substitutionally, by replacing a solvent particle in the lattice, or interstitially, by fitting into the space between solvent particles. Solid solutions consist of fractional composition of one or more of its constituent ions between pure, isostructural extremes, known as end members or parents. For example, parent compounds sodium chloride (NaCl) and potassium chloride (KCl) have the same cubic crystal structure, so it is possible to make a solid solution with any ratio of sodium to potassium (Na1-xKx)Cl, eg. by dissolving that ratio of NaCl and KCl in water and then removing the water by evaporation. An example of a solid solution in this family is sold under the brand name Lo Salt which is (Na0.33K0.66)Cl, hence it contains 66% less sodium than pure NaCl. Similarly, iodised salt is often composed of around 50-100 ppm of potassium iodide (KI) dissolved in a NaCl solvent. In contrast, an example of a physical mixture is the mineral sylvinite - this contains separate, large chunks of NaCl and KCl, and is therefore inhomogenous and not a solid solution. Because minerals are natural materials they are prone to large variations in composition. In many cases specimens are members for a solid solution family and geologists find it more helpful to discuss the composition...