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solid solution[′säl·əd sə′lü·shən]
a solid phase of variable composition in which atoms of different elements are mixed in a finite or infinite range of proportions in a common crystal lattice.
All crystalline solids are soluble in the solid state. In most cases, the solubility is confined to a narrow range, but systems with a continuous series of solid solutions are known—for example, Cu-Au, Ti-Zr, Ge-Si, and GaAs-GaP. All crystalline substances that are said to be pure are essentially solid solutions with a very low impurity content, since absolute purity is virtually unachievable. Solid solutions of minerals are widespread in nature (seeISOMORPHISM). The existence of solid solutions based on compounds or, in particular, on metals is of great importance in technology, since the alloys thereby formed have better mechanical, physical, and other properties than the original components. The decomposition of the solid solutions results in the acquisition of new and often distinctive properties by the alloys (see, and TEMPERING).
Impurity atoms or atoms of alloying elements may form with the matrix of the basis crystal either substitutional or interstitial solid solutions. Which type of solid solution is formed depends mainly on size and electrochemical factors.
According to the semiempirical Hume-Rothery rules, substitutional solid solutions are formed only by atoms that satisfy two conditions. First, the atoms must be similar in radius: their radii must differ by not more than 15 percent (8 percent in the case of iron-based solid solutions). Second, the atoms must be electro-chemically similar: they must be not far from one another in the electromotive series.
Interstitial solid solutions are formed when the component atoms are substantially different in size and the atoms of one species can occupy spaces, or interstices, in the crystal lattice formed by the atoms of the other species. The formation of such solid solutions is typical of the solution in metals of such nonmetals as boron, oxygen, nitrogen, and carbon (see AUSTENITE and MARTENSITE).
Substitutional and interstitial solid solutions may be disordered, partially ordered, or ordered. In disordered solid solutions, the atoms are randomly distributed upon the lattice sites. In ordered solid solutions, unlike atoms are located at definite positions relative to one another. The atoms in an ordered solid solution form a superlattice structure. In some cases, atoms of the same species may tend to form clusters, which may be oriented or ordered in a certain way. Experimental data on the ordering of solid solutions are obtained mainly through the study of diffuse X-ray scattering (seeX-RAY DIFFRACTION ANALYSIS). On a macroscopic scale, solid solutions in thermodynamic equilibrium may be regarded as truly homogeneous, but they are not necessarily homogeneous when examined on an atomic scale.
In addition to the substitutional and interstitial types of solid solutions, a third type can be distinguished. This type, which may be called a subtractional solid solution, is formed by the presence of vacant sites in the crystal lattice (seeVACANCY; DEFECTS, CRYSTAL).
Some nonmetallic systems classified as solid solutions have very valuable properties and are widely used in modern technology. Examples are semiconductors and ferrites.
G. V. INDENBAUM