Isomorphism, (from the Greek isos — equal, the same, and morphe — form, shape), the complete similarity in the atomic and crystal structure and in the external faceting of the crystals of substances that have the same chemical formula (in the ratio of their components) and the same type of chemical bond. Discovered in 1819 by the German chemist E. Mitscherlich in the crystals of KH2PO4, KH2AsO4 and NH4H2PO4. Isomorphism is also the name for the property, bound up with the existence of isomorphous crystals, by which different atoms, ions and combinations of them replace one another in the crystal lattice, forming crystals of variable composition (substitutional solid solutions). An example of perfect isomorphism is the crystals of alum KAl(SO4)*12H2O, in which the monovalent K ions can be replaced in any proportion by the monovalent ions Rb, NH4 and others, whose crystallochemical radius is approximately the same as that of the K ions, and the trivalent Al ions — by the trivalent ions Fe, Cr and others with radii close to that of Al. The difference between the crystallochemical radii of the atoms in isomorphous crystals does not exceed 10—15%.
Besides perfect (complete) isomorphism, with the formation of solid solutions at any ratio of the components, a limited isomorphism is possible, limited in the concentrations it allows; the compounds BaSO4 and KMnO4 may serve as an example of this kind. A distinction is drawn between isovalent isomorphism, where the atoms or groups that replace one another have the same valency (for example, Na+ , K+ , NH4+), and heterovalent isomorphism, where their valency differs (for example, Ca2+ and Y3+ ). In the latter case the closeness in size of the atoms that replace one another matters, while the difference in their charges is compensated by vacancies.
Isomorphism is observed in many minerals and crystals and is used in the synthesis of crystals, where the introduction of small additions substantially alters the properties or creates new ones. Thus the introduction of small isomorphous additions, for example Cr3+ into corundum Al2O3 or Nd3+ into the garnet Y3Al5O12, turns them into an active medium for lasers; the introduction of isomorphous impurities into semiconductor crystals changes the type of conductivity. Isomorphous impurities are used, for example, to change the colour of gemstone crystals.
Besides perfect (complete) isomorphism, with the formation of solid solutions at any ratio of the components, a limited isomorphism is possible, limited in the concentrations it allows; the compounds BaSO4 and KMnO4 may serve as an example of this kind. A distinction is drawn between isovalent isomorphism, where the atoms or groups that replace one another have the same valency (for example, Na+ , K+ , NH4+), and heterovalent isomorphism, where their valency differs (for example, Ca2+ and Y3+ ). In the latter case the closeness in size of the atoms that replace one another matters, while the difference in their charges is compensated by vacancies.
Isomorphism is observed in many minerals and crystals and is used in the synthesis of crystals, where the introduction of small additions substantially alters the properties or creates new ones. Thus the introduction of small isomorphous additions, for example Cr3+ into corundum Al2O3 or Nd3+ into the garnet Y3Al5O12, turns them into an active medium for lasers; the introduction of isomorphous impurities into semiconductor crystals changes the type of conductivity. Isomorphous impurities are used, for example, to change the colour of gemstone crystals.