A class of crystals in which the lattice-site occupants are charged ions held together primarily by their electrostatic interaction. Such binding is called ionic binding. Empirically, ionic crystals are distinguished by strong absorption of infrared radiation, good ionic conductivity at high temperatures, and the existence of planes along which the crystals cleave easily. See Crystal structure
Compounds of strongly electropositive and strongly electronegative elements form solids which are ionic crystals, for example, the alkali halides, other monovalent metal halides, and the alkaline-earth halides, oxides, and sulfides. Crystals in which some of the ions are complex, such as metal carbonates, metal nitrates, and ammonium salts, may also be classed as ionic crystals.
As a crystal type, ionic crystals are to be distinguished from other types such as molecular crystals, valence crystals, or metals. The ideal ionic crystal as defined is approached most closely by the alkali halides (see illustration). Other crystals often classed as ionic have binding which is not exclusively ionic but includes a certain admixture of covalent binding. Thus the term ionic crystal refers to an idealization to which real crystals correspond to a greater or lesser degree, and crystals exist having characteristics of more than one crystal type.
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Ionic crystals, especially alkali halides, have played a very prominent role in the development of solid-state physics. They are relatively easy to produce as large, quite pure, single crystals suitable for accurate and reproducible experimental investigations. In addition, they are relatively easy to subject to theoretical treatment since they have simple structures and are bound by the well-understood Coulomb force between the ions. This is in contrast to metals and covalent crystals, which are bound by more complicated forces, and to molecular crystals, which either have complicated structures or are difficult to produce as single crystals. Being readily available and among the simplest known solids, they have thus been a frequent and profitable meeting place between theory and experiment. These same features of ionic crystals have made them attractive as host crystals for the study of crystal defects: deliberately introduced impurities, vacancies, interstitials, and color centers. See Color centers, Crystal defects
Most ionic crystals have large band gaps, and are therefore generally good electronic insulators. However, electrical conduction occurs by the motion of ions through these crystals. The presence of point defects, that is, deviations from ideal order in the crystalline lattice, facilitates this motion, thus giving rise to transport of electric charge. In an otherwise perfect lattice where all lattice sites are fully occupied, ions cannot be mobile.
Many so-called normal ionic crystals possess conductivities of about 10-10 (ohm · cm)-1 or lower at room temperature. However, a relatively small number of ionic materials, called superionic conductors or fast ionic conductors, display conductivities of the order of 10-1 to 10-2 (ohm · cm)-1, which imply ionic liquidlike behavior. In most of these crystals, only one kind of ionic species is mobile, and its diffusion coefficient and mobility attain values such as found otherwise only in liquids. Due to their high value of ionic conductivity as well as their ability to selectively transport ionic species, superionic conductors have successfully been employed as solid electrolytes in many applications. See Diffusion
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