The Packing of Spherical Ions

The atoms in many crystalline metals form close-packed structures similarly, networks of individual molecules at temperatures at which free rotation is allowed may be nearly close-packed structures (for example, HC1, CH4). The close packing of oxide ions or halide ions in compounds 

Translation of ions within crystals is less frequently observed than is rotation. Perhaps one of the most interesting cases is that of silver iodide which may actually be said to melt in halves. When this solid is heated to 145.8° C, the crystal structure then changes and the ionic conductivity increases tremendously the iodide ions are hexagonally closest-packed below the transition temperature but at this temperature they rearrange to form a more open structure, and the silver atoms are allowed to move within the lattice. At 555° C, the network of iodide ions collapses, and the compound becomes a liquid. The solids Cul and Ag2Se show similar behavior. 

if any, real crystals are perfect. Even those with the smoothest faces almost invariably consist of mosaic units 7 (actually ultramicro crystals) whose dimensions reach several thousand Angstrom units. Such units line up snugly to form a crystal, but the fit is not perfect. 

It should not be supposed that crystal defects enter into the picture only as nuisances which the chemist seeks to avoid or eliminate. Actually, certain optical and electrical properties of oxides, sulfides, and halides have been found to depend strongly on the nature and extent of crystal defects. Indeed, semiconductivity, fluorescence (absorption of radiation and emission of less energetic radiation), and phosphorescence (delayed fluorescence) of some salts may be spectacularly increased, not only by a small stoichiometric excess of one of the constituents, but also by addition of very tiny quantities of a foreign ion. Perhaps the best known example is the case of zinc sulfide which, when precipitated from aqueous solution and dried at low temperatures, shows negligible fluorescence upon exposure to ultraviolet light. When the sulfide is heated to 

900° C, however, and then recooled, the ratio of zinc to sulfur is found to increase very slightly ultraviolet irradiation of this sulfur-deficient product produces an intense blue fluorescence. Further, if one thousandth of 1-percent CuS is thoroughly mixed in with the ZnS before heating, irradiation of the cooled product produces a green fluorescence that persists after the radiation has been removed (phosphorescence). 

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