Structure of Simple Binary Ionic Compounds

Structure of Simple Binary Ionic Compounds.—To be electrically neutral, every ionic compound must contain some positive and some negative ions. The simplest ones are those binary compounds that contain one positive and one negative ion. Obviously the positive ion must have lost the same number of electrons that the negative one has gained. Thus monovalent metals form such compounds with monovalent bases, divalent with divalent, and so on. In other words, this group includes compounds of Li+, Na+, K+, Rb+, Cs+, Cu+, Ag+, and Au+ with F-, C1-, Br, I- of Be++, Mg++, Ca++, Sr++, Ba++, Zn++, Cd++, Hg++, with 0—, S—, Se—, Te of B+++, A1+++, Ga+++, In+++, T1+++, with 

P------, As--, Sb---, Bi--. Even this list contains some negative 

The second common structure is the caesium chloride structure. In this structure, ions of one type are located at the corners of a cubic lattice 

The third and fourth structures are sometimes called the zincblende and the wurtzite structures, on account of two forms of ZnS. The zinc-blende structure is also often called the diamond structure, since it is found in diamond and some other crystals. The fundamental features of both structures are similar each ion is tetrahedrally surrounded by four ions of the opposite sign, as in Fig. XXIII-3 (a). There arc a number of ways of joining such tetrahedra to form regular crystals, however. The diamond, or zincblende, lattice is the simplest of these. In the first place, tetrahedra like Fig. XXIII-3 (a), can be formed into sheets like 

XXIII-3.—The zincblende and wurtzite structures, (a) one ion tetra-hedrally surrounded by four others. (6) and (c) sheets formed from such tetrahedra, in perspective and plan, (d) the zincblende structure (which is identical with the diamond structure if all atoms are alike), (e) the wurtzite structure. 

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