Zintl phases an introduction

1 Zintl phases an introduction. The Zintl phases are a large number of substances which in a way may represent a bridge, a transition, between the ionic and the intermetallic compounds. The expression Zintl Phases was introduced by Laves (1941) in recognition of the work carried out by E. Zintl. 

A comprehensive book on chemistry, structure and bonding of Zintl compounds has been edited by Kauzlarich (1996) in this book several aspects of Zintl phases such as structural patterns, molecular transition complexes of Zintl ions, transition metal Zintl compounds are deeply discussed. 

A traditional example of a Zintl phase is represented by NaTl which may be considered as a prototype of the Zintl rules. The structure of this compound (face centred cubic, cF16, a = 747.3 pm) can be described (see also 7.4.2.2.) as resulting from two interpenetrating diamond type lattices corresponding to the arrangements of the Na and T1 atoms respectively (Zintl and Dullenkopf 1932). Each T1 atom therefore is coordinated to other four T1 at a distance a)3/4 = 747.3)3/4 = 323.6pm which is shorter than that observed in elemental thallium (d = 341-346 pm in aTl, hP2-Mg type, CN = 6 + 6) and d = 336pm in /3 Tl, (cI2-W type, CN = 8). 

As an indication, however, of the limits of the Zintl interpretation and with reference to the prototypal NaTl structure, notice that the description of the filled tetrahedral structures given by Zintl cannot be considered valid generally (with all the elements). An example is the occurrence of this structure in compounds such as LiZn or LiCd in which the diamond-like framework of Zn and Cd atoms cannot obtain the four electrons necessary to give sp3, hybrids. 

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