Ternary compounds Zintl phases

The paradigm of how molecular-based Zintl compounds can be used to make porous materials has been demonstrated. The potential of Zintl phases to act as starting materials for the synthesis of a wide variety of open frameworks is enormous because of the broad diversity of such phases. Not only Zintl anions such as Eg and E4 (E = Ge, Si, Sn, Pb), Asy, E 14, Sby, and Inn composed of single elements but also binary and ternary ones, such as SnxE9 x" (E = Ge, Pb),... 

The NaTl-type structure is the prototype for Zintl phases, which are inter-metallic compounds which crystallize in typical non-metal crystal structures. Binary AB compounds LiAl, LiGa, Liln and Naln are both isoelectronic (isovalent) and isostructural with NaTl. In the Li2AlSi ternary compound, A1 and Si form a diamond-like framework, in which the octahedral vacant sites of the A1 sublattice are filled by Li atoms, as shown in Fig. 13.7.2(b). 

Ternary Zintl phases that incorporate arsenic are exemplified by Ba4 SiAs4 (s ee Zintl Compounds). The material is made of tetrahedral SiAs4 units whose charge is compensated for by the Ba cations. 

Further examples of formally subvalent main group compounds that contain element-element bonds but not necessarily clusters are the Zintl phases. The bonding in these has been described as the octet rule for all atoms . The archetypal Zintl compound is NaTl, in which charges are assigned as Na+ and Tl, representing a formal transfer of electrons from the more to the less electropositive element. The Tl ion can be considered to be a group 14 pseudoelement, and in fact exists in NaTl as a three-dimensional polyanionic diamond framework (TN) stuffed with Na+ cations. The Zintl concept is extended more broadly to other binary and ternary solid-state compounds, whose structures show the formation of element-element bonds in one, two, or three dimensions. ... 

The present work will only consider the electronic properties of those Zintl phases which crystallize in the B32 structure. Binary B32 Zintl phases are formed by the AB compounds LiAl, LiGa, Liln and Naln, by the prototype NaTl, and by the AB compounds LiZn and LiCd. Ternary systems, which crystallize in this structure, are found for alloys ABi-,Cx, for which at least one boundary compound AB or AC crystallizes in the NaTl type of structure. 

The arrangement of the atoms in the NaTl structure and its relation to some other simple cubic structures is illustrated in Fig. 1 and Table 1. In Fig. la a unit cell is shown with four different lattice sites A to D. The A2 structure results if all four sites are occupied by the same kind of atoms or statistically by different atoms. For AB compounds the CsCl structure results for the configurations Aa, Ab, Bq and Bp (cf. Fig. lb). The NaTl structure follows from Aa, Bg, Ac and Bp as displayed in Fig. Ic. For ternary Zintl phases A2BC one finds that either the B and C atoms are statistically distributed on the B and D sites given in Fig. la (B32A structure) or they are ordered and the XA type of structure results 5 (cf. Table 1). 

Finally ternary systems shall be considered. The structure of a large number of ternary systems of the composition LiBi-xC, has been investigated " 5 1. For those systems, for which both boimdary phases of the quasibinary intersection LiB and LiC are B32 type compounds, the B32A structure is found within the whole range of composition 0 < X < 1. The lattice constants vary linearly as a function of x (Vegards law). For those compounds, for which either LiB or LiC does not crystallize in the B32 structure, the XA structure is often realized. For these compounds LiEj-xC, Zintl phases are only detected for the range of composition, in which the valence electron concentration remains in the range of 1.5 Cve 2.0. 

Since the X-ray diffraction studies of Zintl et al. , these members of the family of intermetallic compounds have been of special interest because some of their chemical properties are unusual for intermetallic phases. Many experimental investigations have been reported for binary and ternary B32 type compounds. Besides the crystal structure " , the thermodynamic behavior , electrical conductivity ", magnetic susceptibility , NMR data elastic constants - and optical properties have been studied. Additionally for LiAl electrochemical investigations have been performed in view of the recent interest in fast ionic conductors " . ... 

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