Zintl layered

The metaUic GasIngSrig can be described a an intergrowth of Zintl and metalHc layers, where the valence states of the Zintl layers lie deep below the Fermi level. The excellent metalHc behavior of Ga5ln9Sng was attributed to the well-dispersed electronic states of the intermetallic layers that dominate the Fermi level. 

Indium clusters have also recently been characterized, notably in intermetallic compounds. Thus, the Zintl phase, Rbzinj, (prepared by direct reaction between the two metals at I530°C) has layers of octahedral closo-lnf, clusters joined into sheets through exo bonds at four coplanar vertices. These four In atoms are therefore each bonded to five neighbouring In atoms at the comers of a square-based pyramid, whereas the remaining two (Irans) In atoms in the Ine cluster... 

The compound LisZn2Ge3 can be expressed as SLi , 2Zn, SGe in terms of oxidation numbers, or as 8 Li , 2 Zn, 3 Ge with regard to the Zintl-Klemm concept and valence considerations. It closely relates to Li5AlSi2 and the corresponding [Li3Al3Si 5] hexagonal layer in which Zn and Ge would play the role of A1 and Si. With the Zn vacancy compensated by a Hthium dumbbell, the [Li4ZrL4Gei5] anionic network is isoelectronic. 

Difluorides such as PbF2 with the fluorite structure exhibit fast ion conduction due to facile F ion transport (Section 6.4.5). An interesting structure showing Li" conduction is that of LijN (Rabenau, 1978). Conduction is two-dimensional. Cooperative basal plane excitations involving the rotation of six Li ions by 30 about a common ion to edge positions (positions midway between ions in the Li2N layer) seem to be responsible for conduction in this nitride. In the fluorite structure, a rotation by 45 of a single cube of F ions seems to be involved. The Zintl alloy LiAl is also a lithium-ion conductor. 

The Structural principles of the elements can also be found in a number of ternary Zintl phases. For example, KSnSb contains (SnSb ) layers as in a-arsenic. In other ternary ZiNTL phases the anionic part of the structures resembles halo or oxo anions or molecular halides. For example, in Ba4SiAs4 there are tetrahedral SiAs particles that are isostructural to SiBr4 molecules. In Ba3AlSb3 dimeric groups Al2Sbg2 are present, with a structure as in A Clg molecules (Fig. 13.4). Ca3AlAs3 contains poly-... 

The steady, gradual variation of the P-P distance would seem to be as inconsistent with the molecular orbital model shown here as it was with the Zintl concept. This is not so. If we turn on the interaction between the P atoms and the metal layer (and we have seen before that this interaction is substantial), we will get a mixing of P and Mn orbitals. The discontinuity of the above picture (either single-bond or no bond) will be replaced by a continuous variation of P o and o orbitals occupation between 2 and 0. 

In the structure we see two associated square nets of M and B atoms, separated by a square net layer of A s. The A layer is twice as dense as the others, hence the MAB stoichiometry. Most interesting, from a Zintl viewpoint, is a consequence of that A layer density, a short A - A contact, typically 2.5 A for Si. This is definitely in the range of some bonding. There are no short B- -B contacts. 

In addition, the reactivity of Zintl ion solntions with layered materials have been reported. Solntions of IQSng in enrednce graphite and Hffe the potassium is intercalated into the structure and tin metal is deposited on the edges of the layered... 

At very negative potentials neither the tetraalkylammonium ions nor the metallic electrode are inert they combine to form reduced TAA-metals [7]. Tetraalkylammonium (TAA) metals are composed of quaternary ammonium ions, electrons, and a post-transistion metal such as Hg, Pb, Sn, Sb, Bi [5-18] or Pt [19] most of them have the composition R4N" MeJ [13] or R4N" Mc4 [20] and have been described as Zintl ion salts or Zintl phases [21,22]. They have been shown to be useful intermediates in the electrochemical reduction of certain substrates that are reducible with difficulty. On reduction of the quaternary ammonium salt, the initial layer of the metal compound is controlled by a two-dimensional nucleation, whereas the bulk phase is initiated by a three-dimensional nucleation and a growth controlled by the diffusion of R4N from the solution. In some cases (A-methylquinuclidinium (MQ" ) mercury) the catalytic efficiency of the initial layer is greater than that of the bulk phase [18], whereas in other cases (A, A-dimethylpyrrolidinium (DMP" ) lead) the opposite is found [16]. 

A unique reaction between a transition metal anion and tin (II) chloride yielded the first octahedral Zintl anion, Sni . The synthesis involves layering the filtered THF solution with diethyl ether and hexane, combining the reagents (a), and then slowly crystallizing the products. Tin(O) precipitates, and the other two products are isolated as crystals in low yield. In the Zintl complex the Cr(CO)s units form a protective layer around the central Sne octahedron. This reaction serves to indicate that low-temperature, solution state routes may be used to obtain Zintl anions, but only under very selective circumstances. 

The Zintl phase CaSi2, with its puckered (Si )n polyanion layers, has been the starting point for an interesting development, which began as long ago as in 1863 with a contribution from F. Wohler in a topochemical reaction with... 

Chemical Transformation of the Layered Zintl Phases of Ca with Si and Ge... 

Table 15.1. Compilation of the Ca Zintl phases with layered polyanions and of typical sheet polymers based on an sp hybridized backbone for the different group IV atoms. The arrows indicate the route for obtaining the polymers.

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