Zintl phase compounds

This approach to the formation of metalloid molecular cluster compounds shows clearly the difference lfom Zintl-like phases which lately have been successfully... 

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. 

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),... 

Zintl compounds to be mentioned in other chapters are compounds or phases that have anions such as Sng-, Sn2Bi2 or Pb1. The alkali metal salts can often be isolated crystalline by complexation of the cation with crown ethers or cryptands. Recent examples are Na jSn16 and Li2Ba4Si6, the latter having a SiJ0- ring.17 Saline hydrides were discussed in Chapter 2. 

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. 

Reactions of metal carbonyls with Zintl phases do not always yield the desired result. An example is the reaction of KSi with the metal carbonyls M(CO)6 (M = Cr, Mo, W) and the chromium complex Cr(CO)sNMe3. Instead of forming a structure with a Si4 anion, KSi reduces these transition metal compounds to form the anions, [M2(CO)io] . In addition, certain organometallic transition metal clusters have been synthesized that contain Zintl anions coordinated to the metal, but Zintl compounds are not used as the reagents. 

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. ... 

After having discussed the three approaches, let us now turn to the results of the calculations. The values for AEband were calculated for all binary B32 type Zintl compounds and for the B2 type phase LiTP >. In the perturbation approach ll and the ASA method ) AEband always favours the B32 type of structure (also in case of the B2... 

Table 15. Ratio of the s partial charge densities q / Ep) to the total partial densities qy(Ep) for electronic states at the Fermi level Ep and Knight shifts K, (in %) for intermetallic Zintl compounds AB The calculated direct contribution K - and the total spin contribution K< are listed. For comparison the Knight shift found experimentally in the Zintl (K ) phases and in the pure metals (KJ" ) are given too. For the AB compounds the theoretical values are shown for both the defect (de) and the stoichiometric (st) phases...

Zintl phases have always been understood within the framework of an elegant and simple scheme, the so-called Zintl-Klemm concept. However, only Zintl compounds with semiconducting properties were assumed to fulfil this concept. In the last decade, important steps have been taken towards an expansion of the Zintl-Klemm concept into the metallic regime while still separating Zintl compounds quite clearly from intermetallic phases. This is due to the fact that in intermetallics the clearcut separation of components at which valence states are centered and those at which they are not, is lost. 

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... 

We shall first review the basic principles of VASP and than describe exemplary applications to alloys and compounds (a) the calculation of the elastic and dynamic properties of a metallic compound (CoSi2), (b) the surface reconstruction of a semiconducting compound (SiC), and (c) the calculation of the structural and electronic properties of K Sbi-j, Zintl-phases in the licpiid state. 

The performance of VASP for alloys and compounds has been illustrated at three examples The calculation of the properties of cobalt dislicide demonstrates that even for a transition-metal compound perfect agreement with all-electron calculations may be achieved at much lower computational effort, and that elastic and dynamic properties may be predicted accurately even for metallic systems with rather long-range interactions. Applications to surface-problems have been described at the example of the. 3C-SiC(100) surface. Surface physics and catalysis will be a. particularly important field for the application of VASP, recent work extends to processes as complex as the adsorption of thiopene molecules on the surface of transition-metal sulfides[55]. Finally, the efficiciency of VASP for studying complex melts has been illustrate for crystalline and molten Zintl-phases of alkali-group V alloys. 

The chemistry of silicon in very low oxidation states is one of the most fascinating research areas, which can be located between molecular compounds of silicon and elemental (perhaps amorphous) silicon [190-194]. Most interesting results have recently been obtained by structural investigations of siliddes in Zintl phases. However, compounds of silicon with negative oxidation states and very low coordination numbers of 1, 2, and 3 are so far only known in the composite of a crystal lattice. 

There seem to be many binary metallic systems in which there are phases of this sort. In the sodium-lead system there are two such phases. One of them, based on the ideal structure Na3Pb, extends from 27 to 30 atomic percent lead, with its maximum at about 28 atomic percent lead and the other, corresponding to the ideal composition NaPb3, extends from 68 to 72 atomic percent lead, with maximum at about 70 atomic percent. The intensities of X-ray reflection have verified that in the second of these phases sodium atoms occupy the positions 0, 0, 0, and the other three positions in the unit cell are occupied by lead atoms isomorphously replaced to some extent by sodium atoms (Zintl Harder, 1931). These two phases are interesting in that the ranges of stability do not include the pure compounds Na8Pb and NaPb3. 

In particular, we have found unusual magnetic properties within a series of Eu-ln-P compounds that we have recently synthesized Eu3lnP3, Eu3ln2P4, and Euln2P2 [24—26]. The first two can be described as classical Zintl phases and the third shows semi-metaUic properties. EuIn2P2 may also be a Zintl phase with the semi-metaUic properties attributed to adventitious crossing of the valence band with the conduction band. 

Temperature-dependent resistivity data (In p vs 1/T) for both Eu3lnP3 and Eu3ln2P4 are shown in Pig. 11.3 and indicate that they are semiconductors. The room-temperature resistivities are on the order of 1-100 cm. Band gaps were determined by fitting the data from about 130-300 K to the relationship. In p= Eg/ Ik T + f, providing a band gap. Eg, of approximately 0.5 eV for both samples. Since these two compounds can be rationalized as electron-precise Zintl phases, semiconducting behavior is expected. 

Regardless of their possible metallic properties, metal-rich Zintl system or phases are defined here as cation-rich compounds exhibiting anionic moieties of metal or metalloid elements whose structures can be generally understood by applying the classical or modern electron counting rules for molecules. 

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