Zintl phase

The major use of sodium cyanamide is in the production of sodium cyanide, a compound that is used extensively in preparing solutions for the electroplating of metals. Another use for NaCN is in extraction processes employed to separate gold and silver from ores as a result of their forming complexes with CN . Sodium cyanide, an extremely toxic compound, is also used in the process known as case-hardening of steel. In this process, the object to be hardened is heated and allowed to react with the cyanide to form a layer of metal carbide on the surface. 

Some of the important compounds containing the group IA and IIA metals are the carbonates, nitrates, sulfates, and phosphates. We have already mentioned the mineral trona as the source of sodium carbonate. Calcium carbonate is found in many forms that include chalk, calcite, aragonite, and marble, as well as in egg shells, coral, and seashells. In addition to its use as a building material, calcium phosphate is converted into fertilizers in enormous quantities (see Chapter 14). 

The carbonates, sulfates, nitrates, and phosphates of the group IA and IIA metals are important materials in inorganic chemistry. Some of the most important compounds of the group IA and IIA elements are organometallic compounds, particularly for lithium, sodium, and magnesium, and Chapter 12 will be devoted to this area of chemistry. 

One of the most common techniques for preparing Zintl phases is by the reaction of a solution of the alkali metal in liquid ammonia with the other element. However, many of these materials are obtained by heating the elements. For example, heating barium with arsenic leads to the reaction 

Numerous Zind anions are formed by selenium and tellurium, with some of the more prominent species being Se 2 (where n 2, 4, 5, 6, 7, 9, or 11). The species with n = 11 contains two rings that have five and six members that are joined by a selenium atom. Those with smaller numbers of selenium atoms generally consist of zigzag chains. Tellurium forms an extensive series of polyanions that are present in such species as NaTe (n = 1 to 4). One tellurium anion contains the Hg4Te124 ion, but other species such as [( lg2Tes) 2 are also known, such as the Te122 anion that is present in some cases where the cation is a +1 metal. 

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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 structural and electronic properties of K-Sb Zintl phases in the crystalline and molten states... 

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. 

In many systems, both single-phase and polyphase behaviors are found in different composition ranges. Intermediate, as well as terminal, phases often have been found to have quite wide ranges of composition. Examples are the broad Zintl phases found in several of the binary lithium systems studied by Wen [29]. 

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. 

Schmidt PC (1987) Electronic Structure of Intermetrallic B 32 Type Zintl Phases. 65 91-133 Schmidt W (1980) Physiological Bluelight Reception. 41 1-44... 

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. 

Chemistry, Structure, and Bonding of Zintl Phases and Ions Kauzlarich, S.M.,... 

S.M. Kauzlarich in Chemistry and Bonding of Zintl Phases and Ions (Ed. S.M. Kauzlarich), VCH Publishers, New York, 1996 p. 245 and the references therein. 

Schmidt, P. C. Electronic Structure of Intermetallic B 32 Type Zintl Phases. Vol. 65, pp. 91-133. 

Polyanionic and Poly cationic Compounds. Zintl Phases... 

As we can see from the last entry in this table, we have deduced only a rule. In InBi there are Bi-Bi contacts and it has metallic properties. Further examples that do not fulfill the rule are LiPb (Pb atoms surrounded only by Li) and K8Ge46. In the latter, all Ge atoms have four covalent bonds they form a wide-meshed framework that encloses the K+ ions (Fig. 16.26, p. 188) the electrons donated by the potassium atoms are not taken over by the germanium, and instead they form a band. In a way, this is a kind of a solid solution, with germanium as solvent for K+ and solvated electrons. K8Ge46 has metallic properties. In the sense of the 8-A rule the metallic electrons can be captured in K8Ga8Ge38, which has the same structure, all the electrons of the potassium are required for the framework, and it is a semiconductor. In spite of the exceptions, the concept has turned out to be very fruitful, especially in the context of understanding the Zintl phases. 

The octet principle, primitive as it may appear, has not only been applied very successfully to the half-metallic Zintl phases, but it is also theoretically well founded (requiring a lot of computational expenditure). Evading the purely metallic state with delocalized electrons in favor of electrons more localized in the anionic partial structure can be understood as the Peierls distortion (cf. Section 10.5). 

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