Binary intermetallic phases

The 5d transition elements molybdenum and tungsten supply the crystal lattice of iron with delocalized electrons. This results in strengthening in the interatomic bonding. 

Let us consider the formation of intermetallic compounds for zirconium. Table 13.2 and Table 13.3 show the melting temperatures of binary phases of the 4d series elements. 

The strength of interatomic bonding is expected to be maximal when the atoms have the maximum number of unlike nearest neighbors. The number of unlike nearest atom pairs A-B goes as B(1 — B), where B is the fraction of the second component, with the maximum at 50at.% composition. As one can see from Tables 13.2 and 13.3, this is true for RuZr but is not for ZrPd. The ZrPd3 phase have 

Compound ZrRu ZrRu2 ZrRhs ZrRh ZrPd ZrPd ZrPds 

The heats of formation A H for ordered stoichiometric alloys were determined with self-consistent linear-augmented calculations [53]. For 50 50% alloys of titanium, zirconium, and hafnium with the heavier 4d and 5d elements the agreement between the theory and experiment was of the order of the scatter of the experimental data. For instance, heat of formation was found for the chemical compound of RuZr to be equal to —0.75 eV/atom according to the calorimetric measurements AH = -0.79eV/atom. 

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A.2.2 Nonpolar Binary Intermetallic Phases. Zintl phases are characterized by the presence of markedly heteropolar bonding between the Zintl ions (electronegative polyatomic clusters) and the more electropositive metal atoms. By contrast, the bonding between heteronuclear atoms within other intermetallic compounds is primarily covalent or metallic. A number of different structure types exist for any given... 

Although the presence of Zintl polyanions appears unquestionable in liquid ammonia their separation from this medium as solid products is not possible. The ammonia-free solids obtained by evaporation of metal saturated solutions at low temperatures followed by vacuum treatment are in general binary intermetallic phases and or simple mixtures of the components. 

Figure 7.20 Empirical relationship between the number of intermetallic phases in binary systems and the enthalpy of formation of AB [8].

If the desired catalyst is to consist of two or more catalytic metals after leaching or if a promoter metal is to be included, the precursor alloy becomes even more complicated with respect to phase diagrams. The approximate proportion of reactive metal (aluminum) in these ternary and higher alloys usually remains the same as for the binary metal system for the best results, although the different catalytic activities, leaching behavior and strengths of the various intermetallic phases need to be considered for each alloy system. 

The situation in the solid state is generally more complex. Several examples of binary systems were seen in which, in the solid state, a number of phases (intermediate and terminal) are formed. See for instance Figs 2.18-2.21. Both stoichiometric phases (compounds) and variable composition phases (solid solutions) may be considered and, as for their structures, both fully ordered or more or less completely disordered phases. This variety of types is characteristic for the solid alloys. After a few comments on liquid alloys, particular attention will therefore be dedicated in the following paragraphs to the description and classification of solid intermetallic phases. 

In the previous chapter we looked at some questions concerning solid intermetallic phases both terminal (that is solubility fields which include one of the components) and intermediate. Particularly we have seen, in several alloy systems, the formation in the solid state of intermetallic compounds or, more generally, intermetallic phases. A few general and introductory remarks about these phases have been presented by means of Figs. 2.2-2.4, in which structural schemes of ordered and disordered phases have been suggested. On the other hand we have seen that in binary (and multi-component) metal systems, several crystalline phases (terminal and intermediate, stable and also metastable) may occur. 

Several thousand binary, ternary and quaternary intermetallic phases have been identified and their structures determined. In a comprehensive compilation such as that by Villars and Calvert, about 2200 (first edition, 1985) or about 2700 (second edition, 1991) different types of intermetallic structures are described. The specific data concerning about 17 500 different intermetallic phases belonging to the aforementioned structural types are reported in the first edition, and 26 000 in the second edition. 

Considering then the phase composition as a significant parameter, we obtain the histogram shown in Fig. 7.1(a) for the distribution of the intermetallic phases according to the stoichiometry of binary prototypes. For instance, the binary Laves phases, the A1B2, Caln2, etc., type phases are all included in the number reported for the 66-67.99 stoichiometry range, even if the real stoichiometry of the specific phase is different, see Fig. 7.1(b). We may note the overall prevalence of phases and, to a certain extent, of structural types, which may be related to simple (1 2, 1 1, 1 3, 2 3, etc.) stoichiometric ratios. 

Table 7.1. Approximate distribution of intermetallic phases among the different structural prototypes ( 8000 binary phases considered), according to the data taken as an example in Villars etal. (1995).

The number and variety of intermetallic phases having more complex structure than the simple ones considered in previous paragraphs is very large. The small groups of prototypes here reported are therefore just a few examples of binary (or ternary) phases having odd or very high stoichiometric ratios. 

An elegant access to larger cluster units is to make use of preformed smaller clusters as they occur, e.g., in Zintl phases [8, 9]. The phase KSi (or K4Si4) contains homoatomic Si4 tetrahedra, and thus a salt-like formulation (Na )4[Si4]" with a formal electron transfer is appropriate (Fig. 1) [10]. Such homoatomic anionic building blocks occur in binary or ternary intermetallic phases A E and A Pn j with A being an alkali and alkaline-earth metal, and E = Si-Pb and Pn = P-Bi. These phases are generally available in good quantities, and - since a few members... 

Recently, we and others demonstrated that appropriate germanide Zintl clusters in non-aqueous liquid-crystalline phases of cationic surfactants can assemble well-ordered mesostructured and mesoporous germanium-based semiconductors. These include mesostructured cubic gyroidal and hexagonal mesoporous Ge as well as ordered mesoporous binary intermetallic alloys and Ge-rich chalcogenide semiconductors. 

To identify the intermetallic phases grown, the interplanar distances (d-spacings) found from the X-ray patterns were compared with those calculated from the known literature data on the crystal-lattice parameters. The X-ray patterns from the first two of the four sections were closely comparable and identifiable as the 8 phase of the Ni-Zn binary system. The first section corresponded to a layer composition of 12.1 at. % Ni and 87.9 at. % Zn, while the second to 12.4 at.% Ni and 87.6 at.% Zn. Comparison showed the experimental values to be in better agreement with... 

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