Liquid intermetallic phases

Another characteristic point is the special attention that in intermetallic science, as in several fields of chemistry, needs to be dedicated to the structural aspects and to the description of the phases. The structure of intermetallic alloys in their different states, liquid, amorphous (glassy), quasi-crystalline and fully, three-dimensionally (3D) periodic crystalline are closely related to the different properties shown by these substances. Two chapters are therefore dedicated to selected aspects of intermetallic structural chemistry. Particular attention is dedicated to the solid state, in which a very large variety of properties and structures can be found. Solid intermetallic phases, generally non-molecular by nature, are characterized by their 3D crystal (or quasicrystal) structure. A great many crystal structures (often complex or very complex) have been elucidated, and intermetallic crystallochemistry is a fundamental topic of reference. A great number of papers have been published containing results obtained by powder and single crystal X-ray diffractometry and by neutron and electron diffraction methods. A characteristic nomenclature and several symbols and representations have been developed for the description, classification and identification of these phases. 

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. 

General characteristics of alloys such as those presented in Fig. 3.3 have been discussed by Fassler and Hoffmann (1999) in a paper dedicated to valence compounds at the border of intermetallics (alkali and alkaline earth metal stannides and plumbides) . Examples showing gradual transition from valence compounds to intermetallic phases and new possibilities for structural mechanisms and bonding for Sn and Pb have been discussed. Structural relationships with Zintl phases (see Chapter 4) containing discrete and linked polyhedra have been considered. See 3.12 for a few remarks on the relationships between liquid and amorphous glassy alloys. 

Structural relations between quasicrystals and other intermetallic phases. As discussed in several sections of the review published by Kelton (1995) on quasicrystals and related structures, numerous studies and observations indicate structural similarities between non-periodic quasicrystal phases with crystalline phases and also, on the other hand, with amorphous, glassy and liquid phases. 

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. 

Fig. 2.16. Free-energy diagram for the Ni/Zr system at 300°C showing the free energy of mixing as a function of composition for the hep, bcc, fee, and liquid/amorphous phases. Intermetallic compounds are shown by dots and are assumed to be line compounds. The common tangents for the hep/liquid, and liquid/fee metastable equilibrium are shown. These define compositions XLXj.X3.X4 [2.76]...

A number of the intermetallic phases formed between the alkali metals and the heavier posttransition elements (Sn, Pb, As, Sb, Bi, Se, Te) have the very remarkable property of dissolving in liquid ammonia to form very intensely colored solutions. The most extensive characterization of these solutions has been by Zintl and co-workers, who by potentiometric titrations established the apparent formation of polyatomic ions such as Sn -, Bif, and Tel-.1,2 Unfortunately, on evaporation of solvent these salts always revert to the more stable intermetallic phases, for example,... 

Occasionally, a pure component (a) phase may exhibit properties markedly different from those of the intermetallic phase which is vicinal to it on the constitutional diagram. Thus, the a-phase may dissolve more readily in a solvent or it may be attacked more readily by a reagent. In such cases it may be possible to use an excess of the pure component during the high-temperature synthesis and then liberate the intermetallic product by leaching out the matrix phase. Occasionally, also, slow cooling of the alloy melt may yield well-formed crystals of the intermetallic phase embedded in the pure component matrix, which may then be removed by some solvent. Dependii on circumstances, the matrix phase may be removed by electrolytic oxidation, by aqueous acids, by bases, or by liquid NHa. For example ... 

The solubility of alkaline earth metals in molten alkali metals is limited. The elements of the third group of the periodic table of elements are still less soluble in alkali metals, only the aluminium-lithium system is an exception. This system contains two intermetallic phases, LiAl and the peritectic phase Li9Al4, and solid alloys of both metals are of technical importance. The metals of the fourth group have a better solubility in alkali metals. They tend to form intermetallics. Some of the liquid alloys have found technical interest. 

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. 

Zintl, E., J. Goubeau, and W. Z. Dullenkopf. Metals and alloys 1. Salt-hke compounds and intermetallic phases of sodium in liquid atmnonia. Z. Phys. Chem., Abst. A 154 (1931) 1. 

---