Intermetallic structures examples

In another chapter concerning the intermetallic crystallochemistry (Chapter 7), a number of selected structural prototypes are described presenting some of their typical features and commenting on their distribution among different types of alloys. Attention is especially given to relationships between different prototype structures, and examples of their possible grouping in structural families are underlined. This chapter, therefore, could possibly be used as a first draft of a gazetteer of intermetallic structure types and could be considered as an introduction (partial and provisional indeed ) to the descriptive systematics of intermetallic crystal chemistry. 

Characteristic for the series, including pseudo-ternary compounds, is the change in unit cell volume when varying the T elements. Some pertinent data are summarized in table 13. This change in unit cell volume has strong effects on the electronic structure of these intermetallics. For example, CeNiSn does not order magnetically, as proven in particular by (xSR data, while CePtSn and CePdSn show AFM order with transition temperatures... 

Tin-lead and lead-free solder alloys exhibit different bulk solder nucrostructures. Tin-lead alloys exhibit distinct Sn- and Pb-rich grains the majority of tin-based lead-free solder alloys exhibit intermetallic structures within the tin-matrix. The intermetallic morphology is not in lamellae form and may be round, plate-like, blocky, and needle-like structures. Examples of the deep etched microstructure of lead-free solder alloys follow. 

The mean-field expression for self-diffusion coefficients has been extended to several LRO intermetallic structures by Bakker and Westerveld (1988). In this case, one has to take into account jumps within each sublattice as well as between different sublattices. For example, for the f.c.c. LI2 structure A3B, assuming near-neighbor jumps, one has... 

Another Example AuSn.—From among the many other intermetallic compounds which might be used as a second illustration, AuSn is chosen to show how the consideration of metallic valence and use of the radii contribute to the explanation of the choice of a suitable structure by a compound. 

The inclusion of both covalent and intermetallic crystals in Chapter 6 is predicated on the close relation between the covalent and metallic bonds, as discussed in Chapter 3. SP 54 and SP 55 are beautiful examples of the complexity of the atomic packing and bonding arrangements in alloy structures, which fascinated Pauling. 

The general geometrical problem of the packing of spheres has not been solved. An example of closest packing of atoms with some variation in effective radius is the icosahedral packing found (13) in the intermetallic compound Mg3B(Al,Zn) (Fig. 1). The successive layers in this structure contain 1, 12, 32, and 117 spheres. These numbers are reproduced (to within 1) by the empirical equation (12)... 

The most straightforward synthesis of compounds (L)AuAr uses the metathesis of (L)AuX precursors with aryllithium reagents, as, for example, executed for the preparation of (Ph3P)AuPh. The crystal structure of this benchmark complex has been determined. The linear coordination geometry is as expected. No aurophilic contacts are discernible in the crystal packing. Short Au- -Au contacts are observed, however, in the dinuclear compound (dppm)(AuPh)2 with an intramolecular intermetallic distance of 3.154(1) A. This complex shows a UV-VIS absorption at 290-300 nm and is luminescent in fluid solution at room temperature.1... 

Where might one find new AC/QC materials Our earlier extensions of the p-element components in Zintl-type compounds to the triels (Al, Ga, In, Tl) (beyond the so-called Zintl boundary ) as well as the inclusions of late transition metals had already given us some significant glimpses of new and appropriate chemistry and structures [46,51]. In addition, some examples were already in the literature for Ga [44], In contrast, the anion or intermetallic chemistry of aluminum [45] stands apart from that of the three heavier group members (Ga, In, and Tl) and will not be considered here significantly. 

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. 

More generally, in many cases of intermetallic compounds, unlike a high number of covalent compounds (compare for instance with the illustrative example of a carbon atom in the diamond structure), we cannot speak of bonds of an atom directed to (and saturated with) a well-defined group of atoms. 

Nowotny phases, chimney-ladder structures. The Nowotny chimney-ladder phases are an example in alloy field chemistry of composite structures. They are a series of intermetallic T X , compounds formed by transition metals T from the 4th to 9th groups with p-block elements X from the 13th to the 15th groups. 

Comments on some trends and on the Divides in the Periodic Table. It is clear that, on the basis also of the atomic structure of the different elements, the subdivision of the Periodic Table in blocks and the consideration of its groups and periods are fundamental reference tools in the description and classification of the properties and behaviour of the elements and in the definition of typical trends in such characteristics. Well-known chemical examples are the valence-electron numbers, the oxidation states, the general reactivity, etc. As far as the intermetallic reactivity is concerned, these aspects will be examined in detail in the various paragraphs of Chapter 5 where, for the different groups of metals, the alloying behaviour, its trend and periodicity will be discussed. A few more particular trends and classification criteria, which are especially relevant in specific positions of the Periodic Table, will be summarized here. 

Thermochemistry of cluster compounds. In this short summary of cluster structures and their bonding, a few remarks on their thermochemical behaviour are given, in view of a possible relationship with the intermetallic alloy properties. To this end we remember that for molecular compounds, as for several organic compounds, concepts such as bond energies and their relation to atomization energies and thermodynamic formation functions play an important role in the description of these compounds and their properties. A classical example is given by some binary hydrocarbon compounds. 

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