Intermetallic prototypes

A SELECTION OF MORE COMMON INTERMETALLIC PROTOTYPES HAVING MORE COMPLEX STOICHIOMETRIES AND STRUCTURES... 

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. 

Notice that most of the indicated prototypes correspond to structures frequently found not only in intermetallic phases but also in ionic compounds. 

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).

Introduction. A number of common structures, ideally corresponding to a 1 1 stoichiometry, are presented in this chapter. Some of them are not specifically characteristic of intermetallic compounds only. The CsCl and NaCl types, for instance, are observed for several kinds of chemical compounds (from typical ionic to metallic phases). Notice that for a number of prototypes a few derivative structures have also been considered and described, underlining crystal analogies and relationships even if with a change in the reference stoichiometry. 

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. 

Table 5.4 The most important families of hydride forming intermetallic compounds including the prototype and the structure.

As mentioned above in the intermetallic Section, beta-tungsten, which is chemically WsO, is the prototype of the A-15 structure. The interest in WsO, or Ws01 x, is not only structural but is also based on the fact that this material is superconducting Further surprising is that, for the first time, this oxide superconductor has a higher transition temperature than that of the metal itself. Pure tungsten metal has a Tc of 15.4 mK, whereas the oxide WsO has a reported Tc of 3.35 K. Other oxide compounds such as CrsO and "MosO", which are isostructural with WsO, do not superconduct above 1.02 K. 

Because of their high volumetric density, intermetallic hydrides are also used as hydrogen storage materials in advanced fuel cell driven submarines, prototype passenger ships, forklifts and hydrogen automobiles as well as auxiliary power units for laptops. 

Hydrogen Storage Materials (Solid) for Fuel Cell Vehicles, Table 1 The most important families of hydride forming intermetallic compounds including the prototype and the structure. A is an element with a high affinity to hydrogen, and B is an element with a low affinity to hydrogen... 

As already mentioned, ternary scandium intermetallics often crystallize in structure types which are superstructures to binary prototypes (table 33, Appendix A). It seems convenient to us to select structure types among them which occur in large groups of elements and their combinations, see table 34. These structure types as well as their binary prototypes are frequently observed among all intermetallics having up to hundreds of known representatives. The atomic size factor has a dominant influence on distribution of atoms in ternary phases. The largest atoms in the structure normally occupy atomic... 

Packing of spheres Polymorphism Alloys and intermetallic compounds Band theory Semiconductors Sizes of ions Prototype structures Lattice energy Born-Haber cycle Defects in solid state structures... 

In the case of beryllium, the phase diagrams contain at least one intermetallic compound which has the same stoichiometry (RBeu and AnBen) and the same structure (prototype NaZnu). In the Sc-Be system other intermetallic compounds have been detected. 

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