Crystal structure of intermetallics

A similar procedure was also used by Villars to find atomic property expressions which could be used to distinguish the crystal structures of intermetallic compounds 182 sets of tabulated physical properties and calculated atomic properties were considered. These were combined, for binary phases, according to the modulus sums, differences and ratios. The best separations were obtained by using three-dimensional maps, which, for a binary AVB,., x [Pg.309]

P. Villars, Crystal Structures of Intermetallic Compounds, Factors Governing Crystal Structures, in J.H. Westbrook, R.L. Fleischer (Eds.), Wiley, Chichester, 2000, p. 1. 

For a comprehensive earlier review of the physical properties, compositions and crystal structures of intermetallic compounds formed between rare-earth elements and non-magnetic metals, including NMR applications, see Buschow (1979). 

Figuie4J0. Some simple crystal structures of intermetallic compounds. 

Fig. 2.3 Crystal structures of intermetallic clathrates represented as packing of cage polyhedrons 20-atom dodecahedron blue), 24-atom tetrakaidecahedron (green), 26-atom pentakai-decahedron (red) and 28-atom hexakaidecahedron (yellow). The example compositions for different clathrate families (hydrate, cationic, and anionic) are given for comparison...

Metals frequently form intermetallic compounds, particularly when the individual pure metals have different structures. Intermetallic compounds exhibit a more (Cu-Mg) or less (Cu-Zn) narrow range of homogeneity. Metallic bonding is the predominant type of bonding in many intermetallic compounds. However, a certain fraction of the other types of bonding must also be taken into consideration. This partially non-metallic character of the bonds, as well as the different sizes of the atoms, is responsible for the large variety of crystal structures of intermetallic compounds. Only some of the most important groups will be mentioned here. 

Details regarding the crystal structure of intermetallic compounds based on rare earth elements can be found in Volume 2 of the Handbook on the Physics and Chemistry of Rare Earths (landelli and Palenzona, 1979). Lattice constants and structure type of these intermetallics have furthermore been compiled in the two reviews published by Buschow (1977a, 1979). As will be shown in the following sections, many investigations of hydrogen sorption in intermetallics pertain to compounds formed between rare earth elements and 3d transition elements. 

Krypyakevych (1977) has proposed the systematics of structure types of intermetallic and related compounds based on the analysis of the coordination polyhedra of the smallest atom. This author distinguishes 17 groups to which the coordination polyhedra of the smallest atoms belong. In analyzing the crystal structures of intermetallic compounds some problems, connected with choice of the smallest atom, arise. In the case of the metal compounds with boron, carbon, silicon, and phosphorus the nonmetal element is always the smallest. Kuz ma (1983) successfully used this concept for the analysis of the structure types of borides. In analyzing the structure types of the binary and ternary phosphides we will hold to the systematics developed by Krypyakevych (1977) also because the CPs of phosphorus atoms are given in the figures. 

Lea] Leake, J.A., The Refinement of the Crystal Structure of Intermetallic Phase ALtMo , Acta Crystallogr., 17, 918-924 (1964) (Crys. Structure, Experimental, 38)... 

Metallurgists originally, and now materials scientists (as well as solid-state chemists) have used erystallographic methods, certainly, for the determination of the structures of intermetallic compounds, but also for such subsidiary parepistemes as the study of the orientation relationships involved in phase transformations, and the study of preferred orientations, alias texture (statistically preferential alignment of the crystal axes of the individual grains in a polycrystalline assembly) however, those who pursue such concerns are not members of the aristocracy The study of texture both by X-ray diffraction and by computer simulation has become a huge sub-subsidiary field, very recently marked by the publication of a major book (Kocks el al. 1998). 

Metals Crystallographic Data File (CRYSTMET). Toth Information Systems Inc., Ottawa, Canada. Electronic database of crystal structures of metals, intermetallic compounds and minerals. WWW.Tothcanada.com. 

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... 

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. 

There are many known structures of intermetallic compounds that involve icosahedral coordination about the smaller atoms. Usually these structures are complex, with 20, 52, 58, 162, 184, or more atoms in a cubic unit of structure. Many of the crystals are cubic. The icosahedron has 12 fivefold axes of symmetry, 20 threefold axes, and 30 twofold axes the fivefold axes cannot be retained in the crystal, but some of the others can be (a maximum of four threefold axes in a cubic crystal). 

Chapter 9 Crystal Structures of Some Intermetallic Compounds... 

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