Intermetallic compounds crystal structures

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

The same arguments apply to interstitials. In pure metals of simple (f.c.c. or b.c.c.) crystal structure, the structure and properties of interstitials are well-known for example, in many f.c.c. metals, it consists of a dumbbell oriented in a <1 OOHype direction, with a large formation energy (several eV) and a small migration energy (typically 0.2 eV). In intermetallic compounds, the structure of interstitials is generally not known in some cases, several types of complex defects occur... 

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). 

Crystals of the intermetallic compound magnesium stannide, MgjSn, have been prepared and investigated by means of Laue and spectral photographs with the aid of the theory of space-groups. The intermetallic compound has been found to have the calcium fluoride structure, with dwo = 6.78 0.02 A. U. The closest approach of tin and magnesium atoms is 2.94 0.01 A. U. 

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... 

An important class of intermetallic phases (generally showing rather wide homogeneity ranges) are the Hume-Rothery phases, which are included within the so-called electron compounds . These are phases whose stable crystal structures may be supposed to be mainly controlled by the number of valence electrons per atom, that is, by the previously defined VEC. 

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]

Villars, P. (1995) Factors governing crystal structures. In Intermetallic Compounds Principles and Practice, eds. Westbrook, J.H. and Fleischer, R.L. (John Wiley Sons Ltd., Chichester), p. 227. 

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. 

To solve a crystal structure by direct methods, difficult data are those which are incomplete in the sampling of reciprocal space, have non-atomic i.e. < 1.3A resolution) and are noisy with large (systematic) errors in the data measurements. As we have seen, this definition spans many electron diffraction data sets, but there are some of sufficient quality that they can be solved routinely using conventional direct methods packages. Often these are of inorganic materials or intermetallic compounds that are relatively resistant to radiation damage. 

Well-ordered intermetallic compounds (alloys), when processed in high-energy ball mills exhibit atomic (chemical) disordering in the early stages of ball milling [153]. Let us take as an example ordered AlRu intermetallic crystals that are of B2 type and P-CuZn structure. This structure consists of two simple cubic interpenetrating... 

Guenee et al. [169] synthesized by ingot metallurgy ternary intermetallic compounds LaNi Mg and NdNi Mg having the cubic MgCu Sn crystal structure. They can absorb reversibly up to four hydrogen atoms per formula unit at 7-8 bar and 50°C. The hydrides are stable at room temperature but desorb quite rapidly at 80°C in vacuum. In air, they decompose by catalytic water formation. 

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