Intermetallic compounds, stoichiometry

A selection of frequent stoichiometries and common structural types found within the binary intermetallic compounds of the alkali metals is given in Table 5.6. 

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. 

Metals, intermetallic compounds, and alloys generally react with hydrogen and form mainly solid metal-hydrogen compounds (MH ). Hydrides exist as ionic, polymeric covalent, volatile covalent and metallic hydrides. Hydrogen reacts at elevated temperatrrres with many transition metals and their alloys to form hydrides. Many of the MH show large deviations from ideal stoichiometry (n= 1, 2, 3) and can exist as multiphase systems. 

Complex Intermetallic Compounds with Significant Variation in Stoichiometry... 

The compositions of the intermetallic compound layer grown and the adjacent phases were determined by EPMA on a few specimens annealed under different conditions. Point-to-point measurements, 10 to 25 on each specimen, across the intermetallic layer at a step of 3-10 pm showed its Ni (25.0 0.5 at.%) and Bi (75.0 0.5 at.%) contents to correspond to the stoichiometry of the NiBi3 intermetallic compound. 

After the third anneal, the distance d between the marker 3 and the Ni-NiBi3 interface increased from 76 to 137 pm, whereas the distance d2 between this marker and the NiBi3-Bi interface remained unchanged (70 pm). Marker 4 almost disappeared as a result of consumption of the Bi phase. The distance between the markers 3 and 5 decreased by 61 pm (from 202 to 141 pm). In terms of thickness, the consumption of nickel (around 6 pm) is seen to be much less than that of bismuth (61 pm). These values well agree with the stoichiometry of the NiBi3 intermetallic compound, as it must be from a chemical viewpoint. Indeed,... 

Figure 16 shows the steady-state limiting current density, ilim, for the oxygen reduction reaction (ORR) on pure Al, pure Cu, and an intermetallic compound phase in Al alloy 2024-T3 whose stoichiometry is Al20Cu2(Mn,Fe)3 after exposure to a sulfate-chloride solution for 2 hours (43). The steady-state values for the Cu-bearing materials match the predictions of the Levich equation, while those for Al do not. Reactions that are controlled by mass transport in the solution phase should be independent of electrode material type. Clearly, this is not the case for Al, which suggests that some other process is rate controlling. 

In most cases, intermetallic compounds are built by alloying a metal which easily forms stable hydrides (A) and another element which does not form stable hydrides (B). The intermetallics thus formed could then be grouped according to their stoichiometry such as AB5 (LaNis, CaNis), AB2 (ZrMn2, ZrV2), AB (TiFe) and A2B (Mg2Ni). 

Copper, nickel, and cobalt were found by Seitz (5) to diminish the height of the zinc current peak by broadening it. Although the concentration of cobalt in seawater was deemed too low to cause serious problems, the eflFect of copper and nickel required further study. The interference by copper in the stripping determination of zinc was extensively investigated by Bradford (8). He concluded that in the mercury film, copper and zinc formed a 1 1 intermetallic compound that dissociated to release zinc during the oxidation. Thus zinc peak areas remained proportional to the zinc concentration even in the presence of copper, and the analysis of zinc by standard addition was not affected. The interference from nickel was found to be similar to that from copper although the stoichiometry of the intermetallic compoimd could not be determined. 

As noted in section 6.2, when the material of interest is an intermetallic alloy, the solution of its crystal structure may be simplified because intermetallics often form series of isostructural compounds. In contrast to conventional inorganic and molecular compounds, stoichiometries of the majority of intermetallic phases are not restricted by normal valence and oxidation states of atoms and ions therefore, crystal structures of metallic alloy phases are conveniently coded using the classification suggested by W.B. Pearson. According to Pearson, each type of the crystal structure is assigned a specific code (symbol), which is constructed from three components as follows ... 

When melts of some metal mixtures solidify, the alloy formed may possess a definite lattice type that is difierent from those of the pure metals. Such systems are classified as intermetallic compounds, e.g. 3-brass, CuZn. At 298 K, Cu has a ccp lattice and Zn has a structure related to an hep array, but 3-brass adopts a bcc structure. The relative proportions of the two metals are crucial to the alloy being described as an intermetallic compound. Alloys labelled brass may have variable compositions, and the a-phase is a substitutional alloy possessing the ccp structure of Cu with Zn functioning as the solute. 3-Brass exists with Cu Zn stoichiometries around 1 1, but increasing the percentage of Zn leads to a phase transition to y-brass (sometimes written as Cu5Zng, although the composition is not fixed), followed by a transition to... 

Almost 100 actinide intermetallic compounds of this stoichiometry are already known. Most of them appear as Laves phases which are formed of actinides with transition metals, A1 or Zn. 

If K3Bi is used, the reaction follows the same course but much more slowly, the entire reaction requiring several months and with more gas evolution, as expected. Finely divided bismuth (or intermediate K-Bi phases) formed by oxidation by solvent are believed to speed equilibration and formation of product in the foregoing. With KBi2, which already has the correct stoichiometry, the initial solution is brown but becomes dichroic after about a week, and crystals appear on the surface of the intermetallic compound after about a month. Some... 

Here M is a metal, a solid solution alloy or an intermetallic compound, MHj is the hydride and s the molar ratio of hydrogen to metal, H/M. The hydrides frequently show large deviations from stoichiometry. The reaction is in most cases exothermic and reversible, i.e. dihydrogen is recovered by application of heat. The elements which form solid binary metal hydrides are shown in Figure 1. 

In Figure 3.21 potential measurements in the system LiSb are shown.While the borders between intermetallic phases and intermetallic compounds in the LiAl system were fluctuant, the system in Figure 3.21 shows two intermetallic compounds Li2Sb and LijSb which are of nearly ideal stoichiometry. 

In Figure 3.21 the second intermetallic compound LijSb also looks like a stoichiometric compound. The high accuracy of the coulometric titration method reveals that this compound has a very small region of variable stoichiometry of Lij+ Sb with = 10". This is shown in Figure 3.22. 

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