Sublattices intermetallics

Reaction 5.45 is at least partly hypothetical. Evidence that the Cl does react with the Na component of the alanate to form NaCl was found by means of X-ray diffraction (XRD), but the final form of the Ti catalyst is not clear [68]. Ti is probably metallic in the form of an alloy or intermetallic compound (e.g. with Al) rather than elemental. Another possibility is that the transition metal dopant (e.g. Ti) actually does not act as a classic surface catalyst on NaAlH4, but rather enters the entire Na sublattice as a variable valence species to produce vacancies and lattice distortions, thus aiding the necessary short-range diffusion of Na and Al atoms [69]. Ti, derived from the decomposition of TiCU during ball-milling, seems to also promote the decomposition of LiAlH4 and the release of H2 [70]. In order to understand the role of the catalyst, Sandrock et al. performed detailed desorption kinetics studies (forward reactions, both steps, of the reaction) as a function of temperature and catalyst level [71] (Figure 5.39). 

N.H. Due and T. Goto, Itinerant electron metamagnetism of Co sublattice in the lanthanide-cobalt intermetallics 177... 

This is an important point. A sublattice phase with the FCC structure should not, generally speaking, be considered CCP with regards to slip. The atoms or ions on one sublattice may very well be in a CCP-hke arrangement, but they can be kept apart by large atoms or ions residing on the other sublattice (the interstitial sites). Slip is easiest along tmly close-packed layers of identically sized spheres that are in contact and, preferably, without obstacles such as interstitials. Thus, another reason for low ductility in intermetallics and ceramics is the lack of a sufficient number of active slip systems to allow plastic deformation. 

The difference between the compound energy model and the simple two-sublattice model can be illustrated with two ternary intermetallic phases from the Al-Mg-Zn system. One of these two phases is known to contain a constant composition of 54.5 atomic percent magnesium and an extended homogeneity range of aluminum and zinc, corresponding to the formula Mg6(Al,Zn)5. However, no crystallographic data is available for this phase. Therefore, it is appropriately thermodynamically modeled by two sublattices, in which one sublattice is exclusively occupied by Mg, while A1 and Zn are allowed to randomly mix on the second sublattice (Liang et al., 1998). 

The intermetallic phase /3-SbSn has the rock-salt structure with one lattice almost exclusively occupied by antimony and the other by tin. Write the Gibbs energy expression using the compound energy model for a ternary phase including bismuth, where it is assumed that Bi goes preferentially into the mostly Sb sublattice. 

In alloys and intermetallic compounds the sublattices of interstitial sites usually have rather complex structures. Furthermore, hydrogen atoms dissolved in alloys and intermetallics can occupy a number of inequivalent types of interstitial sites. 

The magnetic structure of FeGc2 was established by combined neutron diffraction and Mossbauer measurements as being antiferromagnetic below 287 K with two equivalent but independent sublattices inclined at 71° to each other [116]. The hyperfine field collapses very slowly below the N6el point in an unusual manner. Once again the parameters are consistent with increased 3d occupation at the iron in the intermetallic phases. 

Gui] Calculation, using sublattice model for intermetallic phases Complete ... 

The y-phase is a solid solution with a face-centered crystal lattice and randomly distributed different species of atoms. By contrast, the y -phase has an ordered crystalline lattice of II2 type (Figure 10.2). In pure intermetallic compound NisAl the atoms of aluminum are placed at the vertices of the cubic cell and form the sublattice A. Atoms of nickel are located at the centers of the faces and form the sublattice B. The y -phase has remarkable properties, in particular, an anomalous dependence of strength on temperature. The y -phase first hardens, up to about 1073 K, and then softens. The interatomic bondings Ni-Al are covalent. 

Figure 10.12 illustrates the effect of replacing A1 atoms in the B-sublattice of intermetallic compoimd NisAl by rhenium, ruthenium, tantalum, and tungsten atoms on distribution of the valence electrons. Comparing Figure 10.12a and b, one can see that the valence electrons resonate between atoms in the solid solution. The degree of a delocalization of the valence electrons increases. 

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