Alloys isostructural

Using NbN as a solvent, Hulm and his associates have systematically studied the effect of alloying isostructural carbides and nitrides on 7, 6, and density of states. Their results are summarized in... 

Figure 4.17. The binary phase diagrams of the magnesium alloy systems with the divalent metals ytterbium and calcium (Ca is a typical alkaline earth metal and Yb one of the divalent lanthanides). Notice, for this pair of metals, the close similarity of their alloy systems with Mg. The compounds YbMg2 and CaMg2 are isostructural, hexagonal hP12-MgZn2 type.

Remarks on the crystal chemistry of the alloys of the 3rd group metals. A large number of intermediate phases have been identified in the binary alloys formed by the rare earth metals and actinides with several elements. A short illustrative list is shown in Tables 5.19 and 5.20. Compounds of a few selected rare earth metals and actinides have been considered in order to show some frequent stoichiometries and crystal structure types. The existence of a number of analogies among the different metals considered and the formation of some isostructural series of compounds may be noticed. 

Remarks on the alloy crystal chemistry of the 4th group metals. Selected groups of isostructural phases, pertaining to simple common structural types have been collected in Table 5.25. A number of them (for instance CsCl, AuCu types, Laves phases, AuCu3 type) correspond to more or less extended solid solution... 

Notes on the crystal chemistry of selected alloys and compounds of the 15th group elements. Among the pnictides, several series of numerous isostructural compounds are found. The 1 1 compounds with Sc, Y, lanthanides and actinides, having the NaCl-type structure, generally correspond to very stable phases, strongly... 

Notice that the structures presented in this paragraph are unary structures, that is one species only is present in all its atomic positions. In the prototypes listed (and in the chemically unary isostructural substances) this species is represented by a pure element. In a number of cases, however, more than one atomic species may be equally distributed in the various atomic positions. If each atomic site has the same probability of being occupied in a certain percentage by atoms X and Y and all the sites are compositionally equivalent, the unary prototype is still a valid structural reference. In this case, from a chemical point of view, the structure will correspond to a two-component phase. Notice that there can be many binary (or more complex) solid solution phases having for instance the Cu-type or the W-type structures. Such phases are formed in several metallic alloy systems either as terminal or intermediate phases. 

One interesting isostructural compound is Y5(Si04)3N its 5813 array is the same as that in the alloy 5813 itself (D8g) ... 

Gd-Ni-Sb. GdNiSb2 is isostructural with the HfCuSi2 type structure with lattice parameters of a = 0.43345, c = 0.9452 (Sologub et al., 1994). The alloy was prepared in the same manner as YCuSb2. 

Chemical alloying is simply replacing parts of the molecules by isostructural and isoelectronic parts. For example, alloys such as (TTF) . .(TSeF TCNQ [163], (TMTSF), t(TMTTF),C104 [164], and (TMTSF)2(C104), (ReO,), [165] have been obtained. 

In the case of the Pt/Ceo.8TbojjQ2-x catalyst, the formation of particles of a LnPts (Ln = Ce, Tb) phase, isostructural with CePts, has been confirmed (155). Figure 4.26(c) shows a HREM in which a particle of this intermetallic is present. The details of the DDP, Figure 4.26(d), can be interpreted as due to a [011] orientation of the alloy phase. HREM thus provides evidence about the incorporation of the lanthanides present in the support to the metal particles but, in the case of the catalysts based on the mixed Ce/Tb oxide, it fails to reveal the extent to which each of them come into the alloyed state. From the analysis of the contrasts in the HREM images of the intermetallic particles it is not possible to precise this point. 

Among ordered bimetallic systems, the Pt-Sn one can be considered at present as the most in-depth studied not only for its surface structural properties, but also for its reactivity and catalytic properties. A comparable detailed knowledge exists only for a few other cases, among platinum alloys we can cite the Ni-Pt and Co-Pt systems, examined for their catalytic properties and the Pt-Ti system studied for their electrocatalytic properties [5]. Sparse data relative to the surface properties of several other Pt alloys exist (e.g. FeaPt and CuaPt -[3] and PtaMn [51]. All these data available pertain to fee phases either random substitutional or ordered compounds. Data exist also for other cubic ordered alloys which are isostructural with the PtaSn compound, e.g. NiaAl [52, 53] and AuaPd [28] and finally the Au-Cu system, which has been object of interest as the prototypical LI2 or Pm3m ordered system in the CuaAu composition [54, 55]. 

In order to better understand ensemble-size requirements for reactions on Pt alloys and the role of Sn as a site-blocking agent, it is important to study other Pt-M surfaces that are isostructural with Pt-Sn surface alloys. However, this has turned out to be a difficult task. Different metals (M=Ge, Zn, Cu, Ag, Fe, and Ti) form different surface alloy structures. For example, on Pt(lll), Zn and Cu alloy but do not form long-range ordered structures, and Ge only forms a dilute, two-domain... 

Vl9xVl9)R23.4° surface alloy with 9 =0.05. CO and NO bond more weakly on these alloyed surfaces than on a clean Pt(lll) surface, similar to Sn/Pt(lll) alloys, but a quantitative comparison is not possible. In an analogous systematic approach, Ross reviewed the adsorption of CO on (111) and (100) oriented single crystals of the isostructural/cc alloys with a bulk stoichiometry of PtjM (M = Ti, Co, and Sn) [30],... 

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 ... 

Pearson s classification is insensitive to both chemical compositions and stoichiometries of metallic alloys. It is quite useful because all known intermetallic crystal structures are grouped according to their structural symbols, which are quite simple. Thus, once the symmetry and the content of the unit cell of a new alloy phase have been established, it only makes sense to search for potentially isostructural compounds among those that have identical Pearson s symbols. 

Although the silicon atom has the same outer electronic structure as carbon its chemistry shows very little resemblance to that of carbon. It is true that elementary silicon has the same crystal structure as one of the forms of carbon (diamond) and that some of its simpler compounds have formulae like those of carbon compounds, but there is seldom much similarity in chemical or physical properties. Since it is more electro-positive than carbon it forms compounds with many metals which have typical alloy structures (see the silicides, p. 789) and some of these have the same structures as the corresponding borides. In fact, silicon in many ways resembles boron more closely than carbon, though the formulae of the compounds are usually quite different. Some of these resemblances are mentioned at the beginning of the next chapter. Silicides have few properties in common with carbides but many with borides, for example, the formation of extended networks of linked Si (B) atoms, though on the other hand few silicides are actually isostructural with borides because Si is appreciably larger than B and does not form some of the polyhedral complexes which are peculiar to boron and are one of the least understood features of boron chemistry. 

The results of the investigation of the MoRu alloy system is thus that it seems to be impossible to reconcile theoretical and experimental values, quite independent of which theoretical method one uses. Although it is possible to get quite close to the experimental curve by using the ab initio isostructural mixing enthalpy with the CALPHAD structural energy difference, there is no obvious reason for using this method. One can also find that no reasonable (non-negative) value of the Mo... 

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