Alloys and Intermetallic Compounds

McAulifee and A. G. Mackie Chemistry of Arsenic, Antimony and Bismuth, Ellis Horwood, Chichester, 1990, 350 pp. 

Some of the alkali metal-group 15 element systems give compounds of stoichiometry ME. Of these, LiBi and NaBi have typical alloy stmc-tures and are superconductors below 2.47 K and 2.22 K respectively. Others, like LiAs, NaSb and KSb, have parallel infinite spirals of As or Sb atoms, and it is tempting to formulate them as M+ (E ) in which the (E ) spirals are iso-electronic with those of covalently catenated Se and Te (p. 752) however, their metallic lustre and electrical conductivity indicate at least some metallic bonding. Within the spiral chains As-As is 246 pm (cf. 252 pm in the element) and Sb-Sb is 285 pm (cf. 291 pm in the element). 

Compounds with Sc, Y, lanthanoids and actinoids are of three types. Those with composition ME have the (6-coordinated) NaCl structure, whereas M3E4 (and sometimes M4E3) adopt the body-centred thorium phosphide structure (Th3P4) with 8-coordinated M, and ME2 are like ThAsi in which each Th has 9 As neighbours. Most of these compounds are metallic and those of uranium are magnetically ordered. Full details of the structures and properties of the several hundred other transition metal-Group 15 element compounds fall outside the scope of this treatment, but three particularly important structure types should be mentioned because of their widespread occurrence and relation to other structure types, namely C0AS3, 

NiAs and structures related to those adopted by FeS2 (marcasite, pyrites, loellingite, etc.). 

Structure of nickel arsenide showing (a) 3 unit cells, (b) a single unit cell NiaAsa and its relation to (c) the unit cell of the layer lattice compound Cdia (see text). 

21 in Comprehensive Inorganic Chemistry, Vol. 2, pp. 547-683, Pergamon Press, Oxford, 1973. 

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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. 

Metals, intermetallic compounds and alloys generally react with hydrogen and form mainly solid metal-hydrogen compounds. Hydrides exist as ionic, polymeric covalent, volatile covalent and metallic hydrides. 

The preparation and characterization of intermetallic compounds and alloys of berkelium should be pursued, as well as the determination of the stability constants of Bk(IV) complexes. The range of oxidation states accessible to berkelium might be expanded by stabilizing Bk(II) and/or Bk(V) in highly complexing aqueous, nonaqueous, or even molten salt media and/or in appropriate solid-state matrices. 

At the present state of the art, most interest centers around the intermetallic compounds and alloys w hich have a combination of high critical temperature and high critical field. From a practical standpoint, the alloys, notably niobium-zirconium, are of more interest to end users and designers because the brittle materials present a greater number of manufacturing and fabrication problems. 

In compounds with metals, isolated boron atoms behave as metal atoms and, with an excess of transition metals, boron can form intermetallic compounds and alloys if n > 2 in the stoichiometry M B. Although boron atoms can exist as metallic atoms in metallic alloys they have the tendency to bind to each other if given the opportunity, even in metals. If the boron content increases to values greater than in M2B covalently bound boron pairs, chains and nets are formed in the metal matrix (Figure 4.9). As in the case of network silicates the stoichiometry of the metal boride is related to the type of boron network in the lattice. A few representative metal borides are listed in Table 4.4. 

Non-fused iron catalysts have been studied earlier. The famous Uhde catalyst was KAl (Fe(CN)6), which was used, to be applied in industry. It was abandoned because of its poor stability, and up to now there are still reports about its modifications. Intermetallic compound and alloy catalysts, such as LaNij, FeTi, Fe2Ce and FeZr etc., were also expected to be prospective, but until now they have not been put into practice. In 1970s, the well-known electron donor-acceptor (EDA) catalysts, e.g., phthalocyanine iron-alkali metal, molysite — graphite — potassium and ferrocene-activated carbon-potassium catalyst systems, were found to have the ability to synthesize ammonia under mild conditions in the laboratory. Unfortunately, their activities declined rapidly in the experiments of scale-up. The application of EDA catalysts in industry turned to be a visionary. Therefore, replacement of fused iron catalyst is not an easy thing for a very long time. 

This chapter deals with the structural chemistry of compounds of the actinide elements from Ac to Es as elucidated mainly by the methods of x-ray and neutron diffraction. Metals, intermetallic compounds, and alloys are not considered here (see Chapter 19 and the review by Lam et al. [250]). 

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