Cerium intermetallic compound

Indium combines with several metals, such as sodium, potassium, magnesium, iron, palladium, platinum, lanthanium and cerium, forming semiconductor-type intermetallic compounds. 

The mechanism by which these deleterious elements are controlled is still a matter of speculation. Some researchers have suggested that the rare earth elements combine with the deleterious elements to form innocuous insoluble intermetallic compounds (31). However, such particles have been observed, using the electron microprobe, only when concentrations of both the rare earths and deleterious elements were well above those levels usually found in commercial practice. Even then, the composition of the particular phases was not determined. Further, the effective level of cerium at which the beneficial effects are observed suggests that the mechanism may not be simply compound formation. 

Defect equilibria in intermetallic compounds are inferred from measured changes of vapor pressure with composition and from other experimental information. Equilibria analogous to those in aqueous solution are found in dissociation, com-plexing, and random solution other equilibria connected with the ordering of defects show a distinctly intermetallic flavor. Techniques for calculating the equilibria are described. Cerium-cadmium phase information is collected. 

The use of mechanical milling is also a suitable method for powder preparation. The feature of this procedure is to obtain powders of small crystallite size of a few nanometers with a high concentration of lattice defects. There are many mechanochemical studies on the synthesis of alloys, solid solutions, nanophasc materials, and intermetallic compounds. Of course, the mechanical alloying also has been applied to prepare mixed oxides containing cerium oxide to enhance catalysis, such as CeOj-TbO, CeOj-HfOj, Ce02 Zr02-Mn0, and CeOt ... 

The spectrum recorded in the particle also contains the cenum lines, whereas only traces of the Tb features could be observed. This suggests a selective incorporation of Ce into the Pi lattice (155). The comparison of the fine structure of the Ce M4, M5 peaks of the support and the particle, Figure 4.28, also reveals some interesting information. Note the 1.8 eV shift to lower enei es, the increase of the M4/M5 intensity ratio and the attenuation of the right side lobes in the spectrum recorded in the particles. All these changes can be interpreted as due to a decrease in the oxidation state of the cerium atoms which have incorporated into the supported particles. In fact these fine structure features are in good agreement with those observed for cerium in intermetallic compounds like CePd, CeAb or in y-Ce. In all these compounds the formal oxidation state of cerium is zero. 

Properties Gray, ductile, highly reactive metal. D 6.78, mp 795C, bp 3257C. Attacked by dilute and concentrated mineral acids and by alkalies. Readily oxidizes in moist air at room temperature. It has four allotropic forms. It is the second most reactive rare-earth metal. Cerium forms alloys with other lanthanides (see misch metal) it also forms a nonmetal with hydrogen, as well as carbides and intermetallic compounds. Decomposes water. 

Group 3 (Sc, Y, La) and Rare Earth Metals. These metals do not form intermediates with tungsten. Only cerium is said to form an intermetallic compound of the composition W2Ce with tungsten, but this is still doubtful. 

In preparing fine particles of inorganic metal oxides, the hydrothermal method consists of three types of processes hydrothermal synthesis, hydrothermal oxidation, and hydrothermal crystallization. Hydrothermal synthesis is used to synthesize mixed oxides from their component oxides or hydroxides. The particles obtained are small, uniform crystallites of 0.3-200 0,m in size and dispersed each other. Pressures, temperatures, and mineralizer concentrations control the size and morphology of the particles. In the hydrothermal oxidation method, fine oxide particles can be prepared from metals, alloys, and intermetallic compounds by oxidation with high temperature and pressure solvent, that is, the starting metals are changed into fine oxide powders directly. For example, the solvothermal oxidation of cerium metal in 2-methoxyethanol at 473-523 K yields ultrafine ceria particles (ca. 2 nm). 

In the anode reaction, charge takes place from left to right and discharge occurs in the opposite direction. The metal is an intermetallic AB5-type compound. Some variation occurs in that a rare-earth mixture (typically cerium or lanthanum) is compounded with nickel, manganese, cobalt, or aluminum (for the B portion). Titanium and vanadium can also be used in AB2 intermetallic compounds with nickel, zirconium cobalt, or chromium (as the B portion), but they are rarely used due to performance issues. 

The best catalyst for the synthesis of methanol from CO + H2 mixtures is copper/zinc oxide/alumina. Intermetallic compounds of rare earth and copper can be used as precursors for low-temperature methanol synthesis as first reported by Wallace et al. (1982) for RCu2 compounds (R = La, Ce, Pr, Ho and Th). The catalytic reaction was performed under 50 bar of CO + H2 at 300°C, and XRD analyses revealed the decomposition of the intermetallic into lanthanide oxide, 20-30 nm copper particles and copper oxide. Owen et al. (1987) compared the catalytic activity of RCux compounds, where R stands mainly for cerium in various amounts, but La, Pr, Nd, Gd, Dy and even Ti and Zr were also studied (table 4). The intermetallic compounds were inactive and activation involved oxidation of the alloys using the synthesis gas itself. It started at low pressures (a few bars) and low temperatures (from 353 K upwards). Methane was first produced, then methanol was formed and it is believed that the activation on, for example, CeCu2, involved the following reaction, as already proposed for ThCu2 (Baglin et al. 1981) ... 

The rare-earth intermetallic compound CeCu2Si2 has, like UBcij and UPtj, the special property of a transition to a superconducting state at low temperatures (T 0.6 K). For the cerium compound, experimental conditions are less favourable for NMR, because /7j2 2kOe is relatively small. 

Two types of metal hydrides electrodes, comprising the AB, and AB2 classes of intermetallic compounds, are currently of interest. The AB, alloys have the hexagonal CaCu, structure where the A component comprises one or more rare earth elements and B consists of Ni, or another transition metal, or a transition metal combined with other metals, The paradigm compound of this class is LaNi, which has been well investigated because of its utility in conventional hydrogen-storage applications. Unfortunately LaNi, is too costly, too unstable, and too corrosion— sensitive for use as a battery electrode. Thus commercial AB, electrodes use mischmetal, a low-cost combination of rare earth elements, as a substitute for La. The B, component remains primarily Ni but is substituted in part with Co, Mn, Al, etc. The partial substitution of Ni increases the thermodynamic stability of the hydride phase fl2] and corrosion resistance. Such an alloy is commonly written as MmB, where Mm represents the mischmetal component. The compositions of normal and cerium-free mischmetal are given in Table 2. 

A number of intermetallic compounds containing cerium or uranium are heavy-fermion systems (Stewart 1984, Fisk et al. 1986). The name arises from several low... 

Lij, absorption spectra of Ce intermetallic compounds were published by Bauchspiess et al. (1981). y-type compounds exhibit strong trivalent absorption lines and a more or less pronounced shoulder at high energies, indicating already the presence of tetravalent cerium, a-type compounds can be clearly distinguished spectroscopically from the y-type compounds through a characteristic double-peaked shape (for the discussion of CeOj cf section 14). According to these spectra cerium in metals occurs never in completely trivalent or tetravalent states it apparently always occurs in two types of mixed valent states. The valence of y-type Ce is 3 < i < 3.14 and the valence of a-type is Ce 3.14 < < 3.3-3.4 (cf. Wohlleben 1982, Wohlleben and Rohler 1984). 

The experimental papers cover the various spectroscopic techniques and a few deal with special materials. The introductory chapter (62) by Baer and Schneider presents an overview of this field and helps tie the various aspects together that are reviewed in detail in the remaining chapters of the volume. Photoemission studies (UPS - ultraviolet photoemission spectroscopy, and XPS - X-ray photoemission spectroscopy) on various materials are discussed by Campagna and Hillebrecht (chapter 63)- intermetallic compounds, by Lynch and Weaver (chapter 66)— cerium and its compounds, and by Hiifner (chapter 67) - chalcogenides. Other experimental techniques covered include BIS (bremsstrahlung isochromat spectroscopy) by Hillebrecht and Campagna (chapter 70), X-ray absorption and X-ray emission by Rohler (chapter 71) and inelastic electron scattering by Netzer and Matthew (chapter 72). 

Among cerium compounds CeAb, CeB4, CeN and a few intermetallic compounds (see table 20.1) are believed to exhibit nonintegral valence state... 

Of great interest is the use of intermetallic compounds of platinum with rare-earth metals such as cerium and praseodymium for anodic methanol oxidation, known from the work of Lux and Cairns (2006). This combination is attractive inasmuch as it involves two metals that differ strongly in their own electrode potentials Pt with = -1-1.2 V and Pr with = —2.3 V(SHE), and thus in their electronic structure. However, for the same reason, traditional methods of preparing joint disperse deposits of these metals by chemical or electrochemical reduction in a solution of the corresponding salts fail in such a situation. Lux and Cairns developed a new technology for preparing disperse powders of such compounds by thermal decomposition of complex cyanide salts of these metals. The catalyst obtained had some activity in ethanol oxidation (although somewhat... 

Subsequent to the discussion of the intermetallic compounds we give a description of the transport properties of cerium monopnictides and monochalcogenides, which are compared with various actinide monopnictide and monochalcogenide systems. In these compoimds, which partly show a seraimetal-like behaviour, a great and... 

We start with the question of what happens to the large orbital moment of f electrons when they are hybridized with other states in solids. This question, of course, is central to understanding the unusual properties of actinide (and cerium) compounds. Form-factor measurements had shown the importance of hybridization effects in compounds such as UGej (Lander et al. 1980), but at that time no theory had been developed to handle these effects in particular the orbital contribution was known to be incorrectly treated in band-structure calculations (Brooks et al. 1984, Brooks 1985). Brooks, Johansson, and their collaborators corrected this deficiency by adding an orbital polarization term in the density-functional approximation (see the chapter by Brooks and Johansson (ch. 112) in this volume). When they made calculations on a series of intermetallic compounds, particularly those with a transition metal in the compact fee Laves phase, they found that the value of was reduced compared to the free-ion values. Loosely speaking, we can associate such a partial quenching of the /j ,-value with the fact that the 5f electrons have become partially itinerant, and we know that fully itinerant electrons (in the 3d metals, for example) have 0. 

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